Unexpected SPI Clock Speed Behavior on Arduino R4

[Jampag]

Issue: Unexpected SPI Clock Speed Behavior on Arduino R4

Source: Arduino Forum Discussion ITA

Summary

When using the SPI library on Arduino R4, the SPI clock speed does not behave as expected after reinitialization:

• SPI is initialized with SPI.begin() and configured to 20MHz using SPISettings.
• After calling SPI.end() and reinitializing, the SPI clock defaults to 1MHz, ignoring the previous 20MHz setting.
• This issue occurs only when reusing the same SPISettings object without modifying it.

Problematic Code Example

SPI.begin();
mySettings = SPISettings(20000000, MSBFIRST, SPI_MODE0);
SPI.beginTransaction(mySettings);
SPI.transfer(0x00);
SPI.endTransaction();
SPI.end();
// Repeating the same cycle causes SPI to default to 1MHz

Technical Insight

• The ArduinoCore-renesas implementation resets the SPI clock to 1MHz every time SPI.begin() is called.
• SPI.beginTransaction() updates the clock only if the new SPISettings differ from the previous ones.
• Reusing the same SPISettings object prevents the clock from being updated, resulting in a default 1MHz speed.

Context

The user needs to frequently release SPI pins and dynamically change SPI configurations during runtime, which requires repeated calls to SPI.begin() and SPI.end().