Change SPI clock polarity between a WRITE and a READ instruction

Hello,

I want to access the TI TRF7960 (a 13.56MHz RFID reader/writer) with Espruino running on a nucleo STM32F401RE. The TI chip can be accessed with SPI, but with an exotic trick : clock polarity has to change between a write and a read instruction. Below is a drawing from the application report "Using the SPI Interface With TRF7960" (http://www.ti.com/lit/an/sloa140a/sloa140a.pdf).

How could I do that ? I tried the following but it doesn't work.

var spi = new SPI();
spi.setup({sck:B6, miso:A6, mosi:A7, mode:integer=0, order:'msb'});

digitalWrite(SS, 0);
spi.write(0b01000100);
spi.setup({sck:B6, miso:A6, mosi:A7, mode:integer=3, order:'msb'});
d = spi.write(0b01000100);

digitalWrite(SS, 1);

Thank you :)

1 Attachment

Any idea ?

not really, because this is odd... may be it saves a line or a few bytes / instructions, but it's more about proprietary than solution.

Adding a new version of soft SPI based on the existing one in the firmware is the only solution I can see. working... or use a attiny## slave with bitbanging...

Hi - I was on holiday last week, hence the lack of replies.

While it's unlikely to be your problem, you'd want to remove integer= from your code, as that's not doing what you expect :)

SPI.setup might be resetting pin states to their default values which would throw things off - you could try a slightly hacky method of just updating the software SPI values directly:

var spi = new SPI();
spi.setup({sck:B6, miso:A6, mosi:A7, mode:integer=0, order:'msb'});
digitalWrite(SS, 0);
spi.write(0b01000100);
spi._options.mode=3;
d = spi.write(0b01000100);
digitalWrite(SS, 1);

Otherwise I'd say to just manually create the waveform using for loops and digitalWrite. Some kind of oscilloscope would probably really help you do see what's going on though.

Hi, sorry for the delay to reply.

I tried and it seems to work, thank you very much ! I will post my code when I will be able to make all the necessary tests to confirm the trick.

Best regards