PIC I/O Port Programming (PIC18F4580)
we discussed in my last post that All digital I/O’s are grouped together into “Ports”, called A, B ,D. Here GPIO means “general purpose input output”. The PIC18F4580 features a total of 36 I/O-pins, called PORTA to PORTD. But there’s more. The device comes with so many additional internal units, that not all features could be connected to dedicated pins of the device package at any one time. The solution is: multiplex. This means, one single physical pin of the device can be used for different functions and it is up to the programmer to decide which function is selected.
Lets dive deep to grasp the some operations which we can perform with I/O PORTS.
Each port has 3 registers for its operation:
• TRIS register (Data Direction register)
• PORT register (reads the logic levels on the pins of the device)
• LAT register often known as (output latch)
The data latch (LAT register) is helpful for read-modify write
Operations on the worth that the I/O pins are driving.
TRIS register job:
Each of Ports A-E within the PIC18F4580 are often used for input or output The TRISx SFR (special function Register) is employed for the aim of creating a given port associate input or output port.as an example, to form a port associate output, we tend to write 0s to the TRISx register.
In different words, set output to any of the pins of the Port B, we tend to should initial place 0s in to the TRISB register to form it associate output port, then send the information to the PORT B SFR itself. It should be noted that unless we tend to activate the TRIS bit (set it to zero), the information won't go from the port register to the pins of PIC. They will be sitting within the SFR of Port B within the CPU. To make a port associate input port, we tend to should initial place one s into the TRISx register for that port.
The easiest way to remember setup to output TRIS bits is that 0 and letter O, 1 and letter I.
Programmer must take care TRIS bits set to 1 if data is needed to brought in to the WREG register from the pins of MC. To get a good grip of how ports are used in PIC18F4580 implement following example and also watch simulation.
Consider bits in Port A and Port B. Lets write a program that shows the count from 0 to FFH (0000 0000 to 1111 1111 in binary) on the PORTS.
Proteus Simulation
watch simulation to get better idea what we are trying to do.
CSS Programming Code shows the count from 0 to FFH (0000 0000 to 1111 1111 in binary) on the PORTS.
int counter = 0;
void main()
{
setup_adc_ports(NO_ANALOGS|VSS_VDD);
setup_adc(ADC_OFF|ADC_TAD_MUL_0);
setup_psp(PSP_DISABLED);
setup_spi(SPI_SS_DISABLED);
setup_wdt(WDT_OFF);
setup_timer_0(RTCC_INTERNAL);
setup_timer_1(T1_DISABLED);
setup_timer_2(T2_DISABLED,0,1);
setup_comparator(NC_NC_NC_NC);
setup_vref(FALSE);
for (;;)
{
delay_ms(100);
OUTPUT_B(counter++);
delay_ms(100);
OUTPUT_A(counter++);
}
}
miKroC Programming Code shows the count from 0 to FFH (0000 0000 to 1111 1111 in binary) on the PORTS.
#define LED PORTB //notice how we can define PORTBvoid main() {
ADCON1 = 0xff; //Configure Analog Pin as a digital IO
CMCON = 0; // Disable Comparators
TRISB = 0; //make PORTB as an Output
TRISA = 0; //make PORTA as an Output
PORTA = 0;
LED = 0 ;
for (;;)
{
delay_ms(100);
LED++;
delay_ms(100);
PORTA++;
}
}
Reading Input PIN From PIC
At this point , we have build concepts of TRISx register and how to access PORTS. Next, we try to read the PIN and take some decisions based on the logic level of the bit, if-else instructions are conditional statements and executed a portion of code if user defined condition is true.
They allow you to observe a pin and build a call to execute code depending on whether it is 0 or 1. once more it should be noted that the if-else are often used for any bit(must be bit addressable) of the register, including the I/O ports A, B, C, D, and so on.
Lets Write a C18 program to monitor bit RA0. If it is HIGH, send 55H to Port C; otherwise,
send AAH to Port C.
Proteus Simulation
watch simulation to get better idea what we are trying to do.
miKroC Programming Code to monitor bit RA0. If it is HIGH, send 55H to Port C; otherwise,send AAH to Port C.
#define mybit PORTA.RA0 //notice how we can define bitvoid main() {
ADCON1 = 0xff; //Configure Analog Pin as a digital IO
CMCON = 0; // Disable Comparators
TRISC = 0; //make Port C as an Output
TRISA.RA0 = 1; //make PIN RA0 as an Input
while(1)
{
if (mybit==1)
PORTC = 0x55;
else
PORTC = 0xAA;
}
}
CSS Programming Code to monitor bit RA0. If it is HIGH, send 55H to Port C; otherwise,send AAH to Port C.
void main()
{
setup_adc_ports(NO_ANALOGS|VSS_VDD);
setup_adc(ADC_OFF|ADC_TAD_MUL_0);
setup_psp(PSP_DISABLED);
setup_spi(SPI_SS_DISABLED);
setup_wdt(WDT_OFF);
setup_timer_0(RTCC_INTERNAL);
setup_timer_1(T1_DISABLED);
setup_timer_2(T2_DISABLED,0,1);
setup_comparator(NC_NC_NC_NC);
setup_vref(FALSE);
while(1)
{
if (input(PIN_A0))
OUTPUT_C(0x55);
else
OUTPUT_C(0xAA);
}
}
Confusion Between Reading input pins vs. LATx port
Dealing with digital circuits like MC ,programmers must take care when reading data PORT, there are instructions which only read the status of ports internally often known as LATx,we should build distinction between these two types of instrctions.
2. On the contrary, instructions that get the data from internal latch of the LAT register.
We will understand this scenario by writing code and simulation.
LATx Register Role in Reading a Port or Latch
Lets design a code and simulation to understand the behavior of above explanation.
Reading LATx for ports
void main() {
ADCON1 = 0xff; //Configure Analog Pin as a digital IO
CMCON = 0; // Disable Comparators
TRISC = 0; //make Port C as an Output
TRISA.RA0 = 0; //make PIN RA0 as an output
while(1)
{
PORTA.RA0 = 0x01;
PORTC.RC0 = LATA.RA0;
}
}
Friends for further learning about PIC18 checkout next.
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