Change SPI clock polarity between a WRITE and a READ instruction

[espruino-discuss]

Posted at 2018-09-12 by benoit

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 :)

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[espruino-discuss]

Posted at 2018-09-18 by benoit

Any idea ?

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Posted at 2018-09-18 by @allObjects

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...

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Posted at 2018-09-18 by @gfwilliams

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.

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Posted at 2018-09-21 by benoit

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