Convert Volts of LM35 with PIC16F877 and MAX187

O_R_E · Guest

Hello, I am using a PIC16F877 and I make the conversion to 12 bits by means of a MAX187. By means of a LM35, receipt the tension in mV, later to show it by display. The problem this in which to the entrance of the ADC I have the correct value, but in display this value oscillates much, and it does not give a correct value. This it is my code:

#include "C:\Program Files C\20060403_spi.h"
#include <max187.c>
#include <lcd.c>
#use fast_io (A)
#use fast_io (B)
#use fast_io (C)
#use fast_io (D)
#use fast_io (E)
#byte port_a=0X05
#byte port_b=0X06
#byte port_c=0X07
#byte port_d=0X08
#byte port_e=0X09

#ifndef MAX4534_EN

#define MAX4534_A0 PIN_A0
#define MAX4534_A1 PIN_A1
#define MAX4534_EN PIN_A2

#endif

void main()
{
float temperatura;
setup_adc_ports(NO_ANALOGS);
setup_adc(ADC_OFF);
setup_psp(PSP_DISABLED);
setup_spi(FALSE);
setup_timer_0(RTCC_INTERNAL|RTCC_DIV_1);
setup_timer_1(T1_DISABLED);
setup_timer_2(T2_DISABLED,0,1);
set_tris_a(0x00); //Port A as exit
set_tris_b(0x00); //Port B as exit
set_tris_c(0b11010111); //Port C as coming and goings
set_tris_d(0xFF); //RD0...RD7 exit
set_tris_e(0x00); //RE0...RE2 gate
port_b=0;

/*Init mux 4:1 MAX4534 */

port_a=0;
delay_ms(5);
output_high(MAX4534_EN);
output_high(MAX4534_A1);
output_low(MAX4534_A0);

/*Init A/D MAX187*/

init_ext_adc();
lcd_init(); //Init LCD
lcd_putc(" Digital\n"); //Extract text
lcd_putc(" Termometer"); //Extract text
delay_ms(3000);
lcd_putc("\f"); //Clean screen
lcd_putc("Temperatura\n"); //Extract text
lcd_putc("actual"); //Extract text
delay_ms(1000);
lcd_gotoxy(14,2); //Arranged cursor LCD
lcd_putc("oC");

while(1)
{
init_ext_adc();
lcd_gotoxy(8,2); //Arranged cursor LCD
lcd_putc(" "); //Clean this sector of screen
lcd_gotoxy(8,2); //Arranged cursor LCD
temperatura = read_ext_adc();
printf(lcd_putc,"%03.1f",temperatura); //xxx.x °C
delay_ms (1000);
temperatura=0;
}
}

And the code off <max187.c>, it's the same off drivers ADS8320.c CCS Compiler

//////////////// Driver for MAX187 A/D Converter ///////////////////////
//// ////
//// init_ext_adc() Call after power up ////
//// ////
//// value = read_ext_adc() Converts to digital number ////
//// and sends to MCU ////
//// ////
/////////////////////////////////////////////////////////////////////////
//// (C) Copyright 1996,2003 Custom Computer Services ////
//// This source code may only be used by licensed users of the CCS ////
//// C compiler. This source code may only be distributed to other ////
//// licensed users of the CCS C compiler. No other use, ////
//// reproduction or distribution is permitted without written ////
//// permission. Derivative programs created using this software ////
//// in object code form are not restricted in any way. ////
/////////////////////////////////////////////////////////////////////////

#ifndef MAX187_CS

#define MAX187_DOUT PIN_C4
#define MAX187_SCLK PIN_C3
#define MAX187_CS PIN_C2

#endif

void init_ext_adc() {
output_high(MAX187_CS);
output_high(MAX187_SCLK);
}

long read_ext_adc() {
int i;
long data;

data=0;
output_low(MAX187_CS);
for(i=0;i<=5;i++) // take sample and send start bit
{
output_low(MAX187_SCLK);
delay_us(1);
output_high(MAX187_SCLK);
delay_us(1);
}
for(i=0;i<8;++i) { // send sample over spi
output_low(MAX187_SCLK);
delay_us(1);
shift_left(&data,2,input(MAX187_DOUT));
output_high(MAX187_SCLK);
delay_us(1);
}

output_high(MAX187_CS);
return(data);
}

float convert_to_volts(long data) {
return ((float)data*2.5/0xffff);
}

Sorry for my basic english

Thank you,

Regards

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

What you have done is to take the CCS driver for the ADS8320
and then edit it to make it look as if CCS has a MAX187 driver
and then post it. That's not going to work. The two chips are
not compatible. You have to make a real driver for the MAX187.

--------
Just a notice to anyone reading this thread from Google.
The code posted above is not a valid driver for the MAX187.
It's made to look as if CCS has a driver for it. They do not.
Do not use the code posted above. It will not work.

O_R_E · Guest

Hello,

I have modified driver of the MAX187, but what I obtain is the value multiplied by two, that is to say, the double. As I can solve it? Thanks This is the code of driver MAX187:

#ifndef MAX187_CS

#define MAX187_DOUT PIN_C4
#define MAX187_SCLK PIN_C3
#define MAX187_CS PIN_C2

#endif

void init_ext_adc() {
output_low(MAX187_CS);
output_low(MAX187_SCLK);
}

long read_ext_adc() {
int i;
long data;
data=0;
delay_us(86); //Delay 86 us, to init conversion (tCONV)
output_low(MAX187_CS);
for(i=0;i<13;++i) { // send sample over spi
output_low(MAX187_SCLK);
delay_us(1);
shift_left(&data,2,input(MAX187_DOUT));
output_high(MAX187_SCLK);
delay_us(1);

}
delay_us(10);
output_high(MAX187_CS);
return(data);
}

float convert_to_volts(long data) {
return ((float)data*5.0/0xFFFF);
}

And this is the main code:

#include "C:\Program Files C\20060405_spi.h"
#include <max187.c>
#include <lcd.c>
#use fast_io (A)
#use fast_io (B)
#use fast_io (C)
#use fast_io (D)
#use fast_io (E)
#byte port_a=0X05
#byte port_b=0X06
#byte port_c=0X07
#byte port_d=0X08
#byte port_e=0X09

#ifndef MAX4534_EN
#define MAX4534_A0 PIN_A0
#define MAX4534_A1 PIN_A1
#define MAX4534_EN PIN_A2
#endif

void main()
{
float value;
setup_adc_ports(NO_ANALOGS);
setup_adc(ADC_OFF);
setup_psp(PSP_DISABLED);
setup_spi(FALSE);
setup_timer_0(RTCC_INTERNAL|RTCC_DIV_1);
setup_timer_1(T1_DISABLED);
setup_timer_2(T2_DISABLED,0,1);
set_tris_a(0x00);
set_tris_b(0x00);
set_tris_c(0b11010111);
set_tris_d(0xFF);
set_tris_e(0x00);
port_b=0;
port_a=0;

/*Init LCD*/
lcd_init();
lcd_putc(" Digital\n");
lcd_putc(" Reference");
delay_ms(2000);
lcd_putc("\f");
lcd_putc("Value\n");
delay_ms(2000);
lcd_gotoxy(14,2);
lcd_putc("V");

while (1)
{
output_high(MAX4534_EN); //Init multiplexor MAX4534
output_low(MAX4534_A1);
output_high(MAX4534_A0);
init_ext_adc(); //Init MAX187
value = read_ext_adc();
lcd_gotoxy(8,2);
lcd_putc(" ");
lcd_gotoxy(8,2);
printf(lcd_putc,"%03.2f",value); //Tension in [Volts]
delay_ms (2000);
value=0;
}
}

Thank you

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Look at some sample drivers on the net. Go to http://www.google.com
and search for: MAX187 int char
That will find you some source code. Here is one of the many
links you will find:
http://www.mikrocontroller.net/attachment.php/206290/GetMAX187.C

Do you notice how their for() loop only does 12 iterations ?

You should then question why you are doing 13 iterations.

You should go to the Maxim web page here, and download the
data sheet:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1033
It shows 12 bits.

Their appnote here, in Figure 2, shows 12 bits.
http://www.maxim-ic.com/appnotes.cfm/appnote_number/827

How did I know to look for this ? Whenever a result is "off" by a
power of 2, the most likely cause is a shift problem. Either you
shifted too much, or too little. So when you mentioned your problem
I instantly zoomed in on the loop count.

O_R_E · Guest

Hello Jdm programer,

This driver off MAX187, it's ok:

//////////////// Driver for MAX187 A/D Converter ///////////////////////
//// ////
//// init_ext_adc() Call after power up ////
//// ////
//// value = read_ext_adc() Converts to digital number ////
//// and sends to MCU ////
//// ////
/////////////////////////////////////////////////////////////////////////
//// (C) Copyright 1996,2003 Custom Computer Services ////
//// This source code may only be used by licensed users of the CCS ////
//// C compiler. This source code may only be distributed to other ////
//// licensed users of the CCS C compiler. No other use, ////
//// reproduction or distribution is permitted without written ////
//// permission. Derivative programs created using this software ////
//// in object code form are not restricted in any way. ////
/////////////////////////////////////////////////////////////////////////

#ifndef MAX187_CS

#define MAX187_DOUT PIN_C4
#define MAX187_SCLK PIN_C3
#define MAX187_CS PIN_C2

#endif

void init_ext_adc() {
output_low(MAX187_CS);
output_low(MAX187_SCLK);
}

long read_ext_adc() {
int i;
long data;
data=0;
delay_us(86); //Espero 86 us, para iniciar la conversion (tCONV)
output_low(MAX187_CS);
for(i=0;i<12;++i) { // send sample over spi
output_low(MAX187_SCLK);
delay_us(1);
shift_left(&data,2,input(MAX187_DOUT));
output_high(MAX187_SCLK);
delay_us(1);

}
delay_us(10);
output_high(MAX187_CS);
return(data);
}

float convert_to_volts(long data) {
return ((float)data*2.5/0xFFFF);
}
The problem was in the curl, that is of 13 clocks. The paragraph of datasheet had confused me that says in page 15:

1. Use a general-purpose I/O line on the CPU to pull CS
low. Keep SCLK low.
2. Wait the for the maximum conversion time specified
before activating SCLK. Alternatively, look for a
DOUT rising edge to determine the end of
conversion.
3. Activate SCLK for a minimum of 13 clock.

Thanks