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vira
Joined: 12 Feb 2007
Posts: 5
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| Help with SPI... |
 Posted: Tue Mar 13, 2007 8:31 am |
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Hi,
sorry to bother but i need help, im trying to use ADC MCP3551 but i just recently started to try SPI, i used 16F877A's internal ADC at first and succeeded but had to change to MCP3551 instead becoz i realised i need a higher bit ADC... now my problem is i dunno how to get the reading from MCP3551 through SPI... im a total newbie on SPI so please bear with me if i sound ridiculous to u ...
before i go on to the code, i want to ask if i really need to control CS if i wanted to use continuous conversion mode?
below is the code that i have written:
u cant skip the void part since its not relevant to the problem.
| Code: |
#define (__PCM__)
#include <16f877.h>
#device *=16
#include <stdlib.h>
#include <math.h>
#fuses XT,NOWDT,NOPROTECT,NOLVP
#use delay (clock=4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7)
#define SPI_MODE_0_0 (SPI_L_TO_H | SPI_XMIT_L_TO_H)
#define SPI_MODE_0_1 (SPI_L_TO_H)
#define SPI_MODE_1_0 (SPI_H_TO_L)
#define SPI_MODE_1_1 (SPI_H_TO_L | SPI_XMIT_L_TO_H)
#byte PORTB=6
#byte PORTC=7
#bit SDI=PORTC.4
#bit CS=PORTC.5
#byte PORTD=8
void seg7()
{
}
void dotseg()
{
}
void dig7()
{
}
void display()
{
}
int32 value;
int byte0, byte1, byte2, byte3;
void read3551()
do
{
CS=0; //drop CS
delay_cycles(1);
CS=1;
delay_cycles(1);
CS=0; //At this point conversion has been triggered.
delay_us(1);
while (SDI==1) //wait for conversion to complete
{
CS=1;
delay_cycles(1);
CS=0;
}
byte2=spi_read(0); //Now read data
byte1=spi_read(0);
byte0=spi_read(0);
byte3=0;
CS=1;
count=make32(byte0,byte1,byte2,byte3);
}while(1);
main()
{
set_tris_b(0); //output for seg7()
set_tris_c(0b10010000); //C3=SCK, C4=SDI, C5=CS, C6=xmit, C7=rcv
set_tris_d(0b00000000); //output for dig7()
setup_adc_ports(NO_ANALOGS);
setup_adc(ADC_OFF);
setup_timer_2(T2_DIV_BY_1,128,8);
setup_spi(SPI_MASTER|SPI_MODE_1_1|SPI_CLK_DIV_4);
do
{
read3551();
display(); //display count value (0 to 4194304)
}while(1);
}
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basically seg7(), dotseg(), dig7() and display() are for displaying purposes, thats why i purposely removed it becoz its quite long and like i said earlier its not relevant to the problem. and theres no problem with the code if ur wondering since it works fine when i was using internal ADC.
im using 7 7segment LED so i need to drive it like that, it'll display the value of count from 0 to 250,0000. this will be done later becoz i want to be able to get 0 to 419,4304(22-bit) first.
on to the code, am i missing anything important? is that the right way to get 22-bit?...
another thing is i am getting a signal coming out from pin RE1, the signal is similar to SCK and SDO(from ADC). i dunno why and ive disabled PORTE, but i might need to use PORTE later so maybe i should just ask as well...
im really running out of time rite now, this is the last thing that i need to do to finish my final year project...
I'll provide more information on the project if u guys think its neccessary..
thanks in advance...
vira |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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Guest
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| Posted: Tue Mar 13, 2007 7:11 pm |
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Nope, im not das... ive read that thread before but dunt really understands it, so i took a bit of here and there and assembled something that i understands  ....
by the way i forgot to tell that i actually got some reading from the code but its nowhere near 0 to 4194304, if i aint mistaken its 0 to 19000... |
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vira
Joined: 12 Feb 2007
Posts: 5
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| Posted: Tue Mar 13, 2007 7:14 pm |
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| not exactly 19000 but somewhere 19xxx.... cant remember the exact value ... |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Tue Mar 13, 2007 10:00 pm |
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| Quote: |
byte2=spi_read(0); // D23-D16 MSB
byte1=spi_read(0); // D15-D8
byte0=spi_read(0); // D7 - D0 LSB
byte3=0;
CS=1;
count=make32(byte0,byte1,byte2,byte3);
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The MCP3551 data sheet says the MSB comes out first.
| Quote: |
From Bit 20 to bit 0, the output code is given
MSb first (MSb is bit 20 and LSB is Bit 0).
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The CCS manual says the make32() function has up to 4 parameters.
It says:
| Quote: | i32 = MAKE32(var1, var2, var3, var4)
The msb is first.
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So it looks to me like you have the bytes in the wrong order
in your make32() statement. You ought to have them in
this order:
| Code: | | make32(byte3,byte2,byte1,byte0); |
There are other issues with the overflow bits, OVL and OVH, that
I didn't really look at. Also, I don't know what your reference
voltage is. That will affect the output value. |
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