slave board: ARDUINO UNO REV3
SPI API: beginTransaction(SPISettings(uint32_t clock, uint8_t bitOrder, uint8_t dataMode))
Who know the the effective range of clock setting?
ATmega328P.pdf has the following content:
• Bit 0 - SPI2X: Double SPI Speed Bit
When this bit is written logic one the SPI speed (SCK Frequency) will be doubled when the SPI
is in Master mode (see Table 18-5). This means that the minimum SCK period will be two CPU
clock periods. When the SPI is configured as Slave, the SPI is only guaranteed to work at fosc/4
or lower.
How many mHZ match "fosc/4"?