I am trying to implement the SPI module on the S32K144 RevA on a demonstration board. My program loops back MOSI on MISO, sending a number over SPI, which is incremented on reception, and sent again. When this number reaches an arbitrary high (20 000), the red LED is toggled and the number is reset (to 0). In pseudo-code:
myNumber = 0
send over SPI(myNumber)
wait for reception()
receivedNumber = read from SPI()
if receivedNumber == 20000
myNumber = 0
increment received number
assign received number to myNumber
The LED toggling frequency should therefore be directly proportional (or at least, linked) to the SPI frequency. Here is where I cease to understand what is happening:
I do not understand why the SPI CLK/bit frequency is seemingly not linked to the LED toggling frequency. (which should be linked to the SPI transmission speed, i.e. SPI CLK frequency)
Could you please help me understand this?
Thanks in advance,
The SPI baudrate is given by
Baudrate = SPI functional clock / 2^TCR[PRESCALE] / (CCR[SCKDIV] +2)
The Functional clock is selected by PPC_LPSPIx register. Not sure what is your configuration in fact, but assuming the XOSC (8Mhz) is selected by PPC_LPSPIx register then using TCR[PRESCALE] =0 and CCR[SCKDIV] = 6 you get
Baudrate = 8Mhz / 1 / 8 = 1MHz, Bit period = 1us
Then if you are using 16bit SPI frame the single SPI transmission takes 16*1us = 16us
In fact will be more depending complete CCR register setting.
Consired there will be no SW and HW overhead then sending of 20000 words takes 20000*16u = 320ms and the LED will toggle with this rate.