Mr. Integral
Embedded notes/Interacting with Hardware/Digital Input and Output

UNIT 07 · INTERACTING WITH HARDWARE

Digital Input and Output

Pins that read, drive and survive the real world.

6 lessons24 guided experiments43 min full read

A general-purpose I/O pin looks like the simplest thing on a microcontroller: write a 1, the LED lights; read a 0, the button is pressed. In practice every pin is a small configurable circuit with a multiplexer, an output driver that can push or only pull, an input buffer with pulls and hysteresis, and protection diodes, and the firmware around it decides whether a press is seen once, twice or not at all.

Start with lesson 1 →
∫ MR-INTEGRAL · 15 UNITS · 90 LESSONS016 lessonsPinssignals and circuits036 lessonsCPU coreand memory026 lessonsRegisters096 lessonsInterruptcontroller136 lessonsSchedulertasks and deadlines046 lessonsFlash imagebuilt by the toolchain156 lessonsSecure bootupdates and keys056 lessonsBoot ROMand reset066 lessonsDebug portprobe, trace146 lessonsPowersleep, brown-out126 lessonsDMA076 lessonsGPIOpins in and out086 lessonsClocksand timers106 lessonsUART · SPI · I²Cserial links116 lessonsADC · DACanalog
∫ MR-INTEGRAL · 15 UNITS01Pins · signals and circuits036 lessonsCPU coreand memory026 lessonsRegistersC close to the metal096 lessonsInterruptcontroller136 lessonsSchedulertasks and deadlines046 lessonsFlash imagebuilt by the toolchain156 lessonsSecure bootupdates and keys056 lessonsBoot ROMand reset066 lessonsDebug portprobe, trace146 lessonsPowersleep, brown-out126 lessonsDMA076 lessonsGPIOpins in and out086 lessonsClocksand timers106 lessonsUART · SPI · I²Cserial links116 lessonsADC · DACanalog

The unit in six ideas

  1. 1A pin is a multiplexer (who owns it: software GPIO or a peripheral) in front of a pad (input buffer, pulls, Schmitt trigger, drive strength, slew, protection).
  2. 2Push-pull drives both levels hard with two transistors; open-drain only pulls low, and a pull-up makes the high level.
  3. 3Every input needs a driver or a pull; use a pull-up with a button to GND (active low) and a pull-down with a button to VDD.
  4. 4Contacts bounce for milliseconds; sample the input periodically and decide from its history instead of reacting to edges.
  5. 5Budget both per-pin and total chip current; anything more than a few milliamps, or a sum near the limit, needs a driver transistor.
  6. 6delay() blocks the whole loop; an input shorter than the delay can be missed entirely.

Lessons

  1. LESSON 01 · 4 EXPERIMENTS · 7 MIN

    Pin Modes and Alternate Functions

    Who decides which peripheral owns a pin, and how do you tell the chip?

  2. LESSON 02 · 4 EXPERIMENTS · 7 MIN

    Push-Pull and Open-Drain Outputs

    What is different about those pins?

  3. LESSON 03 · 4 EXPERIMENTS · 7 MIN

    Reading Inputs and Handling Floating Pins

    What else does a reliable input need?

  4. LESSON 04 · 4 EXPERIMENTS · 7 MIN

    Debouncing Buttons and Switches

    How do you turn that burst into exactly one event, and what does it cost?

  5. LESSON 05 · 4 EXPERIMENTS · 8 MIN

    Driving Loads and Protecting Pins

    What does a pin need around it to drive real loads safely, and to survive the outside world?

  6. LESSON 06 · 4 EXPERIMENTS · 7 MIN

    Designing a Nonblocking Input-Output Loop

    How should a loop be structured so that inputs are never missed and one new feature cannot break the others?