path: root/portable/ThirdParty/GCC/ATmega/port.c

/*
 * FreeRTOS Kernel V10.5.0
 * Copyright (C) 2021 Amazon.com, Inc. or its affiliates.  All Rights Reserved.
 *
 * SPDX-License-Identifier: MIT
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of
 * this software and associated documentation files (the "Software"), to deal in
 * the Software without restriction, including without limitation the rights to
 * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
 * the Software, and to permit persons to whom the Software is furnished to do so,
 * subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
 * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
 * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
 * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 *
 * https://www.FreeRTOS.org
 * https://github.com/FreeRTOS
 *
 */


#include <stdlib.h>

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/wdt.h>

#include "FreeRTOS.h"
#include "task.h"

/*-----------------------------------------------------------
 * Implementation of functions defined in portable.h for the AVR port.
 *----------------------------------------------------------*/

/* Start tasks with interrupts enabled. */
#define portFLAGS_INT_ENABLED           ( (StackType_t) 0x80 )

#if defined( portUSE_WDTO)
    #warning "Watchdog Timer used for scheduler."
    #define portSCHEDULER_ISR           WDT_vect

#elif defined( portUSE_TIMER0 )
/* Hardware constants for Timer0. */
    #warning "Timer0 used for scheduler."
    #define portSCHEDULER_ISR           TIMER0_COMPA_vect
    #define portCLEAR_COUNTER_ON_MATCH  ( (uint8_t) _BV(WGM01) )
    #define portPRESCALE_1024           ( (uint8_t) (_BV(CS02)|_BV(CS00)) )
    #define portCLOCK_PRESCALER         ( (uint32_t) 1024 )
    #define portCOMPARE_MATCH_A_INTERRUPT_ENABLE    ( (uint8_t) _BV(OCIE0A) )
    #define portOCRL                    OCR0A
    #define portTCCRa                   TCCR0A
    #define portTCCRb                   TCCR0B
    #define portTIMSK                   TIMSK0
    #define portTIFR                    TIFR0

#else
    #error "No Timer defined for scheduler"
#endif

/*-----------------------------------------------------------*/

/* We require the address of the pxCurrentTCB variable, but don't want to know
any details of its type. */
typedef void TCB_t;
extern volatile TCB_t * volatile pxCurrentTCB;

/*-----------------------------------------------------------*/

/**
    Enable the watchdog timer, configuring it for expire after
    (value) timeout (which is a combination of the WDP0
    through WDP3 bits).

    This function is derived from <avr/wdt.h> but enables only
    the interrupt bit (WDIE), rather than the reset bit (WDE).

    Can't find it documented but the WDT, once enabled,
    rolls over and fires a new interrupt each time.

    See also the symbolic constants WDTO_15MS et al.

    Updated to match avr-libc 2.0.0
*/

#if defined( portUSE_WDTO)

static __inline__
__attribute__ ((__always_inline__))
void wdt_interrupt_enable (const uint8_t value)
{
    if (_SFR_IO_REG_P (_WD_CONTROL_REG))
    {
        __asm__ __volatile__ (
                "in __tmp_reg__,__SREG__"   "\n\t"
                "cli"                       "\n\t"
                "wdr"                       "\n\t"
                "out %0, %1"                "\n\t"
                "out __SREG__,__tmp_reg__"  "\n\t"
                "out %0, %2"                "\n\t"
                : /* no outputs */
                : "I" (_SFR_IO_ADDR(_WD_CONTROL_REG)),
                "r" ((uint8_t)(_BV(_WD_CHANGE_BIT) | _BV(WDE))),
                "r" ((uint8_t) ((value & 0x08 ? _WD_PS3_MASK : 0x00) |
                        _BV(WDIF) | _BV(WDIE) | (value & 0x07)) )
                : "r0"
        );
    }
    else
    {
        __asm__ __volatile__ (
                "in __tmp_reg__,__SREG__"   "\n\t"
                "cli"                       "\n\t"
                "wdr"                       "\n\t"
                "sts %0, %1"                "\n\t"
                "out __SREG__,__tmp_reg__"  "\n\t"
                "sts %0, %2"                "\n\t"
                : /* no outputs */
                : "n" (_SFR_MEM_ADDR(_WD_CONTROL_REG)),
                "r" ((uint8_t)(_BV(_WD_CHANGE_BIT) | _BV(WDE))),
                "r" ((uint8_t) ((value & 0x08 ? _WD_PS3_MASK : 0x00) |
                        _BV(WDIF) | _BV(WDIE) | (value & 0x07)) )
                : "r0"
        );
    }
}
#endif

/*-----------------------------------------------------------*/
/**
    Enable the watchdog timer, configuring it for expire after
    (value) timeout (which is a combination of the WDP0
    through WDP3 bits).

    This function is derived from <avr/wdt.h> but enables both
    the reset bit (WDE), and the interrupt bit (WDIE).

    This will ensure that if the interrupt is not serviced
    before the second timeout, the AVR will reset.

    Servicing the interrupt automatically clears it,
    and ensures the AVR does not reset.

    Can't find it documented but the WDT, once enabled,
    rolls over and fires a new interrupt each time.

    See also the symbolic constants WDTO_15MS et al.

    Updated to match avr-libc 2.0.0
*/

#if defined( portUSE_WDTO)

static __inline__
__attribute__ ((__always_inline__))
void wdt_interrupt_reset_enable (const uint8_t value)
{
    if (_SFR_IO_REG_P (_WD_CONTROL_REG))
    {
        __asm__ __volatile__ (
                "in __tmp_reg__,__SREG__"   "\n\t"
                "cli"                       "\n\t"
                "wdr"                       "\n\t"
                "out %0, %1"                "\n\t"
                "out __SREG__,__tmp_reg__"  "\n\t"
                "out %0, %2"                "\n\t"
                : /* no outputs */
                : "I" (_SFR_IO_ADDR(_WD_CONTROL_REG)),
                "r" ((uint8_t)(_BV(_WD_CHANGE_BIT) | _BV(WDE))),
                "r" ((uint8_t) ((value & 0x08 ? _WD_PS3_MASK : 0x00) |
                        _BV(WDIF) | _BV(WDIE) | _BV(WDE) | (value & 0x07)) )
                : "r0"
        );
    }
    else
    {
        __asm__ __volatile__ (
                "in __tmp_reg__,__SREG__"   "\n\t"
                "cli"                       "\n\t"
                "wdr"                       "\n\t"
                "sts %0, %1"                "\n\t"
                "out __SREG__,__tmp_reg__"  "\n\t"
                "sts %0, %2"                "\n\t"
                : /* no outputs */
                : "n" (_SFR_MEM_ADDR(_WD_CONTROL_REG)),
                "r" ((uint8_t)(_BV(_WD_CHANGE_BIT) | _BV(WDE))),
                "r" ((uint8_t) ((value & 0x08 ? _WD_PS3_MASK : 0x00) |
                        _BV(WDIF) | _BV(WDIE) | _BV(WDE) | (value & 0x07)) )
                : "r0"
        );
    }
}
#endif

/*-----------------------------------------------------------*/

/*
 * Macro to save all the general purpose registers, the save the stack pointer
 * into the TCB.
 *
 * The first thing we do is save the flags then disable interrupts. This is to
 * guard our stack against having a context switch interrupt after we have already
 * pushed the registers onto the stack - causing the 32 registers to be on the
 * stack twice.
 *
 * r1 is set to zero (__zero_reg__) as the compiler expects it to be thus, however
 * some of the math routines make use of R1.
 *
 * r0 is set to __tmp_reg__ as the compiler expects it to be thus.
 *
 * #if defined(__AVR_HAVE_RAMPZ__)
 * #define __RAMPZ__ 0x3B
 * #endif
 *
 * #if defined(__AVR_3_BYTE_PC__)
 * #define __EIND__ 0x3C
 * #endif
 *
 * The interrupts will have been disabled during the call to portSAVE_CONTEXT()
 * so we need not worry about reading/writing to the stack pointer.
 */
#if defined(__AVR_3_BYTE_PC__) && defined(__AVR_HAVE_RAMPZ__)
/* 3-Byte PC Save  with RAMPZ */
#define portSAVE_CONTEXT()                                                              \
        __asm__ __volatile__ (  "push   __tmp_reg__                             \n\t"   \
                                "in     __tmp_reg__, __SREG__                   \n\t"   \
                                "cli                                            \n\t"   \
                                "push   __tmp_reg__                             \n\t"   \
                                "in     __tmp_reg__, 0x3B                       \n\t"   \
                                "push   __tmp_reg__                             \n\t"   \
                                "in     __tmp_reg__, 0x3C                       \n\t"   \
                                "push   __tmp_reg__                             \n\t"   \
                                "push   __zero_reg__                            \n\t"   \
                                "clr    __zero_reg__                            \n\t"   \
                                "push   r2                                      \n\t"   \
                                "push   r3                                      \n\t"   \
                                "push   r4                                      \n\t"   \
                                "push   r5                                      \n\t"   \
                                "push   r6                                      \n\t"   \
                                "push   r7                                      \n\t"   \
                                "push   r8                                      \n\t"   \
                                "push   r9                                      \n\t"   \
                                "push   r10                                     \n\t"   \
                                "push   r11                                     \n\t"   \
                                "push   r12                                     \n\t"   \
                                "push   r13                                     \n\t"   \
                                "push   r14                                     \n\t"   \
                                "push   r15                                     \n\t"   \
                                "push   r16                                     \n\t"   \
                                "push   r17                                     \n\t"   \
                                "push   r18                                     \n\t"   \
                                "push   r19                                     \n\t"   \
                                "push   r20                                     \n\t"   \
                                "push   r21                                     \n\t"   \
                                "push   r22                                     \n\t"   \
                                "push   r23                                     \n\t"   \
                                "push   r24                                     \n\t"   \
                                "push   r25                                     \n\t"   \
                                "push   r26                                     \n\t"   \
                                "push   r27                                     \n\t"   \
                                "push   r28                                     \n\t"   \
                                "push   r29                                     \n\t"   \
                                "push   r30                                     \n\t"   \
                                "push   r31                                     \n\t"   \
                                "lds    r26, pxCurrentTCB                       \n\t"   \
                                "lds    r27, pxCurrentTCB + 1                   \n\t"   \
                                "in     __tmp_reg__, __SP_L__                   \n\t"   \
                                "st     x+, __tmp_reg__                         \n\t"   \
                                "in     __tmp_reg__, __SP_H__                   \n\t"   \
                                "st     x+, __tmp_reg__                         \n\t"   \
                             );
#elif defined(__AVR_HAVE_RAMPZ__)
/* 2-Byte PC Save  with RAMPZ */
#define portSAVE_CONTEXT()                                                              \
        __asm__ __volatile__ (  "push   __tmp_reg__                             \n\t"   \
                                "in     __tmp_reg__, __SREG__                   \n\t"   \
                                "cli                                            \n\t"   \
                                "push   __tmp_reg__                             \n\t"   \
                                "in     __tmp_reg__, 0x3B                       \n\t"   \
                                "push   __tmp_reg__                             \n\t"   \
                                "push   __zero_reg__                            \n\t"   \
                                "clr    __zero_reg__                            \n\t"   \
                                "push   r2                                      \n\t"   \
                                "push   r3                                      \n\t"   \
                                "push   r4                                      \n\t"   \
                                "push   r5                                      \n\t"   \
                                "push   r6                                      \n\t"   \
                                "push   r7                                      \n\t"   \
                                "push   r8                                      \n\t"   \
                                "push   r9                                      \n\t"   \
                                "push   r10                                     \n\t"   \
                                "push   r11                                     \n\t"   \
                                "push   r12                                     \n\t"   \
                                "push   r13                                     \n\t"   \
                                "push   r14                                     \n\t"   \
                                "push   r15                                     \n\t"   \
                                "push   r16                                     \n\t"   \
                                "push   r17                                     \n\t"   \
                                "push   r18                                     \n\t"   \
                                "push   r19                                     \n\t"   \
                                "push   r20                                     \n\t"   \
                                "push   r21                                     \n\t"   \
                                "push   r22                                     \n\t"   \
                                "push   r23                                     \n\t"   \
                                "push   r24                                     \n\t"   \
                                "push   r25                                     \n\t"   \
                                "push   r26                                     \n\t"   \
                                "push   r27                                     \n\t"   \
                                "push   r28                                     \n\t"   \
                                "push   r29                                     \n\t"   \
                                "push   r30                                     \n\t"   \
                                "push   r31                                     \n\t"   \
                                "lds    r26, pxCurrentTCB                       \n\t"   \
                                "lds    r27, pxCurrentTCB + 1                   \n\t"   \
                                "in     __tmp_reg__, __SP_L__                   \n\t"   \
                                "st     x+, __tmp_reg__                         \n\t"   \
                                "in     __tmp_reg__, __SP_H__                   \n\t"   \
                                "st     x+, __tmp_reg__                         \n\t"   \
                             );
#else
/* 2-Byte PC Save */
#define portSAVE_CONTEXT()                                                              \
        __asm__ __volatile__ (  "push   __tmp_reg__                             \n\t"   \
                                "in     __tmp_reg__, __SREG__                   \n\t"   \
                                "cli                                            \n\t"   \
                                "push   __tmp_reg__                             \n\t"   \
                                "push   __zero_reg__                            \n\t"   \
                                "clr    __zero_reg__                            \n\t"   \
                                "push   r2                                      \n\t"   \
                                "push   r3                                      \n\t"   \
                                "push   r4                                      \n\t"   \
                                "push   r5                                      \n\t"   \
                                "push   r6                                      \n\t"   \
                                "push   r7                                      \n\t"   \
                                "push   r8                                      \n\t"   \
                                "push   r9                                      \n\t"   \
                                "push   r10                                     \n\t"   \
                                "push   r11                                     \n\t"   \
                                "push   r12                                     \n\t"   \
                                "push   r13                                     \n\t"   \
                                "push   r14                                     \n\t"   \
                                "push   r15                                     \n\t"   \
                                "push   r16                                     \n\t"   \
                                "push   r17                                     \n\t"   \
                                "push   r18                                     \n\t"   \
                                "push   r19                                     \n\t"   \
                                "push   r20                                     \n\t"   \
                                "push   r21                                     \n\t"   \
                                "push   r22                                     \n\t"   \
                                "push   r23                                     \n\t"   \
                                "push   r24                                     \n\t"   \
                                "push   r25                                     \n\t"   \
                                "push   r26                                     \n\t"   \
                                "push   r27                                     \n\t"   \
                                "push   r28                                     \n\t"   \
                                "push   r29                                     \n\t"   \
                                "push   r30                                     \n\t"   \
                                "push   r31                                     \n\t"   \
                                "lds    r26, pxCurrentTCB                       \n\t"   \
                                "lds    r27, pxCurrentTCB + 1                   \n\t"   \
                                "in     __tmp_reg__, __SP_L__                   \n\t"   \
                                "st     x+, __tmp_reg__                         \n\t"   \
                                "in     __tmp_reg__, __SP_H__                   \n\t"   \
                                "st     x+, __tmp_reg__                         \n\t"   \
                             );
#endif

/*
 * Opposite to portSAVE_CONTEXT().  Interrupts will have been disabled during
 * the context save so we can write to the stack pointer.
 */
#if defined(__AVR_3_BYTE_PC__) && defined(__AVR_HAVE_RAMPZ__)
/* 3-Byte PC Restore with RAMPZ */
#define portRESTORE_CONTEXT()                                                           \
        __asm__ __volatile__ (  "lds    r26, pxCurrentTCB                       \n\t"   \
                                "lds    r27, pxCurrentTCB + 1                   \n\t"   \
                                "ld     r28, x+                                 \n\t"   \
                                "out    __SP_L__, r28                           \n\t"   \
                                "ld     r29, x+                                 \n\t"   \
                                "out    __SP_H__, r29                           \n\t"   \
                                "pop    r31                                     \n\t"   \
                                "pop    r30                                     \n\t"   \
                                "pop    r29                                     \n\t"   \
                                "pop    r28                                     \n\t"   \
                                "pop    r27                                     \n\t"   \
                                "pop    r26                                     \n\t"   \
                                "pop    r25                                     \n\t"   \
                                "pop    r24                                     \n\t"   \
                                "pop    r23                                     \n\t"   \
                                "pop    r22                                     \n\t"   \
                                "pop    r21                                     \n\t"   \
                                "pop    r20                                     \n\t"   \
                                "pop    r19                                     \n\t"   \
                                "pop    r18                                     \n\t"   \
                                "pop    r17                                     \n\t"   \
                                "pop    r16                                     \n\t"   \
                                "pop    r15                                     \n\t"   \
                                "pop    r14                                     \n\t"   \
                                "pop    r13                                     \n\t"   \
                                "pop    r12                                     \n\t"   \
                                "pop    r11                                     \n\t"   \
                                "pop    r10                                     \n\t"   \
                                "pop    r9                                      \n\t"   \
                                "pop    r8                                      \n\t"   \
                                "pop    r7                                      \n\t"   \
                                "pop    r6                                      \n\t"   \
                                "pop    r5                                      \n\t"   \
                                "pop    r4                                      \n\t"   \
                                "pop    r3                                      \n\t"   \
                                "pop    r2                                      \n\t"   \
                                "pop    __zero_reg__                            \n\t"   \
                                "pop    __tmp_reg__                             \n\t"   \
                                "out    0x3C, __tmp_reg__                       \n\t"   \
                                "pop    __tmp_reg__                             \n\t"   \
                                "out    0x3B, __tmp_reg__                       \n\t"   \
                                "pop    __tmp_reg__                             \n\t"   \
                                "out    __SREG__, __tmp_reg__                   \n\t"   \
                                "pop    __tmp_reg__                             \n\t"   \
                             );
#elif defined(__AVR_HAVE_RAMPZ__)
/* 2-Byte PC Restore with RAMPZ */
#define portRESTORE_CONTEXT()                                                           \
        __asm__ __volatile__ (  "lds    r26, pxCurrentTCB                       \n\t"   \
                                "lds    r27, pxCurrentTCB + 1                   \n\t"   \
                                "ld     r28, x+                                 \n\t"   \
                                "out    __SP_L__, r28                           \n\t"   \
                                "ld     r29, x+                                 \n\t"   \
                                "out    __SP_H__, r29                           \n\t"   \
                                "pop    r31                                     \n\t"   \
                                "pop    r30                                     \n\t"   \
                                "pop    r29                                     \n\t"   \
                                "pop    r28                                     \n\t"   \
                                "pop    r27                                     \n\t"   \
                                "pop    r26                                     \n\t"   \
                                "pop    r25                                     \n\t"   \
                                "pop    r24                                     \n\t"   \
                                "pop    r23                                     \n\t"   \
                                "pop    r22                                     \n\t"   \
                                "pop    r21                                     \n\t"   \
                                "pop    r20                                     \n\t"   \
                                "pop    r19                                     \n\t"   \
                                "pop    r18                                     \n\t"   \
                                "pop    r17                                     \n\t"   \
                                "pop    r16                                     \n\t"   \
                                "pop    r15                                     \n\t"   \
                                "pop    r14                                     \n\t"   \
                                "pop    r13                                     \n\t"   \
                                "pop    r12                                     \n\t"   \
                                "pop    r11                                     \n\t"   \
                                "pop    r10                                     \n\t"   \
                                "pop    r9                                      \n\t"   \
                                "pop    r8                                      \n\t"   \
                                "pop    r7                                      \n\t"   \
                                "pop    r6                                      \n\t"   \
                                "pop    r5                                      \n\t"   \
                                "pop    r4                                      \n\t"   \
                                "pop    r3                                      \n\t"   \
                                "pop    r2                                      \n\t"   \
                                "pop    __zero_reg__                            \n\t"   \
                                "pop    __tmp_reg__                             \n\t"   \
                                "out    0x3B, __tmp_reg__                       \n\t"   \
                                "pop    __tmp_reg__                             \n\t"   \
                                "out    __SREG__, __tmp_reg__                   \n\t"   \
                                "pop    __tmp_reg__                             \n\t"   \
                             );
#else
/* 2-Byte PC Restore */
#define portRESTORE_CONTEXT()                                                           \
        __asm__ __volatile__ (  "lds    r26, pxCurrentTCB                       \n\t"   \
                                "lds    r27, pxCurrentTCB + 1                   \n\t"   \
                                "ld     r28, x+                                 \n\t"   \
                                "out    __SP_L__, r28                           \n\t"   \
                                "ld     r29, x+                                 \n\t"   \
                                "out    __SP_H__, r29                           \n\t"   \
                                "pop    r31                                     \n\t"   \
                                "pop    r30                                     \n\t"   \
                                "pop    r29                                     \n\t"   \
                                "pop    r28                                     \n\t"   \
                                "pop    r27                                     \n\t"   \
                                "pop    r26                                     \n\t"   \
                                "pop    r25                                     \n\t"   \
                                "pop    r24                                     \n\t"   \
                                "pop    r23                                     \n\t"   \
                                "pop    r22                                     \n\t"   \
                                "pop    r21                                     \n\t"   \
                                "pop    r20                                     \n\t"   \
                                "pop    r19                                     \n\t"   \
                                "pop    r18                                     \n\t"   \
                                "pop    r17                                     \n\t"   \
                                "pop    r16                                     \n\t"   \
                                "pop    r15                                     \n\t"   \
                                "pop    r14                                     \n\t"   \
                                "pop    r13                                     \n\t"   \
                                "pop    r12                                     \n\t"   \
                                "pop    r11                                     \n\t"   \
                                "pop    r10                                     \n\t"   \
                                "pop    r9                                      \n\t"   \
                                "pop    r8                                      \n\t"   \
                                "pop    r7                                      \n\t"   \
                                "pop    r6                                      \n\t"   \
                                "pop    r5                                      \n\t"   \
                                "pop    r4                                      \n\t"   \
                                "pop    r3                                      \n\t"   \
                                "pop    r2                                      \n\t"   \
                                "pop    __zero_reg__                            \n\t"   \
                                "pop    __tmp_reg__                             \n\t"   \
                                "out    __SREG__, __tmp_reg__                   \n\t"   \
                                "pop    __tmp_reg__                             \n\t"   \
                             );
#endif
/*-----------------------------------------------------------*/

/*
 * Perform hardware setup to enable ticks from relevant Timer.
 */
static void prvSetupTimerInterrupt( void );
/*-----------------------------------------------------------*/

/*
 * See header file for description.
 */
StackType_t *pxPortInitialiseStack( StackType_t *pxTopOfStack, TaskFunction_t pxCode, void *pvParameters )
{
uint16_t usAddress;
    /* Simulate how the stack would look after a call to vPortYield() generated by
    the compiler. */

    /* The start of the task code will be popped off the stack last, so place
    it on first. */
    usAddress = ( uint16_t ) pxCode;
    *pxTopOfStack = ( StackType_t ) ( usAddress & ( uint16_t ) 0x00ff );
    pxTopOfStack--;

    usAddress >>= 8;
    *pxTopOfStack = ( StackType_t ) ( usAddress & ( uint16_t ) 0x00ff );
    pxTopOfStack--;

#if defined(__AVR_3_BYTE_PC__)
    /* The AVR ATmega2560/ATmega2561 have 256KBytes of program memory and a 17-bit
     * program counter. When a code address is stored on the stack, it takes 3 bytes
     * instead of 2 for the other ATmega* chips.
     *
     * Store 0 as the top byte since we force all task routines to the bottom 128K
     * of flash. We do this by using the .lowtext label in the linker script.
     *
     * In order to do this properly, we would need to get a full 3-byte pointer to
     * pxCode. That requires a change to GCC. Not likely to happen any time soon.
     */
    *pxTopOfStack = 0;
    pxTopOfStack--;
#endif

    /* Next simulate the stack as if after a call to portSAVE_CONTEXT().
    portSAVE_CONTEXT places the flags on the stack immediately after r0
    to ensure the interrupts get disabled as soon as possible, and so ensuring
    the stack use is minimal should a context switch interrupt occur. */
    *pxTopOfStack = ( StackType_t ) 0x00;    /* R0 */
    pxTopOfStack--;
    *pxTopOfStack = portFLAGS_INT_ENABLED;
    pxTopOfStack--;

#if defined(__AVR_3_BYTE_PC__)
    /* If we have an ATmega256x, we are also saving the EIND register.
     * We should default to 0.
     */
    *pxTopOfStack = ( StackType_t ) 0x00;    /* EIND */
    pxTopOfStack--;
#endif

#if defined(__AVR_HAVE_RAMPZ__)
    /* We are saving the RAMPZ register.
     * We should default to 0.
     */
    *pxTopOfStack = ( StackType_t ) 0x00;    /* RAMPZ */
    pxTopOfStack--;
#endif

    /* Now the remaining registers. The compiler expects R1 to be 0. */
    *pxTopOfStack = ( StackType_t ) 0x00;    /* R1 */

    /* Leave R2 - R23 untouched */
    pxTopOfStack -= 23;

    /* Place the parameter on the stack in the expected location. */
    usAddress = ( uint16_t ) pvParameters;
    *pxTopOfStack = ( StackType_t ) ( usAddress & ( uint16_t ) 0x00ff );
    pxTopOfStack--;

    usAddress >>= 8;
    *pxTopOfStack = ( StackType_t ) ( usAddress & ( uint16_t ) 0x00ff );

    /* Leave register R26 - R31 untouched */
    pxTopOfStack -= 7;

    return pxTopOfStack;
}
/*-----------------------------------------------------------*/

BaseType_t xPortStartScheduler( void )
{
    /* Setup the relevant timer hardware to generate the tick. */
    prvSetupTimerInterrupt();

    /* Restore the context of the first task that is going to run. */
    portRESTORE_CONTEXT();

    /* Simulate a function call end as generated by the compiler. We will now
    jump to the start of the task the context of which we have just restored. */
    __asm__ __volatile__ ( "ret" );

    /* Should not get here. */
    return pdTRUE;
}
/*-----------------------------------------------------------*/

void vPortEndScheduler( void )
{
	/* It is unlikely that the ATmega port will get stopped. */
}
/*-----------------------------------------------------------*/

/*
 * Manual context switch. The first thing we do is save the registers so we
 * can use a naked attribute.
 */
void vPortYield( void ) __attribute__ ( ( hot, flatten, naked ) );
void vPortYield( void )
{
    portSAVE_CONTEXT();
    vTaskSwitchContext();
    portRESTORE_CONTEXT();

    __asm__ __volatile__ ( "ret" );
}
/*-----------------------------------------------------------*/

/*
 * Manual context switch callable from ISRs. The first thing we do is save
 * the registers so we can use a naked attribute.
 */
void vPortYieldFromISR(void) __attribute__ ( ( hot, flatten, naked ) );
void vPortYieldFromISR(void)
{
    portSAVE_CONTEXT();
    vTaskSwitchContext();
    portRESTORE_CONTEXT();

    __asm__ __volatile__ ( "reti" );
}
/*-----------------------------------------------------------*/

/*
 * Context switch function used by the tick. This must be identical to
 * vPortYield() from the call to vTaskSwitchContext() onwards. The only
 * difference from vPortYield() is the tick count is incremented as the
 * call comes from the tick ISR.
 */
void vPortYieldFromTick( void ) __attribute__ ( ( hot, flatten, naked ) );
void vPortYieldFromTick( void )
{
    portSAVE_CONTEXT();
    if( xTaskIncrementTick() != pdFALSE )
    {
        vTaskSwitchContext();
    }
    portRESTORE_CONTEXT();

    __asm__ __volatile__ ( "ret" );
}
/*-----------------------------------------------------------*/

#if defined(portUSE_WDTO)
/*
 * Setup WDT to generate a tick interrupt.
 */
void prvSetupTimerInterrupt( void )
{
    /* reset watchdog */
    wdt_reset();

    /* set up WDT Interrupt (rather than the WDT Reset). */
    wdt_interrupt_enable( portUSE_WDTO );
}

#elif defined (portUSE_TIMER0)
/*
 * Setup Timer0 compare match A to generate a tick interrupt.
 */
static void prvSetupTimerInterrupt( void )
{
uint32_t ulCompareMatch;
uint8_t ucLowByte;

    /* Using 8bit Timer0 to generate the tick. Correct fuses must be
    selected for the configCPU_CLOCK_HZ clock.*/

    ulCompareMatch = configCPU_CLOCK_HZ / configTICK_RATE_HZ;

    /* We only have 8 bits so have to scale 1024 to get our required tick rate. */
    ulCompareMatch /= portCLOCK_PRESCALER;

    /* Adjust for correct value. */
    ulCompareMatch -= ( uint32_t ) 1;

    /* Setup compare match value for compare match A. Interrupts are disabled
    before this is called so we need not worry here. */
    ucLowByte = ( uint8_t ) ( ulCompareMatch & ( uint32_t ) 0xff );
    portOCRL = ucLowByte;

    /* Setup clock source and compare match behaviour. */
    portTCCRa = portCLEAR_COUNTER_ON_MATCH;
    portTCCRb = portPRESCALE_1024;


    /* Enable the interrupt - this is okay as interrupt are currently globally disabled. */
    ucLowByte = portTIMSK;
    ucLowByte |= portCOMPARE_MATCH_A_INTERRUPT_ENABLE;
    portTIMSK = ucLowByte;
}

#endif

/*-----------------------------------------------------------*/

#if configUSE_PREEMPTION == 1

    /*
     * Tick ISR for preemptive scheduler. We can use a naked attribute as
     * the context is saved at the start of vPortYieldFromTick(). The tick
     * count is incremented after the context is saved.
     *
     * use ISR_NOBLOCK where there is an important timer running, that should preempt the scheduler.
     *
     */
    ISR(portSCHEDULER_ISR, ISR_NAKED) __attribute__ ((hot, flatten));
/*  ISR(portSCHEDULER_ISR, ISR_NAKED ISR_NOBLOCK) __attribute__ ((hot, flatten));
 */
    ISR(portSCHEDULER_ISR)
    {
        vPortYieldFromTick();
        __asm__ __volatile__ ( "reti" );
    }
#else

    /*
     * Tick ISR for the cooperative scheduler. All this does is increment the
     * tick count. We don't need to switch context, this can only be done by
     * manual calls to taskYIELD();
     *
     * use ISR_NOBLOCK where there is an important timer running, that should preempt the scheduler.
     */
    ISR(portSCHEDULER_ISR) __attribute__ ((hot, flatten));
/*  ISR(portSCHEDULER_ISR, ISR_NOBLOCK) __attribute__ ((hot, flatten));
 */
    ISR(portSCHEDULER_ISR)
    {
        xTaskIncrementTick();
    }
#endif