Reading the status of an input pin and displaying onto an LED - LPC1115

Question

this code is supposed to read the state of a digital input pin via a pushbutton and output the state to an LED. i.e. when input is high, LED is on and vice versa Since the pushbutton is connected to pull-up resistor, When the pushbutton is pressed the input is supposed to read a LOW and vice versa.

My code:

    #include "board.h"
    #include <stdio.h>

    //setting pointers
    #define Port0 ((LPC_GPIO_T *) 0x50000000) //Port 0
    #define IOCON ((LPC_IOCON_T *) 0x40044000) //IO configuration

    int main(void)
    {

        /* Initialize pins */       
        Port0->DIR &= ~((1 << 1)); //PIO0_1 input - onboard switch (unpressed state is pulled-up)
        Port0->DIR |= (1<<7);      //PIO0_7 output - onboard LED

        //Pin configuration
        IOCON->REG[IOCON_PIO0_7] &= 0x0 << 3; //No addition pin function
        IOCON->REG[IOCON_PIO0_1] &= 0x0 << 3; // "

        Port0->DATA[1<<7] &= ~(1<<7); // output initially low 

        while (1) {

            if((Port0->DATA[1<<1]) & (1<<1)) //When input is high
            {
                Port0->DATA[1<<7] |= (1<<7); //drive PIO0_7 High

            }
            else
            {
                 Port0->DATA[1<<7] &= ~(1<<7); //Drive PIO0_7 Low
            }
        }

        return 0;
    }

When this part of the code is executed PIO0_7 remains remains low unless the button is pressed..However isn't it meant to work the opposite way since switch is pulled-up? I also double checked this with the voltmeter.

I tried changing

     if((Port0->DATA[1<<1]) & (1<<1)) //When input is high

to

     if(!(Port0->DATA[1<<1]) & (1<<1)) //When input is Low

The LED output remains High, even when the button is pressed.

veeman veeman · Accepted Answer · 2018-12-10T12:06:44

Assuming your Port0->DATA[0] is pointing to Base-Address 0x5000 0000 and defined as aligned 8bit array then your Pin-Port addressing/masking is wrong.

See LPC111x user manual UM10398 Rev. 12.4 p196 Chapter 12.4.1 write/read data operation:

In order for software to be able to set GPIO bits without affecting any other pins in a single write operation, bits [13:2] of a 14-bit wide address bus are used to create a 12-bit wide mask for write and read operations on the 12 GPIO pins for each port.

So there is an offset of 2 bit in the address to get/set the value of your desired pin. Therefore you must shift your addressing by 2 bit, the following should do the trick:

Port0->DATA[1<<(7+2)] &= ~(1<<7); // output initially low 

while (1) {
    if((Port0->DATA[1<<(1+2)]) & (1<<1)) //When input is high
    {
        Port0->DATA[1<<(7+2)] |= (1<<7); //drive PIO0_7 High
    }
    else
    {
         Port0->DATA[1<<(7+2)] &= ~(1<<7); //Drive PIO0_7 Low
    }
}

Reading the status of an input pin and displaying onto an LED - LPC1115

1 Answers