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ADS8344 works with DAC7634 on one SPI bus, but why?

 
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Zer0flag
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ADS8344 works with DAC7634 on one SPI bus, but why?
PostPosted: Mon Jul 23, 2007 2:28 pm     Reply with quote
Hi all!

A former colleague of mine has developed a device based on a 16F877A @ 20 Mhz. In this device the PIC is the SPI master and communicates to two slave SPI devices: an ADS8344 (an ADC) and a DAC7634 (a DAC).

I am more a programmer than an electronics guy but looking at the datasheets of the ADC and the DAC as far as I can see the ADS8344 should work in SPI mode 0,0 while the DAC7634 should work in SPI mode 1,1.

Surprisingly the ADC and the DAC seem to work ok together. The SPI setup string of my former colleague is the following:

Code:
setup_spi(SPI_MASTER | SPI_H_TO_L | SPI_XMIT_L_TO_H | SPI_CLK_DIV_16 | SPI_SAMPLE_AT_END);


This setup string seems to represent SPI mode 1,1 (actually the one of the DAC7634) with the addition SPI_SAMPLE_AT_AND. If I refer to the 16f877a datasheet SPI_SAMPLE_AT_AND defines that the PIC's SMP bit is set high, which means "input data sampled at end of data output time". I had trouble figuring out what that exactly means but it sounds like it influences only the way the data that comes from the ADC is sampled by the PIC (or am I wrong?).

Now I am confused why the ADS8344 works at all in the above config (SPI mode 1,1). Or are there maybe some hidden problems?

Thanks for any help.

Best regards,
Zer0flag
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