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SPI throughput problem MK22

Question asked by Miguel Asselbergs on Jul 29, 2019
Latest reply on Jul 31, 2019 by Jing Pan

Dear all,


I have got a custom PCB design with the MK22FN512CAP12. I have got a SPI device connected to the SPI1 peripheral. The MK22 acts as the master. The baudrate is at 20 MHz. I used the dspi_edma_b2b_transfer_master example project, but I changed from SPI0 to SPI1. I am using SDK version 2.5.0.


I noticed that the data took longer to send than I anticipated, so I went on an investigation. I am sending 500 bytes of 0xAA to test.


I measured the SOUT pin of SPI1 when I sent data and saw that the MK22 sends out one byte of data and then a pause of about  a bytes length and then the next byte of data. See the attached picture: SPI1-SOUT.
I changed from the SPI1 peripheral back to the SPI0 perhipheral and did another measurement on SPI0-SOUT and noticed that the deadtime between bytes is much shorter. This is what I would expect from SPI. See attached picture: SPI0-SOUT.


When I increased the core clock speed from 80 MHz to 120 MHz I noticed that the deadtime between bytes is a little bit less.


I did the same experiment on the FRDM-K22FN dev kit and noticed the same result.


Does anyone know why this happens?