Read temperature and send over RS232 right?

casandralam · Joined: 26 Jan 2008 Posts: 15

hi,i am a student doing my project now..
My project is about interface designed circuit to PC..
My first stage is to read temperature using the LM35,an analog sensor and sends it over the RS232.
Here is my code:
I hope u can help me to check whether my code is valid to meet my objective.
I am very appreciate of your help

#include<16F877A.h>
#device*=16
#device adc=10
#fuses HS,NOWDT,NOPROTECT,NOLVP
#use delay(clock=4000000)
#use rs232(baud=9600,xmit=PIN_B2,rcv=PIN_B1)

void main()
{
int16 value; //set data type for analog value
//defines a 16bits number

int_temp;

setup_port_a(ALL_ANALOGS);
setup_adc(ADC_CLOCK_INTERNAL);
// setup_psp(PSP_DISABLED);
//setup_spi(FALSE);
//setup_timer_0(RTCC_INTERNAL|RTCC_DIV_64);
// setup_timer_1(T1_DISABLED);
//setup_timer_2(T2_DISABLED,0,1);
// setup_comparator(NC_NC_NC_NC);
// setup_vref(FALSE);

setup_adc_channel(0); //channel AN0 is adc input port
delay_ms(1000);

do{
value=read_adc();
read_adc(ADC_START_ONLY);
value=read_adc(ADC_READ_ONLY);
delay_ms(1000);

temp=(value*48)/100; //convert to degree Celcius
delay_ms(1000);

printf("%u\r\n",value);
}
while(TRUE)

Thanks for all of your help
:-)

Ttelmah · Guest

Start with the ADC reading. The line:

casandralam · Joined: 26 Jan 2008 Posts: 15

thanks you very much..
i understand now.your explanation is excellent
one thing i am not so clear is the ADC internal clock....
why i cannot just write ADC_CLOCK_INTERNAL?But, ADC_CLOCK_DIV_8?
Which data sheet can i refer to?i try to look into my microcontroller datasheet and also ccs manual already,but i cannot find it..

Ttelmah · Guest

The microcontroller data sheet.
Section 11, on the ADC module.
Table 11.1
Note2, below the table.

The problem is that the RC clock, is asynchronous to the processor clock. As the processor gets faster, the internal processor noise rises, and since the RC oscillator changes in speed significantly with temperature, you will get varying numbers of machine instructions executed in the ADC cycle, and more variability in the noise seen. In fact the RC clock, is the _best_ clock for really accurate result, if you put the processor to sleep for the conversion. However unless this is done, above 1MHz, you should select a clock based on the master oscillator.

Best Wishes

Foppie · Joined: 16 Sep 2005 Posts: 138 Location: The Netherlands

I might have a small hint.

I don't know your exact design specification. But what I would do in this case is sending the raw value form the adc to the pc and let the pc software handle the conversion. This has in my opinion 2 big advantages.
1. The conversion can be done much 'better' on a PC. You save time on your PIC.
2. The communication is easier when you send in hex format.
The disadvantage is that your PIC doesn't know the temperature.

I hope this helps you.