1. Home
  2. Field-Programmable Gate Arrays
  3. Reversing SPI shift out order on Microblaze design

Reversing SPI shift out order on Microblaze design

Hi, I have a design that connects a Spartan 3e running a Microblaze soft-core with an SPI unit attached to the OPB bus to a 16 Bit DAC unit (DAC8534). The data I send to the DAC unit has the bit order reversed. The SPI_OPB pheripheral has no way of reversing the shift out order from what I can see. Below is a program snippet of sending data to the dac unit:

------------------------------------------------------------------------------------------------------------------------- // Select the DAC Unit XSpi_mSetSlaveSelectReg(XPAR_SPI_DAC_BASEADDR, SPI_DAC_SELECT);

// Set the message data. send_data[0]=0x20; // Control Register send_data[1]=0x0F; // Upper 8 bit value send_data[2]=0x0F; // Lower 8 bit value

// Send the message data. spi_transfer(send_data, 3);

// Deselect the DAC Unit XSpi_mSetSlaveSelectReg(XPAR_SPI_DAC_BASEADDR, SPI_NONE_SELECT);

-------------------------------------------------------------------------------------------------------------------------

The data sent to the dac unit appears as 0xF0F0 instead of 0x0F0F disregarding the control byte. Does anyone know of how I can reverse the bit order at a minimal overhead cost. Can you give an example of bit order reversing?

Thanks, Aaron Curtin

Reply to
Aaron Curtin
Loading thread data ...

You may use a look-up table of 256 bytes.

Or you may use two look-up tables of 16 bytes, one for low nibble and one for high-nibble

Or you could use the trick of intergchanging adjacebts bits, then bit pairs...

x= ( (x&0x55)1); x= ( (x&0x33)2); x= ( (x&0x0F)4);

Whichever pelases you most

Zara

Reply to
Zara

-------------------------------------------------------------------------------------------------------------------------

-------------------------------------------------------------------------------------------------------------------------

Is the SPI selecting the correct register?

If it was a consistent issue, then the register would also be reversed, one might think (so in your snippet you would address 0x04 for bit reversal).

Interesting.

A simple (very simplistic) bit order reverser (single cycle) would take

8 regs plus maybe a LUT.

Something like

reg [7:0] DataIn; reg [7:0] DataOut;

... in some always block

DataOut[7:0]

Reply to
PeteS

I can't see a parameter for reversing the bits either.

The source for opb_spi is in c:/EDK81/hw/XilinxProcessorIPLib/pcores/opb_spi_v1_00_d/ Copy it to your local pcores directory and modify it. Then use your local copy.

Alan Nishioka

Reply to
Alan Nishioka

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.