First time SPI questions

[Guest]

Oooops. Of course you are right..... Gomen.

[Guest]

Well it was the missing '0' in clock speed and things have moved quite quickly since this was pointed out to me.

I am getting results in hyperterm now but I can't work out what they mean

The data sheet is a little unhelpful when it comes to looking at the data returned after a sample, for instance it does not make it clear what is returned when I disable averaging! I am assuming that it's two bytes containing 12bits MSB first (really only a nibble) but I am showing results like the one's below from my little prog and I cannot figure out what how this correlates to the 1.5v battery I have connected.

0CDA
0CCC
0CCA
0CBC
0CCD

I am measuring a 1.5v battery, again I am a little unsure which way round the battery is supposed to go! from what I can tell AN0 should be batt+ and AN1 batt- (just search for IN+ in datasheet, you will see what I mean)

Well here is the program any help will be appreciated very much as I'm a little stuck.



#include <16F876.H>
#fuses HS, NOWDT, NOPROTECT,PUT,BROWNOUT,NOLVP
#use delay(clock = 20000000)
#use rs232(baud = 9600, xmit=PIN_C6, rcv=PIN_C7,PARITY=N,BITS=8,ERRORS)

#define EOC PIN_B6
#define CS PIN_B7

void main()
{
int16 Data;
int LSB,MSB;

printf("Start\n\r");
setup_spi(SPI_MASTER | SPI_L_TO_H | SPI_CLK_DIV_16);
output_low(CS); //CS Low enables serial interface

//Setup Register
//01 = bits 7-6 select setup reg
//10 = bits 5-4 CKSEl1/2 Clock mode internally timed, no need for CNVST
//10 = bits 3-2 REFSEL1/2 Internal, reference always on, no wake up delay
//11 = bits 1-0 DEFSEL1/2 One byte follows, write to Unipolar register

spi_write(0b01101010);

//Setup Register
//111111 = bits 7-2 AN0-AN11 as unipolar differential pairs 0+1,2+3 etc.
//00 = bits 1-0 These are only applicaable for MAX1231

spi_write(0b11111100);

//Averaging Register
//001 = bits 7-5 Select Averaging register
//0 = bit 4 AVGON 0 = disable Averaging
//00 = bits 3-2 NAVG1/2 dont care
//00 = bits 1-0 NSCAN1/0 dont care

spi_write(0b00100000);


output_high(CS);

delay_ms(1);

while(1)
{
output_low(CS);

//Conversion register
//1 = bits 7 select conversion register
//0000 = bits 6-3 CHSEL3-0 Select Channel AN0
//11 = bits 2-1 SCAN1-0 No scan, Convert channel 0 once only
//0 = bits 0 TEMP, no temp sample.

spi_write(0b10000110);

//Signal we have finished and sample can begin

output_high(CS);

//Wait to the End Of Conversion pin to go low


while(input(EOC)){
}

//Enable serial interface

output_low(CS);

//Clock out the data

MSB = spi_read(0);
LSB = spi_read(0);
Data = MSB << 8;
Data = Data + LSB;

printf("%LX\n\r",Data);

//Disable serial interface and wait a little bit

output_high(CS);
delay_ms(1);
}
}

[Guest]

Hi,

Well after much playing I finally got it reading values but this being my first A/D and me being a little new to all this I don't understand the results

I used a bench power supply to get some readings, here they are

0.5v = 0x242
1v = 0x55E
1.5v = 0x896
2v = 0xBCB

and here is the link to the pdf
http://pdfserv.maxim-ic.com/en/ds/MAX1227-MAX1231.pdf

Can anyone explain this in simple terms because I can work out why 2v is not twice the value of 1v (approx)

[Ttelmah]

[rhodotron]

I have to write a software to communicate to a dac mcp4822 and I am a bit confused on how start writing my code.
May be, my question is a lot stupid but I see few coding possibility.
One is to use the #use spi().
But I see also some example with setup spi() which apparently do the same. (not yet sure of that...)

when should we use one or the other option ?

thanks

[PCM programmer]

This CCS driver should work for the MCP4822. The two chips are
very similar.

[rhodotron]

thanks pcm but about my question on #use spi and setup_spi what do you think ?

[PCM programmer]

Apparently you want to use hardware SPI. In that case, I think it's
easier to use the setup_spi() and spi_write().

Look at the timing diagrams in the MCP4822 data sheet. It lists the
SPI modes that can be used. Here are the constants:

[ckielstra]