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IO Ports

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Tristate Input Output Ports

PORTA, PORTB and PORTC

The AQA processor has three input / output ports called PORTA, PORTB and PORTC.
They are eight bit ports.
The port pins can be configured to be inputs or outputs.
Tristate logic is used to achieve this.

TRISA, TRISB and TRISC

Three data direction registers called TRISA, TRISB and TRISC are used to control the I/O pins on the three I/O data ports.
Binary data in the TRIS registers controls the direction of the PORT registers.
1 = I = Input.
0 = O = Output. This is logical and easy to remember.

Data I/O Example

CTRL+Click here to run the simulator.

Type in the code below or copy and paste it.
Click the "A" button to assemble the code (translate it into binary).
Click the "S" button to step through the program.

Send data to four of the eight LEDs. Click (red highlight) to connect the LEDs to PORTA.

INIT:
    MOVW    0xF0    ; IIIIOOOO
    MOVWR   TRISA   ; PORTA has four
                    ; inputs and four
                    ; outputs
LOOP:
    MOVW    0xAA    ; 10101010 test data
    MOVWR   PORTA   ; Write data to PORTA
    MOVW    0x55    ; 01010101 test data
    MOVWR   PORTA   ; Write data to PORTA
    JMP     LOOP

The program sends test data to PORTA.
The test data is 0x55 = 01010101 and 0xAA = 10101010
Since only four bits are set as outputs, the data that reaches the ports is
0x05 = 00000101 and 0x0A = 00001010
Four of the LEDs should flash in this alternating pattern.

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