Sending Multiple Bytes with SPI.Transfer using Arduino Due

Hardware Arduino Due
1

Hi all :slight_smile:

I am a little bit stuck on the need to send out 24 bytes of information over the SPI bus and the best way to do it. I am using the TLC5940 LED driver (datasheet). I know there is a library for it but I really want to try and work it out myself if I can as the data arrays are slightly different.

I have an array with 16 x 12 bit values, 192 bits in total. I need to send out MSB first so it sits as 15MSB .15LSB . 14MSB ...... 0LSB.

I am struggling on how best to convert the array of 16 12 bit values into 24 individual 8 bit values to send over the SPI bus.

As a side note, using the Arduino Dues extended SPI functions, when I setup the following,

void loop(){
  byte response = SPI.transfer(4, 0xFF);
  byte response2 = SPI.transfer(10, 0xFF);
  byte response3 = SPI.transfer(52, 0xFF);
}

Will this execute simultaneously or send/receive the first, then the second then the third?

Thanks in advance

2

your code example will transmit everything serially. a byte will be sent out and the transfer() function will wait until it receives an a response.

the easiest thing to do is keep using these SPI.transfer() calls and just serialize your 12 bit data to a buffer (a load of bytes). then just transfer all the bytes one by one.

such serialization code will look something like this

uint8_t bytes[24];
uint16_t twelvebit[16];

//..
//... fill twelvebit with meaningful stuff..
//....

int j = 0;
// 16 12-bit words -> 24 bytes
for (int i=0; i<16; i+=2)
{
  bytes[j++] = twelvebit[i] >> 4;
  bytes[j] = (twelvebit[i] & 15)<<4;
  bytes[j++] |= twelvebit[i+1] >> 8;
  bytes[j++] = twelvebit[i+1] & 255;
}
3

You need to read this:

In particular the use of SPI_CONTINUE

4

Hi,

Thanks very much for your help. I think I have it sorted as far as converting the 12 bit values to an 8 bit array.

I had seen that page regarding extended usage but I'm not entirely sure on how to go through each of the 8 but values, would I do it with something like this? (Excuse the code, it's very rough idea)

for(i=0; i <=23-1; i++) {
  SPI.transfer(4, bytes[i], SPI_CONTINUE)
}

SPI.transfer(4, bytes[23])

Or would a while loop suit this task better? I'm not overly familiar with while loops.

Thanks again for everyone's help!

5

the loop you describe should be fine.. omission of the SPI.transfer() 's third parameter is implicitly SPI_LAST. just forget about while loops, for loops can do everything while loops can and more.. and they are typically less programmer error prone.

6

Thanks again everyone.

I will try it out and let you know how I go :slight_smile: