Microcontroller Architecture and GPIO Output

Complete Lab Manual

For the complete experiment including learning objectives, theoretical background, and detailed explanations, download the PDF manual: Download Experiment 4 PDF

Examples

The following examples illustrate the complete process of initializing Port F and controlling the on-board LEDs using Assembly language.

Example 1: Simple LED Blink

This example initializes Port F and toggles the Red LED (PF1) on and off using address masking with a software delay.

Address Masking for PF1:

0x40025000 + (0x02 << 2) = 0x40025008

        AREA    RESET, CODE, READONLY    ; Code section
        THUMB                            ; Use Thumb instruction set
        EXPORT  __main                   ; Export symbol

__main  PROC
        LDR     R1, =0x400FE608          ; RCGCGPIO address
        LDR     R0, [R1]                 ; Read current value
        ORR     R0, R0, #0x20            ; Enable Port F clock
        STR     R0, [R1]                 ; Write back
        NOP                              ; Small delay
        NOP

        LDR     R1, =0x40025400          ; GPIODIR address
        LDR     R0, [R1]                 ; Read current value
        ORR     R0, R0, #0x02            ; Set PF1 as output
        STR     R0, [R1]                 ; Write back

        LDR     R1, =0x4002551C          ; GPIODEN address
        LDR     R0, [R1]                 ; Read current value
        ORR     R0, R0, #0x02            ; Enable digital function for PF1
        STR     R0, [R1]                 ; Write back

loop    LDR     R1, =0x40025008          ; Masked DATA address for PF1
        MOV     R0, #0x02                ; Turn ON red LED (PF1)
        STR     R0, [R1]                 ; Write to GPIODATA

        LDR     R0, =1000000             ; Load delay count
        BL      DELAY                    ; Call delay

        MOV     R0, #0x00                ; Turn OFF red LED (PF1)
        STR     R0, [R1]                 ; Write to GPIODATA

        LDR     R0, =1000000             ; Load delay count
        BL      DELAY                    ; Call delay

        B       loop                     ; Repeat
        ENDP

DELAY   PROC
        SUBS    R0, R0, #1               ; Decrement counter
        BNE     DELAY                    ; Loop until zero
        BX      LR                       ; Return
        ENDP
        END

Example 2: Cycle Through RGB Colors

This example extends the previous one by cycling through RGB LED colors (Red, Green, Blue) using a software delay.

Address Masking for PF1, PF2, PF3:

0x40025000 + ((0x02 | 0x04 | 0x08) << 2) = 0x40025038

        AREA    RESET, CODE, READONLY    ; Code section
        THUMB                            ; Use Thumb instruction set
        EXPORT  __main                   ; Export symbol
__main  PROC
        BL      PF_Init                  ; Initialize Port F
loop    
        LDR     R1, =0x40025038          ; Masked DATA address for PF1-PF3
        
        MOV     R0, #0x02                ; Turn ON red LED (PF1)
        STR     R0, [R1]                 ; Write to GPIODATA	
        LDR     R0, =10000000            ; Load delay count
        BL      DELAY                    ; Call delay

        MOV     R0, #0x04                ; Turn ON blue LED (PF2)
        STR     R0, [R1]
        LDR     R0, =10000000
        BL      DELAY

        MOV     R0, #0x08                ; Turn ON green LED (PF3)
        STR     R0, [R1]
        LDR     R0, =10000000
        BL      DELAY
        B       loop                     ; Repeat
        ENDP

DELAY   PROC
        SUBS    R0, R0, #1               ; Decrement counter
        BNE     DELAY                    ; Loop until zero
        BX      LR                       ; Return
        ENDP

PF_Init PROC
        LDR     R1, =0x400FE608          ; RCGCGPIO address
        LDR     R0, [R1]                 ; Read current value
        ORR     R0, R0, #0x20            ; Enable Port F clock
        STR     R0, [R1]
        NOP                              ; Small delay
        NOP
        LDR     R1, =0x40025400          ; GPIODIR address
        LDR     R0, [R1]
        ORR     R0, R0, #0x0E            ; Set PF1-PF3 as outputs
        STR     R0, [R1]

        LDR     R1, =0x4002551C          ; GPIODEN address
        LDR     R0, [R1]
        ORR     R0, R0, #0x0E            ; Enable digital function for PF1-PF3
        STR     R0, [R1]
        BX      LR                       ; Return
        ENDP
        END

Tasks

Task 1: Adjust the Blink Rate

Modify the delay routine in Example 1 to change the blinking speed of the Red LED. Experiment with different delay values until the LED blinks at approximately 1 Hz (about one second ON and one second OFF).

Task 2: Cycle Through Multiple Colors

Expand Example 2 to include additional colors by combining the Red, Green, and Blue LEDs. Create a program that automatically cycles through all color combinations listed in the table above.

Hint: Use a loop to step through the color sequence repeatedly instead of writing separate code for each color.