• Task Get_voltage reads the analog voltage from channel 0 (pin RA0 or AN0) of the microcontroller. The value is then converted into millivolts by multiplying by 5000 and dividing by 1024 (in a 10-bit A/D there are 1024 quantization levels, and when working with a reference voltage of +5V, each quantization level corresponds to 5000/1024mV). The voltage is stored in a global variable called Volts.
• Task To_RS232 reads the measured voltage from common variable Volts and sends it to the RS232 port using the C printf statement. The result is sent in the following format:
Measured voltage = nnnn mV
The HyperTerminal program is run on the PC to get an output from the program. A typical screen output is shown in Figure 10.15.
Figure 10.15: Typical output from the program
Using a Semaphore
The program given in Figure 10.14 is working and displays the measured voltage on the PC screen. This program can be improved slightly by using a semaphore to synchronize the display of the measured voltage with the A/D samples. The modified program (RTOS4.C) is given in Figure 10.16. The operation of the new program is as follows:
• The semaphore variable (sem) is set to 1 at the beginning of the program.
• Task Get_voltage decrements the semaphore (calls rtos_wait) variable so that task To_RS232 is blocked (semaphore variable sem=0) and cannot send data to the PC. When a new A/D sample is ready, the semaphore variable is incremented (calls rtos_signal) and task To_RS232 can continue. Task To_RS232 then sends the measured voltage to the PC and increments the semaphore variable to indicate that it had access to the data. Task Get_voltage can then get a new sample. This process is repeated forever.
//////////////////////////////////////////////////////////////////////////////
//
// SIMPLE RTOS EXAMPLE - VOLTMETER WITH RS232 OUTPUT
// ---------------------------------------------------------------------------
//
// This is a simple RTOS example. Analog voltage to be measured (between 0V
// and +5V) is connected to analog input AN0 of a PIC18F8520 type
// microcontroller. The microcontroller is operated from a 10MHz crystal. In
// addition, an LED is connected to port in RD7 of the microcontroller.
//
// RS232 serial output of the mirocontroller (RC6) is connected to a MAX232
// type RS232 voltage level converter chip. The output of this chip can be
// connected to the serial input of a PC (e.g., COM1) so that the measured
// voltage can be seen on the PC screen.
//
// The program consists of 3 tasks called "live", "Get_voltage", and
// "To_RS232".
//
// Task "Live" runs every 200ms and it flashes the LED connected to port RD7
// of the microcontroller to indicate that the program is running and is
// ready to measure voltages.
//
// task "Get_voltage" reads analog voltage from port AN0 and then converts the
// voltage into millivolts and stores in a variable called Volts.
//
// Task "To_RS232" gets the measured voltage and then sends to the PC over
// the RS232 serial line. The serial line is configured to operate at 2400
// Baud (higher Baud rates can also be used if desired).
//
// In this modified program, a semaphore is used to synchronize
// the display of the measured value with the A/D samples.
//
// Programmer: Dogan Ibrahim
// Date: September, 2007
// File: RTOS4.C
//
//////////////////////////////////////////////////////////////////////////////
#include <18F8520.h>
#device adc=10
#use delay (clock=10000000)
#use rs232(baud=2400,xmit=PIN_C6,rcv=PIN_C7)
unsigned int16 adc_value;
unsigned int32 Volts;
int8 sem;
//
// Define which timer to use and minor_cycle for RTOS
//
#use rtos(timer=0, minor_cycle=100ms)
//
// Declare TASK "Live" - called every 200ms
// This task flashes the LED on port RD7
//
#task(rate=200ms, max=1ms)
void Live() {
output_toggle(PIN_D7); // Toggle RD7 LED
}
//
// Declare TASK "Get_voltage" - called every 10ms
//
#task(rate=2s, max=100ms)
void Get_voltage() {
rtos_wait(sem); // decrement semaphore
adc_value = read_adc(); // Read A/D value
Volts = (unsigned int32)adc_value*5000;
Volts = Volts / 1024; // Voltage in mV
rtos_signal(sem); // increment semaphore
}
//
// Declare TASK "To_RS232" - called every millisecond
//
#task(rate=2s, max=100ms)
void To_RS232() {
rtos_wait(sem); // Decrement semaphore
printf("Measured Voltage = %LumV\n\r",Volts); // Send to RS232
rtos_signal(sem); // Increment semaphore
}
//
// Start of MAIN program
//
void main() {
set_tris_d(0); // PORTD all outputs
output_d(0); // Clear PORTD
set_tris_a(0xFF); // PORTA all inputs
setup_adc_ports(ALL_ANALOG); // A/D ports
setup_adc(ADC_CLOCK_DIV_32); // A/D clock
set_adc_channel(0); // Select channel 0 (AN0)
delay_us(10);
sem = 1; // Semaphore is 1
rtos_run(); // Start RTOS
}
Figure 10.16: Modified program listing