SPI.setClockSpeed/setClockDividerReference

[m-mcgowan]

SPI.setClockSpeed, SPI.setClockDividerReference for more portable SPI clock speeds.

Examples:

Set the clock divider reference for arduino compatibility

SPI.setClockDividerReference(ARDUINO);
SPI.setClockDivider(SPI_CLOCK_DIV4);

Set an absolute clock speed (or the highest supported speed without going over)

unsigned acutalSpeed = SPI.setClockSpeed(20, MHZ);

[m-mcgowan]

The reference clock on SPI is 60MHz and on SPI1 is 120MHz, this needs to be reflected in the code.

[satishgn]

Just to add in here from the STM32F2 datasheet: "SPI1 can communicate at up to 30 Mbits/s, while SPI2 and SPI3 can communicate at up to 15 Mbit/s. The 3-bit prescaler gives 8 master mode frequencies and the frame is configurable to 8 bits or 16 bits."

[satishgn]

Some calculations to show how the above can be achieved:
HCLK = SYSCLK (120MHz)
PCLK2 = HCLK/2 (60MHz) - base clock for SPI1
PCLK1 = HCLK/4 (30MHz) - base clock for SPI2/3

SPI1 - using minimum prescaler 1 gives 60/(1+1) = 30 Mbits/s (max) [using SPI_CLOCK_DIV2]
SPI2/3 - using minimum prescaler 1 gives 30/(1+1) = 15 Mbit/s (max) [using SPI_CLOCK_DIV2]

In spi_hal.c, if user doesn't set SPI.setClockDivider() explicitly then it defaults to Arduino's SPI_CLOCK_DIV4 which corresponds to "SPI_BaudRatePrescaler_4 in stm32f2xx_spi.h"

Default SPI speed on Photon as per latest code would be:
SPI (SPI1) = 60/4 = 15 Mbit/s
SPI1 (SPI3) = 30/4 = 7 Mbit/s (This should be changed to default SPI_CLOCK_DIV2 right for 15 Mbit/s)

[satishgn]

A fix for default 15MBit/s on all SPI interfaces on photon: #583