topic Re: SPI Clock problem in 8-bit Microcontrollers

SPI Clock problem

longliveboy — Wed, 08 Jul 2009 17:40:06 GMT

Hi everybody;

I'm using LL16 and try to sent data to a DAC via SPI.

The problem is LL16 generates SPI clock only for 8bits but my hardware(DAC) needs 24bit continuos clock for successful data transfer. Is there any way to generate 24 bits clock with LL16

Thanks,

Re: SPI Clock problem

CarlFST60L — Wed, 08 Jul 2009 17:53:08 GMT

As its async can you just send three bytes one after the other?

Re: SPI Clock problem

peg — Wed, 08 Jul 2009 17:55:04 GMT

Hi longliveboy,

As the SPI data register is buffered, by keeping it fed, you can produce an infinitely long continous clock and data. Basically you can transfer into the buffer while the actual shift register is still shifting out. When the shifting is done the buffer will transfer into the shift register and you have continuous operation.

Re: SPI Clock problem

longliveboy — Wed, 08 Jul 2009 18:24:30 GMT

PEG;

At the datasheet writes that do not send second data before transmit buffer empty flag is set.

I wrote this codes:

void sendSPI(byte adres, byte data1, byte data2){
while (!SPIS_SPTEF);// check to see if SPI TX ready
SPIS;       // clear status flags

SPID = adres;   // load data into SPI register

while (!SPIS_SPTEF);// check to see if SPI TX ready
SPIS;       // clear status flags

SPID = data1;   // load data into SPI register

while (!SPIS_SPTEF);// check to see if SPI TX ready
SPIS;       // clear status flags

SPID = data2;   // load data into SPI register

while (!SPIS_SPRF); // wait for receive full flag
SPID;       // dummy read to clear flags
}

If u have any codes to implement your suggest i'll try and feedback again

Re: SPI Clock problem

jag — Wed, 08 Jul 2009 20:35:52 GMT

longliveboy wrote:
PEG;

At the datasheet writes that do not send second data before transmit buffer empty flag is set.

[CUT]

If u have any codes to implement your suggest i'll try and feedback again

It's why it exists an interrupt connected to the trasmit buffer empty flag. I suggest to use it.

Bye Jack

Re: SPI Clock problem

bigmac — Thu, 09 Jul 2009 22:44:05 GMT

Hello,

There is an additional requirement, that an overrun condition must not occur, or further byte transfers will fail. Therefore the SPRF flag must be cleared three times, each prior to the completion of transfer of the next byte. This would apply even if you have no interest in the return data.

Some untested code -

while (!SPIS_SPTEF);

SPID = adres;

while (!SPIS_SPTEF);

SPID = data1; // Second byte queued to buffer

while (!SPIS_SPRF); // Wait for completion of first byte

(void)SPID; // Clear flag to prevent overrun

while (!SPIS_SPTEF);

SPID = data1; // Third byte queued to buffer

while (!SPIS_SPRF); // Wait for completion of second byte

(void)SPID; // Clear flag to prevent overrun

while (!SPIS_SPRF); // Wait for completion of third byte

(void)SPID; // Clear flag

Interrupts should be disabled prior to starting this process, otherwise it is possible for an overrun to occur while an unrelated interrupt is serviced, especially with a fast SPI clock.

Regards,

Mac