ADS7843 SPI settings and output data format

Natali Slinko

Other Parts Discussed in Thread: ADS7843

I have a problem with ads7843. I saw in internet that the spi settings are CPHA=0, CPOL=0. I set them, SPI psc=256(fpclk/256, fpclk=72MHz). And in result I get from SPI only zeroes. I read X in such way:

k=WriteSPI(SPI1, 0xDC);

msb= WriteSPI(SPI1, 0x00);

lsb=WrieSPI(SPI1,0x00);

And I get Y in the same way, but send 0x9C.

When I set CPHA=1, CPOL=0, and SPI psc=2, then I get data from spi. But what is the output data format? In different forums I saw

1) result=(msb<<8)| lsb;

2)result=((msb<<8) | lsb)>>4;

3)result=(((msb&0x7f)<<8)| lsb)>>3

what variant (1, 2, 3 ) is correct or the other one?

I use stm32f107 as a master. TFT is wf43btibedotp(evaluation board sk-wf43btibedotp ).

over 13 years ago

0 Wendy Fang over 13 years ago

TI__Genius 9725 points

Hello Natali,

The ADS7843 SPI timing/protocol has been provided on the Figure 7 (pg 10) of the d/s, where the SCLK (i.e.: ADS7843 's DCLK) is low when the SPI bus has not be activated and starts to send clock when activated; and the MOSI (i.e.: ADS7843's DIN) or MISO (i.e: ADS7843's DOUT) reads/latches a bit at the rising edge of the SCLK and writes/changes a bit at the falling edge of the SCLK, which is the so-called SPI mode with the SPI clock polarity (CPOL) = 0 and clock phase (CPHA) = 0. You may capture your SPI bus (4 channels) waveform and send to me if you have problem with the timing so that I may help with the debug.

Since you command X reading with 0xDC and Y reading with 0x9C, it shows you are using 8-bit mode and single-ended input.

For the 8-bit mode, the data format, in fact, should be -

result = ( ((msb & 0x7F) <<256) | lsb) >> 7

BTW#1: under the 12-bit mode, the data format is: result = ( ((msb & 0x7F) <<256) | lsb) >> 3

BTW#2: for better performance, please use differential mode for touch data reading.

Regards,
Wendy F.

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ADS7843 SPI settings and output data format