spi_question

[dipen]

Im trying to use the MCP6s26 PGA multiplexer with a 877 from microchip to recieve 6 analog signals. my program starts out by defining the channels and the channel register like:
#define channel0 b'00000000' //as channel 0
#define channel1 b'00000001'// as channel 1...etc
#define PrgChannel b'01000001' // as channel register

according to the data sheet i need to write to the channel register before i select the channel,yet when i try:
spi_write (PrgChannel);
value = spi_read (channel0);
i get character syntax errors
i think i can only recall a defined statment with:
#ifdef PrgChannel
(and then some statement)
.
.
but that doesnt seem very practicle. If anyone has used this device or has more knowledge on how to use the spi bus i would greatly appreciate your help.
_________________
nick

[plehman]

Not quite sure what a character syntax error is, but the definitions you made for the channels in binary look a little off. I was under the impression that binary numbers were written like 0b11010101 instead of b'11010101', but it may be correct. when I tried to compile one of my projects with that type of define, I got a "Character constant constructed incorrectly" error.

Also, I don't think that you are correctly writing to the Channel Register. I would think that to set the channel, you would need to actually do something like the following:

[Charlie U]

Since you didn't post any of your code, I'll make the assumption that you are using the basic spi set up commands. Try searching on "spi" in this forum and reviewing the recent posts. Especially the mysterious right shift post. My guess is that you are another victim of CCS's breaking the spi routines a couple of years ago. The basic spi setup does not support the majority of the spi devices in mode 0,0 or 1,1.

[dipen]

thanks for both of your help
_________________
nick

[dipen]

I'm still having problems swtiching channels and completing an ADC on each channel of the PGA

my code goes like:

void main() {

int16 v[41];

setup_spi(spi_master |0x4000 |spi_clk_div_16); // MODE 0, 0

do {
OUTPUT_low(PIN_C1); // chip select

set_adc_channel(0); //set up ad on channel a0
spi_write(0x41); //initiallizing write to channel register
spi_write(0x00); //set mux\pga channel 1 here

v[0] = Read_ADC();
delay_us(15);
printf("A/D VALUE = %2x\n\r", v[1]);

spi_write(0x41); //initiallizing write to channel register
spi_write(0x01); //set mux\pga channel 2 here

v[1] = Read_ADC();
delay_us(15);
printf("A/D VALUE = %2x\n\r", v[2]);

spi_write(0x41); //initiallizing write to channel register
spi_write(0x02); //set mux\pga channel 3 here

v[3] = Read_ADC();
delay_us(15);
printf("A/D VALUE = %2x\n\r", v[3]);

only the 1st channel is read, so its not switching channels of the Pga. Ive even tried to spi_read(0); in between writing to the channel and reading the adc value. That doesnt work. Please help.
_________________
nick

[Ttelmah]

[dipen]

I did that, and my channel still doesnt change for the PGA.
the data sheet for the PGA has a low impedance output resistance, yet they do not give any values. Ive tried to delay it up to 125us. I think my main problem could involve the clock. I have connected sck pin of PGA to C3 of the uC and defined it as sck. The measured frequency on that pin is only measure at 25Hz. When i reality it should be clock frequency/ sck divisions = 4Mhz/16 = 250kHz. Way way off. Please help. thanks
_________________
nick

[Kieran]

Microchip list an application note (AN248) for connecting this device to a PIC, may be worth a look!