Embedded Lab

Digital Input-Output – DIO

Digital Input-Output (DIO) a.k.a General-Purpose Input-Output (GPIO) coding is the basic and simplest requirement for any microcontroller. This is one area where a microcontroller (MCU) differs from a microprocessor (MPU).

MSP430F5529LP Launchpad comes with 40-pin dual-in-line headers. Of these 40 pins, 35 pin headers are connected to DIO pins. There seven DIO ports but they are not evenly divided, i.e. not all ports are 8-bit wide as one would typically expect. Most of the DIOs have more than one functionality, i.e. timer, ADC, communication, etc. DIO pins can be individually programmed as either inputs or outputs. There are independent input-output data registers. In input mode, DIOs can be pulled up or pulled down using internal pull resistors just like other MSP430s. To take care of Electromagnetic Interference (EMI) issues, output drive strength can be altered in output mode, adding incredible robustness. Additionally, pins can operate at high frequency (25MHz) conditions.

Note in the diagram above that ports are grouped as PA, PB, PC and PD. Ports are usually 8-bit wide but after adjacently grouping them as such, they become 16-bit wide. PA port pins additionally have interrupt capability unlike other ports. PA port consists of P1 and P2, PB port consists of P3 and P4 and so forth. Port grouping is helpful when we need to drive parallel port interfaces like TFT displays.

It is advised not to exceed output drive current or input voltage ranges/polarity. Best practices are to avoid driving loads directly via output pins and using buffer ICs or isolation for input pins. We must check that whether we are not overloading the microcontroller pins any how because this may have undesired consequences. Try to keep MCU’s total current consumption as low as possible. Any device is more stable at low power and optimum temperature conditions than otherwise. Remember MSP430s are low-power device.

Code Example

#include "driverlib.h"

void GPIO_init(void);

void main(void)
{
    WDT_A_hold(WDT_A_BASE);

    GPIO_init();

    while(1)
    {
        if(GPIO_getInputPinValue(GPIO_PORT_P1,
                                 GPIO_PIN1) == 0)
        {
            GPIO_setOutputHighOnPin(GPIO_PORT_P4,
                                    GPIO_PIN7);

            GPIO_setOutputLowOnPin(GPIO_PORT_P1,
                                   GPIO_PIN0);
        }

        if(GPIO_getInputPinValue(GPIO_PORT_P2,
                                 GPIO_PIN1) == 0)
        {
            GPIO_setOutputHighOnPin(GPIO_PORT_P1,
                                    GPIO_PIN0);

            GPIO_setOutputLowOnPin(GPIO_PORT_P4,
                                   GPIO_PIN7);
        }
    };
}

void GPIO_init(void)
{
    GPIO_setAsInputPinWithPullUpResistor(GPIO_PORT_P1,
                                         GPIO_PIN1);

    GPIO_setAsInputPinWithPullUpResistor(GPIO_PORT_P2,
                                         GPIO_PIN1);

    GPIO_setAsOutputPin(GPIO_PORT_P1,
                        GPIO_PIN0);

    GPIO_setDriveStrength(GPIO_PORT_P1,
                          GPIO_PIN0,
                          GPIO_FULL_OUTPUT_DRIVE_STRENGTH);

    GPIO_setAsOutputPin(GPIO_PORT_P4,
                        GPIO_PIN7);

    GPIO_setDriveStrength(GPIO_PORT_P4,
                          GPIO_PIN7,
                          GPIO_FULL_OUTPUT_DRIVE_STRENGTH);
}

Hardware Setup

Explanation

The demo here alternatively turns on/off the pair of on-board LEDs with on-board button presses. Since this is the first example, I did not configure clocks and so the default clock settings are used. There is nothing dependent on timing.

Inputs are configured as inputs with internal pull-up resistor. This is so because the on-board buttons are directly connected the GPIO pins without any pull resistor.

GPIO_setAsInputPinWithPullUpResistor(GPIO_PORT_P1, GPIO_PIN1);

Outputs are configured full output drive output pins.

GPIO_setAsOutputPin(GPIO_PORT_P4, GPIO_PIN7); GPIO_setDriveStrength(GPIO_PORT_P4, GPIO_PIN7, GPIO_FULL_OUTPUT_DRIVE_STRENGTH);

In the main loop, the LED pins are toggle alternatively with button presses.

if(GPIO_getInputPinValue(GPIO_PORT_P1, GPIO_PIN1) == 0)
{
    GPIO_setOutputHighOnPin(GPIO_PORT_P4, GPIO_PIN7);

    GPIO_setOutputLowOnPin(GPIO_PORT_P1, GPIO_PIN0);
}

if(GPIO_getInputPinValue(GPIO_PORT_P2, GPIO_PIN1) == 0)
{
    GPIO_setOutputHighOnPin(GPIO_PORT_P1, GPIO_PIN0);

    GPIO_setOutputLowOnPin(GPIO_PORT_P4, GPIO_PIN7);
}

Demo

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