14.4.1 Data Register

PORTx is a bidirectional port, and its corresponding data direction register is TRISx. Setting a TRISx bit (‘1’) will make the corresponding PORTx pin an input (i.e., disable the output driver). Clearing a TRISx bit (‘0’) will make the corresponding PORTx pin an output (i.e., it enables output driver and puts the contents of the output latch on the selected pin). The example below shows how to initialize PORTA.

Reading the PORTx register reads the status of the pins, whereas writing to it will write to the PORT latch. All write operations are Read-Modify-Write operations. Therefore, a write to a port implies that the PORT pins are read, this value is modified and then written to the PORT data latch (LATx).

The PORT data latch LATx holds the output port data and contains the latest value of a LATx or PORTx write. The examples below show how to initialize PORTA.

; This code example illustrates initializing the PORTA register. 
; The other ports are initialized in the same manner.

    BANKSEL    PORTA        ;
    CLRF       PORTA        ;Init PORTA
    BANKSEL    LATA         ;Data Latch
    CLRF       LATA         ;
    BANKSEL    ANSELA       ;
    CLRF       ANSELA       ;digital I/O
    BANKSEL    TRISA        ;
    MOVLW      B'00111000'  ;Set RA[5:3] as inputs
    MOVWF      TRISA        ;and set RA[2:0] as outputs

// This code example illustrates initializing the PORTA register. 
// The other ports are initialized in the same manner.

    PORTA  = 0x00;       // Clear PORTA
    LATA   = 0x00;       // Clear Data Latch 
    ANSELA = 0x00;       // Enable digital drivers
    TRISA  = 0x38;       // Set RA[5:3] as inputs and set others as outputs