docs: Note mcu types with i2c_speed support in Config_Reference.md

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

1 files changed, 11 insertions, 2 deletions

diff --git a/docs/Config_Reference.md b/docs/Config_Reference.md
index ac77b9419..be33c7237 100644
--- a/docs/Config_Reference.md
+++ b/docs/Config_Reference.md
@@ -1536,9 +1536,9 @@ number of sections with an "mpu9250" prefix).
 #   Default is 104 (0x68).
 #i2c_mcu:
 #i2c_bus:
-#i2c_speed:
+#i2c_speed: 400000
 #   See the "common I2C settings" section for a description of the
-#   above parameters.
+#   above parameters. The default "i2c_speed" is 400000.
 #axes_map: x, y, z
 #   See the "adxl345" section for information on this parameter.
 ```
@@ -4224,6 +4224,15 @@ support for error recovery varies between each micro-controller type.
 It is generally recommended to only use i2c devices that are on the
 same printed circuit board as the micro-controller.
 
+Most Klipper micro-controller implementations only support an
+`i2c_speed` of 100000. The Klipper "linux" micro-controller supports a
+400000 speed, but it must be
+[set in the operating system](RPi_microcontroller.md#optional-enabling-i2c)
+and the `i2c_speed` parameter is otherwise ignored. The Klipper
+"rp2040" micro-controller supports a rate of 400000 via the
+`i2c_speed` parameter. All other Klipper micro-controllers use a
+100000 rate and ignore the `i2c_speed` parameter.
+
 ```
 #i2c_address:
 #   The i2c address of the device. This must specified as a decimal