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/**
******************************************************************************
* @file tsl_filter.c
* @author MCD Application Team
* @version V2.2.0
* @date 01-february-2016
* @brief This file contains all functions to manage the signal or delta filters.
******************************************************************************
* @attention
*
* <h2><center>© COPYRIGHT 2014 STMicroelectronics</center></h2>
*
* Licensed under MCD-ST Liberty SW License Agreement V2, (the "License");
* You may not use this file except in compliance with the License.
* You may obtain a copy of the License at:
*
* http://www.st.com/software_license_agreement_liberty_v2
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
******************************************************************************
*/
/* Includes ------------------------------------------------------------------*/
#include "tsl_filter.h"
/* Private typedefs ----------------------------------------------------------*/
/* Private defines -----------------------------------------------------------*/
/* Private macros ------------------------------------------------------------*/
/* Private variables ---------------------------------------------------------*/
/* Private functions prototype -----------------------------------------------*/
/* Noise Filter description
------------------------
The noise filter is a first order IRR digital filter based on the following formula:
S(n) = (1-k).S(n-1)+ k.N(n)
S(n) : sample number n of the filtered signal
N(n) : sample number n of the raw signal
k : filter coefficient parameter in [0..1]
The filter sampling rate is the acquisition rate.
In order to optimize the implementation in the firmware, the above formula is
modified in order to have only one multiply operation:
S(n) = S(n-1) + k.(N(n) - S(n-1))
Additionally, we use k = K/256 with K an unsigned 8-bit integer.
The K is given by the ACQ_FILTER_COEFF constant.
S(n) = S(n-1) + K.(N(n) - S(n-1))/(2^8)
and the division can be done easily with bit shifting.
As we are in the digital world, this formula presents a drawback:
if the difference between S(n-1) and N(n) is less than 1/k, there will be no
difference between S(n-1) and S(n).
As a consequence, there will be a static error of up to 1/k.
In the STMTouch Driver, the S(n) is stored in the Meas element of the data
structure after each acquisition:
Meas(n) = S(n) = N(n)
The formula is then:
Meas(n) = Meas(n-1) + K.(Meas(n) - Meas(n-1))/(2^8)
In order to reduce the static error, we can use "Meas(n) = S(n).2^P".
The P is given by the ACQ_FILTER_RANGE constant.
This would shift the signal value left and provides a few additional low
significant bits useful to reduce the static error.
Warning: all thresholds must be shifted accordingly if the parameter P is
different from 0.
If we report this into the filter formula we obtain:
Meas(n) = Meas(n-1) + K.[ Meas(n)*2^P - Meas(n-1)]/2^8
In this case the static error is reduced to 1/(k.2^P)
*/
#define ACQ_FILTER_RANGE (0) /* Range[0..5] - Warning: all thresholds must be shifted if different from 0 */
#define ACQ_FILTER_COEFF (128) /* Range[1..255] - First order filter coefficient (k = ACQ_FILTER_COEFF/256) */
/**
* @brief Example of measure value filter
* @param[in] measn1 Previous measure value
* @param[in] measn Current measure value
* @retval Filtered measure
*/
TSL_tMeas_T TSL_filt_MeasFilter(TSL_tMeas_T measn1, TSL_tMeas_T measn)
{
TSL_tMeas_T val;
val = (TSL_tMeas_T)(measn << ACQ_FILTER_RANGE);
if (measn1 != 0)
{
if (val > measn1)
{
val = measn1 + ((ACQ_FILTER_COEFF * (val - measn1)) >> 8);
}
else
{
val = measn1 - ((ACQ_FILTER_COEFF * (measn1 - val)) >> 8);
}
}
return(val);
}
/**
* @brief Example of delta value filter
* @param[in] delta Delta value to modify
* @retval Filtered delta
*/
TSL_tDelta_T TSL_filt_DeltaFilter(TSL_tDelta_T delta)
{
return(delta);
}
/************************ (C) COPYRIGHT STMicroelectronics *****END OF FILE****/
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