SPI custom clock frequency value calculation

RE: SPI custom clock frequency value calculation

Nick_RA — Tue, 13 Aug 2024 15:32:08 GMT

Sadly 5kbps wasn't achievable, although it will go low enough the frequency setting doesn't have enough resolution - it jumps from 4k to 6k

    spi_config.frequency =   0x0017fffeUL; // 250uS 4kbps
    spi_config.frequency =   0x0017ffffUL; // 165uS 6kbps

Still it may be a moot point after all: looking at hardware options for the 433MHz RF side many devices now incorporate a TX FIFO so I may be able to just use SPI as-is.

RE: SPI custom clock frequency value calculation

hmolesworth — Wed, 07 Aug 2024 17:24:31 GMT

Some of the peripherals use common hardware implementations, so for example a peripheral can be used as either SPI or I2C depending on the enable bits; that can create difficulties if switching between them but maybe the baud rate generator is similar to the Uart baud rate generator; I don't remember trying this on the SPIM but maybe worth a look at my (unofficial) code:

   // Baud rate calculation on nRF51 and nRF52832
   // ===========================================
   //
   // Calculate BAUDRATE value settings for the black-magic 20-bit baud rate generator

   uint32_t CalculatedRegisterValue;
   uint64_t SystemClock = 16000000ULL;    // Typically 16MHz
   uint64_t MagicScaler = 32; // Preserves bits on divisions, shift 32 bits

   // Baudrate generator for the UART is the top 20 bits of a 32-bit field; add in rounding 0.5 ls bit
   CalculatedRegisterValue = (uint32_t)(((((uint64_t)RequiredBaudRate << MagicScaler) + (SystemClock>>1)) / SystemClock) + 0x800) & 0xFFFFF000;

RE: SPI custom clock frequency value calculation

Kenneth — Wed, 07 Aug 2024 13:46:53 GMT

It's not that I don't want to answer really, but I would have to go into the design description for the serial box that include the spi ip to find the details, and that is a bit out of scope.

Kenneth

RE: SPI custom clock frequency value calculation

Nick_RA — Wed, 07 Aug 2024 12:54:53 GMT

Thanks, but as I highlighted in my original post, I'm not after tested & qualified values! Especially as I'm not actually using it as a SPI interface per se.

Oh well, if you can't/won't divulge the workings behind the FREQUENCY register I'll have to resort to a fudge-it-and-see approach.

RE: SPI custom clock frequency value calculation

Kenneth — Wed, 07 Aug 2024 12:12:52 GMT

[quote user="Nick_RA"]I was hoping you could advise how the SPIM FREQUENCY register relates to the actual frequency rather than me doing trial-and-error,[/quote]

I can understand that, but unfortunately we have only tested and qualifed those values listed in the FREQUENCY register.

Kenneth

RE: SPI custom clock frequency value calculation

Nick_RA — Wed, 07 Aug 2024 11:45:17 GMT

Hi Kenneth

Actually I don't need the clock itself. I just need to clock out data at 5kbps; the data itself will be Manchester encoded, which is self-clocking.

I was hoping you could advise how the SPIM FREQUENCY register relates to the actual frequency rather than me doing trial-and-error,

RE: SPI custom clock frequency value calculation

Kenneth — Wed, 07 Aug 2024 11:24:21 GMT

Hello,

I would have looked at using PWM, should be no problem to make a 5kHz PWM signal that is continuous. But if you only want to send 56 "clock" periods, then I agree that using SPIM might also work, but you will need to test it, because you may not get a very accurate frequency of 5kHz, since i believe not all LSB bits of the FREQUENCY register impact the frequency. So you will need to simply test how close you get to 5kHz:
https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.ps.v1.1/spim.html#register.FREQUENCY

Kenneth