PIC 18XXX PORT LAT

PIC18 series microcontrollers are high-end series of 8-bit microcontrollers of Microchip, USA. The maximum sink current or maximum source current allowed by any I/O pin is 25mA, which can directly drive LEDs and relays. The maximum sink current or maximum source current of PORTA, PORTB and PORTE is 200mA, the maximum sink current or maximum source current of PORTC and PORTD is 200mA, and the maximum sink current or maximum source current of PORTF and PORTG is 100mA (Note: PIC18F4550 does not have these two ports). 

           

The interaction between the microcontroller and peripherals is carried out through the I/O port. Each I/O port has three operation registers:

1. TRISx ——— Data direction register

Used to control the direction of the I/O pin, that is, to control whether PORTx is input or output.

2. PORTx ——— Port register

Used to latch output data. When reading PORTx, the device reads the I/O pin level directly (not the latched value).

3. LATx ——— Output data latch

Writing to the port is writing to this latch (LATx). The data latch can also be read and written directly. If the peripheral does not use the pin and the TRISx bit configures the pin as an output, the data in the latch is output to the pin. 

    In the reset state, the reset value of TRISx is 0xff, that is, the values ​​of the 8 bits (D0 ~ D7) of the TRISx register are all 1. At this time, the corresponding PORTx pin is defined as input, and the corresponding output driver is in high impedance state. When set to 0, it means that the corresponding pin is defined as output.

 

It should be noted here that writing PORT is writing LAT, but reading PORT is different from reading LAT. Reading PORT reads the state of the pin, whether the pin is set as an input pin or an output pin. Reading LAT, on the other hand, gets the stored value of the output data latch. The value obtained by reading LAT may be different from the value obtained by reading PORT.

                     

In Microchip C18, the three operation registers of the I/O port can be operated bit by bit or byte by byte.

For example, the direction register of port B is represented by TRISB (or DDRB), and a certain bit is represented by TRISBbits.TRISB0 (or DDRB bits.RB0). Bytes are represented by TRISB (or DDRB).

For example, the PORT register of port B is represented by PORTB, and a certain bit is represented by PORTBbits.RB0. The byte is represented by PORTB.

For example, the output data latch of port B is represented by LATB, a certain bit is represented by LATBbits. LATB0. Bytes are represented by LATB.

    After the chip is reset, the value of the LATx (PORTx) latch is random. In order to eliminate the possibility of glitches on the I/O pin level, when initializing the port, first initialize the PORT data latch (LAT or PORT register) and then initialize the data direction register TRIS.

    The following example illustrates a specific application. In the figure below, PIC18F4550, power supply, crystal oscillator and light-emitting diodes form the simplest 8-bit microcontroller system, which requires lighting up 8 light-emitting diodes at the same time.   

  

    First, we can choose the bitwise operation method to implement it. It is not difficult to see that the bitwise operation does not actually realize the simultaneous lighting of the 8 LEDs connected to PORTB. It is just that the delay effect of the LEDs covering up the fact of sequential lighting makes the final effect reach the simultaneous lighting. The following is the implementation code of the bitwise operation method.

#include

void main(void)
{
 PORTBbits.RB0=1;
 TRISBbits.TRISB0=0; //Turn on the 1st LED
 PORTBbits.RB1=1;
 TRISBbits.TRISB1=0; //Turn on the 2nd LED
 PORTBbits.RB2=1;
 TRISBbits.TRISB2=0; //Turn on the 3rd LED
 PORTBbits.RB3=1;
 TRISBbits.TRISB3=0; //Turn on the 4th LED
 PORTBbits.RB4=1;
 TRISBbits.TRISB4=0; //Turn on the 5th LED
 PORTBbits.RB5=1;
 TRISBbits.TRISB5=0; //Turn on the 6th LED
 PORTBbits.RB6=1;
 TRISBbits.TRISB6=0; //Turn on the 7th LED
 PORTBbits.RB7=1;
 TRISBbits.TRISB7=0; //Light up the 8th LED
 
 while(1); 
} 

    Secondly, it can be implemented by byte operation. The code is much simpler than bit operation, and it can truly achieve the requirement of lighting up at the same time. The following is the implementation code of byte operation.
#include

void main(void)
{
 PORTB=0xff;
 TRISB=0x00; //Light up 8 LEDs
 
 while(1); 
}