Set chipselects for DMA SPI transfer

rdamoise · **Joined:** Fri Aug 13, 2010 11:38 am **Posts:** 9

Hi all,

I am using 3 different chip selects on my spi bus, all transferred within one DMA cycle. This is the way I set the chip selects for different registers in my SPI Buffers:

Code:

    for(int i=0; i<16;i++){
        fpgaTxBuffer[i] &= 0x0FFFF;
        linkTxBuffer[i] |= 0x10000;
    }
    for(int i=16;i<48;i++){
        fpgaTxBuffer[i] |= 0x30000;
        linkTxBuffer[i] |= 0x10000;
    }

This takes about 35 uS to complete, and together with other interrupt routines, in a 100uS frame it is quite a lot of time.

Is there a way to set my chipselects quicker?

Thanks in advance,
Ruben

blue_z · **Posted:** Wed Jul 25, 2012 10:59 pm

rdamoise wrote:

for(int i=0; i<16;i++){
fpgaTxBuffer[i] &= 0x0FFFF;
linkTxBuffer[i] |= 0x10000;
}
for(int i=16;i<48;i++){
fpgaTxBuffer[i] |= 0x30000;
linkTxBuffer[i] |= 0x10000;
}

If nobody comes up with a faster algorithm/technique, then the least you could do is code the loops more efficiently, i.e. use pointers instead of indexing into arrays.

Code:

        struct foo     fpgaTxBuffer, *p, *z;
        struct bar     linkTxBuffer, *q;

        p = fpgaTxBuffer;
        z = fpgaTxBuffer + 16;
        q = linkTxBuffer;
        do {
                *p++ = *p & 0x0FFFF;
                *q++ = *q | 0x10000;
        } while (p < z);
        z = fpgaTxBuffer + 48;
        do {
                *p++ = *p | 0x30000;
                *q++ = *q | 0x10000;
        } while (p < z);

Regards

rdamoise · **Joined:** Fri Aug 13, 2010 11:38 am **Posts:** 9

Thanks, already gained 3uS.

Set chipselects for DMA SPI transfer

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