NOTE: The program was wrote using the CCS C compiler PIC C with a license PCWHD
The next program reads the 10-bit ADC of the PIC18F2550, channel 0(pin 2), and send the higher 8 bits of the result through the port B
The sampling rate is 1kHz approximately.
1: #include <18F2550.h> //Selecting the microcontroller
2: #fuses HS,NOWDT,NOPROTECT,NOLVP //Selecting set-up (high freq oscillator, no watch dog timer, et)
3: #use delay(clock=20000000) //We gonna use a 20MHz crystal
4: void main() //Main function
5: { unsigned int8 value; //Defines a variable called "value", length 8 bits
6: setup_adc_ports(AN0); //Defines wich port will be used for the ADC module, ANO (pin 2)
7: set_tris_b(0xFF); //Sets the whole port B as output
8: setup_adc(ADC_CLOCK_DIV_32); //Selects the TAD=1.6us (Xtal=20MHz)
9: set_adc_channel(0); //Selects the channel from were the measures will be taken
10: do //Initiates a loop
11: {
12: delay_ms(1); //Delay of 1ms between measures
13: value = read_adc(); //Starts the ADC module which return a value and this value is saved.
14: output_b(value); //The ADC returned value is sent to the port B
15: }
16: while (TRUE); //The loop never ends
17: }
Important issues:
The ADC module takes 11TAD to gat a 10-bit conversion. The TAD time is defined by the instruction
setup_adc(ADC_CLOCK_DIV_32);
In this case we divide the clock (20MHz) into 32 to get a time of 1.6us. This is the minimum time of adquisition for the ADC module. As the module takes 11 TAD to performs the conversion in 10-bit, then the total time of conversion is:
Total conversion time=11*TAD=11*(1.6uS)=17.6uS
If we wish a more accurate sampling rate we should take this value into account and subtract it from our delay routine. In this case, for a 1kHz sampling rate we should put a delay of:
Delay=1ms-17.6uS=982.4uS
This value without take into account the time that each instruction take to be executed by the micro.
The image above shows the results of the program in a simulation made using the PIC18 simulator.
Note that for a 468 in decimal, the output in binary is 0111010100 That is the value showed by the LEDS without the two bits least significant.
The circuit in breadboard working.
References:
I bet some day we have needed to or wanted to save a large amount of data using the common SD cards and some microcontroller.
