Hello,

I would like to know the maximum frequency that can be used with the SPI bus. Where can I find this information?

Thank you.

Depends on the slave device. I think mbed can do a max of 12.5Mbit/s as per datasheet.

Most SPI devices (i.e. sensors/memory) will not go near this rate though. http://www.nxp.com/documents/data_sheet/LPC1769_68_67_66_65_64_63.pdf

The SPI clock is divide from the 96 MHz clock.

I use a clock of 48 MHz to drive a TFT LCD, but you will not find a lot of devices working as fast. (I check the speed with the scope)

The actual frequency you get on the SPI is the next less frequency entered to (96 MHz / n)

If you enter 13Mhz you will get 96 MHz / 8 = 12 MHz. If you enter 11MHz you will get 96 / 9 = 10.6 MHz ...

For the LPC1768, the datasheet says the maximum SPI rate for the SPS peripheral is 33 MHz. The firmware uses CCLK (96 mhz) for the SPS clock, and the minimum prescale is 2, so the max SPI rate could be 48mhz (but the peripheral limit is 33 mhz). I tested unconnected SPI speeds (megabits/second mbs) with spi.write() of 1024 bytes.

SPS  clock    write             DMA
4MHz          2.6 mbs           3.3 
8MHz          3.9 mbs           6.6
12MHz         4.9 mbs           9.8
16MHz         5.0 mbs          12.9   
24MHz         6.1 mbs          19.1
32MHz         6.6 mbs          19.1

With a logic analyzer, I measured the mbed's SPI CLK frequency, and CLK rates matched the spi.frequecy() request up to 24 mhz. My analyzer can only sample up to 50mhz.

(EDIT) added DMA. DMA should be added to SPI library, many devices could profit from high speed SPI block transfers (wiznet Ethernet, SPI SD, SPI serial flash, ...) Other MCUs utilize DMA/FIFOs. For some comparative results visit https://github.com/manitou48/DUEZoo/blob/master/SPIperf.txt

DMA timed from modifications to the following https://developer.mbed.org/users/okini3939/code/SPIRAM_23LC1024_DMA/