%{
  title: "Monitor a Normally-Open Pushbutton",
  description: "Control an LED with a normally-open pushbutton connected to a GPIO pin.",
  topics: ["gpio"],
  level: "Beginner"
}
---

## Overview

Monitoring and responding to user input is a fundamental skill. This epxeriment will demonstrate program control
with a normally-open pushbutton. In this experiment the button is connected to ground; this is known as an 
"active-low" input as the GPIO pin will see 0 when the button is pressed, and 1 when the button is released 
(to the internal pull-up).

## Schematic

Connect a 1K resistor and a nomrally-open pushbutton between **P1** and **GND** as shown. The 1K resistor will protect
the GPIO pin if it is accidentally made an output and high while the button is pressed. 

```
  P1 ----[ 1K ]----[N.O. BTN]---- GND
  P0 ----[220R]----[+ LED -]----- GND
```

## SPIN2 Code

```spin
CON

  _clkfreq = 200_000_000

  BTN_PIN  = 1
  LED_PIN  = 0


PUB main() | bstate

  pinlow(LED_PIN)                             ' star with LED off
  wrpin(BTN_PIN, P_HIGH_15K)                  ' add pull-up to button pin
  pinhigh(BTN_PIN)                            ' activate pull-up

  repeat
    bstate := pinread(BTN_PIN)                ' get state of button
    if (bstate == 0)                          ' if pressed
      pinhigh(LED_PIN)                        '  turn the LED on
    else                                      ' else
      pinlow(LED_PIN)                         '  turn the LED off


## SPIN2 Code (Refined)

This version introduces the pinwrite() function to update the LED with the state of the button input. As button is
configured as active-low, the program XORs (^) the input with 1 to invert the input state to match the LED.

```spin
CON

  _clkfreq = 200_000_000

  BTN_PIN  = 1
  LED_PIN  = 0


PUB main() 

  pinlow(LED_PIN)                             ' star with LED off
  wrpin(BTN_PIN, P_HIGH_15K)                  ' add pull-up to button pin
  pinhigh(BTN_PIN)                            ' activate pull-up

  repeat
  ' copy inverted state of button to LED
    pinwrite(LED_PIN, pinread(BTN_PIN) ^ 1) 


## SPIN2 Code (Refined, Version 2)

This version demonstrates how the input state of a button input can be inverted with configuration, allowing the 
value read from the pin to be used directly.

```spin
CON

  _clkfreq = 200_000_000

  BTN_PIN  = 1
  LED_PIN  = 0

PUB main()

  pinlow(LED_PIN)                             ' star with LED off
  wrpin(BTN_PIN, P_HIGH_15K | P_INVERT_IN)    ' add pull-up to button pin, invert input state
  pinhigh(BTN_PIN)                            ' activate pull-up

  repeat
  ' copy state of button to LED
    pinwrite(LED_PIN, pinread(BTN_PIN))