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zilog
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| interfacing AD7796 to PIC using SPI |
 Posted: Thu Jun 07, 2007 2:10 pm |
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I am interfacing two AD7796 sigma-delta AD:s to a PIC16F883, the AD:s CLK-lines are tied together using the first to clock the second. PIN_C0 and PIN_C1 are tied to their /CS lines respectively. It seems like I can talk _to_ the AD:s, but I cannot read back stuff that looks sane. On the oscilloscope it looks like I can enable the continous conversion and readout mode (using the code below, I can get 16.7Hz /RDY pace on the scope), and I usually am successful in reading back the device ID:s (but not always as things tend to get a bit random).
I have added
| Code: |
#define DEBUG_READTEMP
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To see if I can read the internal temperature sensor or not, but the output from the SPI bus just keeps flipping randomly between 0x0000 and 0xFFFF just as before. Has anybody done this before, and can guide me a bit?
I use this code to enable the SPI interface:
| Code: |
/* input and output data is valid at rising edge of SCLK, SCLK idle high, Fosc/4 */
SSPSTAT = 0x00;
set_tris_c(0b10010100);
SSPCON = 0x30;
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This is a function to setup the both AD:s, and make sure they start in sync
| Code: |
void start_external_adc(void)
{
/* reset both AD:s */
output_low(PIN_C0 | PIN_C1);
spi_write(0xFF); spi_write(0xFF); spi_write(0xFF); spi_write(0xFF);
delay_us(500);
output_high(PIN_C0 | PIN_C1);
/* setup LAST */
output_low(PIN_C0);
spi_write(0x08); //next write is MODE REGISTER
delay_cycles(10);
spi_write(0x00); spi_write(0x8A); //cont, use external clock, 16.7HZ, 50+60Hz suppression
#ifdef DEBUG_READTEMP
delay_cycles(10);
spi_write(0b00010000); //config register
delay_cycles(10);
spi_write(0b00010111); spi_write(0b00010110);
#endif
delay_cycles(10);
spi_write(0x5C); //cont data output mode
output_high(PIN_C0);
output_low(PIN_C1);
/* setup HANDTAG */
spi_write(0x08); //next write is MODE REGISTER
spi_write(0x00); spi_write(0x4A); //cont, output internal clock, 16.7HZ, 50+60Hz suppression
spi_write(0x5C); //cont data output mode
output_high(PIN_C1);
return;
}
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And this is a function I call repeatedly to check if both AD:s have finished the conversion by examining the state of the /RDY pin (SDI on the PIC). It also takes timestamps to help monitoring if the AD:s drift or not.
| Code: |
/* returns true if both AD:s were ready to sample out */
int1 read_external_adc(unsigned int16 *last, unsigned int16 *ht, unsigned int8 *tAD1, unsigned int8 *tAD2, int1 wasteSample)
{
int1 DTR1 = 0, DTR2 = 0;
if (wasteSample) {
/* let both AD:s start, waste the first conversion result */
output_low(PIN_C0);
while (input(PIN_C4));
spi_read(0); spi_read(0);
output_high(PIN_C0);
output_low(PIN_C1);
while (input(PIN_C4));
spi_read(0); spi_read(0);
output_high(PIN_C1);
reset_timExtAD();
} else {
output_low(PIN_C0);
DTR1 = !input(PIN_C4);
if (DTR1) {
*tAD1 = read_timExtAD();
}
output_high(PIN_C0);
output_low(PIN_C1);
DTR2 = !input(PIN_C4);
if (DTR2)
*tAD2 = read_timExtAD();
output_high(PIN_C1);
if (DTR1 && DTR2) {
output_low(PIN_C0);
*last = (unsigned int16)spi_read(0) << 8;
*last |= (unsigned int16)spi_read(0);
output_high(PIN_C0);
output_low(PIN_C1);
*ht = (unsigned int16)spi_read(0) << 8;
*ht |= (unsigned int16)spi_read(0);
output_high(PIN_C1);
reset_timExtAD();
}
}
return (DTR1 && DTR2);
}
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[/code] |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Thu Jun 07, 2007 2:51 pm |
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| Quote: | | output_low(PIN_C0 | PIN_C1); |
This syntax is totally incorrect. You need to look at other CCS
programs before you write your own code. |
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Guest
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| Posted: Thu Jun 07, 2007 3:36 pm |
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Thanks for pointing that out - had totally missed it.
Now that I get some kind of reading from the internal temperature sensor - does anyone know how temperature translates into output value? There is 0.81mV/C specified, but since an offset is missing I cant use it.. |
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