SPI Setup for Linear Tech 16 Bit ADC LTC1864

lguancho · Joined: 08 Sep 2003 Posts: 5

Hi, all. I was experimenting on LTC1864, particularly on how to setup my PIC18F452 (as spi master) to read the 16 bit data. Here are my codes:

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Look at Figure 1 in the LTC1864 data sheet (page 9) and compare
it to the diagrams of the SPI modes shown in the following link:
http://www.totalphase.com/docs/articles/article03/#modes
The data sheet shows that the idle state of the clock is a high level,
and that it samples the data on the rising edge of clock.
In the link, this is shown as SPI mode 3. The following chart
shows the settings for the CKP and CKE bits for each SPI mode.

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

As additional info, I always add the following to my code:

lguancho · Joined: 08 Sep 2003 Posts: 5

Thanks, PCM Programmer and Ckielstra!. I will implement the spi modes as recommended.

About this LTC1864, I find that if I just do two spi-read, the SDATA line will just toogle between HIGH and LOW after the 16 bit data transfer. If I do another spi_read (third ones) while holding the CONV of LTC1864, SDATA line will stay at a nice LOW!!. In fact, this is exactly as per described in the datasheet! Missing this third SPI-READ will cause my code to go astray!

But anyone encounter this kind of issues?

guest · Guest

I modify lguancho's code to use LTC1864 with 16F877a. ADC reading is 0. I have verified the clock (C3), and CONV(D7), but just no signal on DI (C4). Can someone tell me what is wrong?

#include <16f877a.h>
#fuses HS,NOWDT,NOPROTECT,NOLVP
#use delay(clock=20000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7)

#define ADC_CONV PIN_D7
#define ADC_DO PIN_C4 // SDI
#define ADC_DI PIN_C5 // SDO
#define ADC_CLK PIN_C3

#bit CKP_BIT=0x14.4
#bit CKE_BIT=0x94.6
#bit SMP_BIT=0x94.7

void init_ext_adc();
int16 read_ext_adc(void);

void main() {

int16 dummydata;
output_high(PIN_C0);

init_ext_adc();

while(1)
{
dummydata= read_ext_adc();
printf("ADC=%lu\r\n", dummydata);
delay_us(50);
}
}

void init_ext_adc() {
output_low(ADC_CONV);

setup_spi(SPI_MASTER|SPI_H_TO_L|SPI_CLK_DIV_16|SPI_SS_DISABLED);
CKP_BIT=1; //SCK IDLE=HIGH
CKE_BIT=0;
}

int16 read_ext_adc(void) {
BYTE data_hi,data_lo,dummy;
int16 data;

output_high(ADC_CONV);
delay_us(10);
output_low(ADC_CONV);
data_hi=spi_read(0);
data_lo=spi_read(0);
dummy=spi_read(0);
data=make16(data_hi,data_lo);
return(data);
}