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1. 

   nplayle
   Member

   Hi all,
   I'm a relatively new user of the maple, so forgive me if this is a simple question.

   I'm trying to read data via the SPI bus for an accelerometer. Writing data works as expected, but using spi.read is causing me some issues.

   When I send the data address out, it uses a full byte as expected. Then I use spi.read, but the chip is no longer being clocked by the spi.write command, so no data is returned to me. Since a full read cycle takes 16 clocks, the accelerometer doesnt send data until after 8 have passed. I can cheat it by writing blank data, and then the accelerometer sends data back to me, but something tells me this isnt a good way of doing it... I'm assuming it has something to do with not detecting there are unread bytes, but I'm not sure how to tell it there are.

   Here is the code I have
   This works:
   

         spi.write(0xE8);     
         spi.write(0x00);
         uint8 data2 = spi.read();

   This doesnt:
   

   spi.write(0xE8);<br />
         uint8 data2 = spi.read();

   Thanks
   Nick

   Posted 4 years ago #
2. 

   mbolivar
   LeafLabs

   Hi Nick,

   I'll look into this.

   Posted 4 years ago #
3. 

   crenn
   Moderator

   nplayle, that's actually the way that you use SPI, if you want data back, you need to send a 'dummy' byte which isn't used for anything. Spi.read() just returns the last byte that came into the SPI data register.

   Posted 4 years ago #
4. 

   robodude666
   Member

   Check the datasheet of the device you're trying to communicate with. Generally speaking, you must always send a dummy byte like crenn said. The byte received then is the response from the device. The datasheet for the device you're communicating with will have a waveform diagram with an example of what bytes must be sent to receive data.

   What you've reported, however, is not a bug -- it's just the way SPI works like crenn said.

   -robodude666

   Posted 4 years ago #
5. 

   nplayle
   Member

   OK thanks guys.
   I was worried that my sending a dummy byte would eventually overwrite a register it should have. I have it working the as described and it works perfectly, so I'll leave it as is.

   Nick

   Posted 4 years ago #

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