Microcontroller Pins: Digital I/O

Pins of an IC

• Pins are physical connections that link the microcontroller to the external environment.
• Certain pins serve specific roles like power supply, clock, or reset.
• Most pins are responsible for transmitting or receiving signals, whether digital or analog.
• For communication purposes, software or hardware components implementing communication protocols control pins.

Digital Inputs

• A digital input acts as an amplifier, more specifically a comparator.
• It uses a constant reference voltage.
• Input pins have a high input impedance.
• Little current flows into or out of the pin.
• It does not load the signal source, ensuring the voltage of the signal source remains unchanged.

Digital Outputs

• A digital output uses a driver circuit to set the pin voltage to high or low.
• Output pins have low output impedance, ideally close to zero.
• It enables current to be driven into or out of the pin, effectively creating a shortcut to ground or to the supply voltage.
• It maintains a constant pin voltage, regardless of current flow, within certain limits.
• It can be disabled to prevent unintended short circuits.

Pull-Up and Pull-Down Resistors

• Pull-up and pull-down resistors connect to a pin.
• Pull-up resistors are linked to a positive voltage supply.
• Pull-down resistors connected to the ground.
• They set a specific pin value when no external driver connects, which is known as a floating pin.
• High resistance is used to avoid overloading the signal source, typically around 50 kΩ.

Voltage Levels of Pins

• Digital pins use different standardized voltage levels.
• Standards define voltage levels for high and low signals, limits for driving a pin, and thresholds for detecting the signal's state.
• Higher voltages improve the signal-to-noise ratio (SNR).
• They maintain signal integrity over long distances.
• Lower voltages reduce power consumption, electromagnetic interference (EMI), and are faster.

LVDS (Low Voltage Differential Signaling)

• LVDS uses a differential signal with two signals of opposite polarity.
• It has a small difference between the two signals, like 350 mV.
• Receivers ignore the common-mode voltage, and the average of the two signals.
• Noise common to both signals does not affect the receiver.
• LVDS commonly is used for high-speed communication, up to 4 Gbit/s.
• It supports video streaming between the CPU and display or camera.

GPIO (General Purpose Input/Output)

• Definition: A GPIO pin is used to read or write individual digital values (high or low).
• Typical applications: controlling LEDs or reading buttons.
• GPIO pins can have peripheral functions.
• Software selects the function through a memory-mapped register inside the CPU's IO multiplexer.
• GPIO pins feature configurable pull-up or pull-down resistors and adjustable drive strength.

ESP32 GPIO example: Arduino framework

• The order differs between this and the next slide, this has no practical reason.
• Configures the digital pin as an input/output.
• Reads and writes the actual value.

ESP32 GPIO example: Memory-mapped access

• Access output by setting the pin to high or low.
• Read the value of the button state or pin input.

ISO/OSI Reference Model

• The ISO Open Systems Interconnection (OSI) model provides a conceptual model for communication systems.
• There are seven layers, each completing a specific function.
• Hardware components (PHY) or software (drivers, protocols) implement the layer functions.
• A layer transfers data to the layer above or below.
• Standardized interfaces make implementations interchangeable.
• It can omit or merge layers for communication.
• Exemplary protocol stack, starting from the bottom: Ethernet (1&2), IP (3), TCP (4), HTTP (5), JSON (6), REST API (7).

UART (Universal Asynchronous Receiver Transmitter)

• Implementation: It implements layer-2 (Data Link).
• Function: Serial communication between two computers.
• Topology: Point-to-point.
• No Clock Signal: Asynchronous communication omits a clock signal.
• Start of message: can occur at any time with the receiver syncing its clock with the data stream.
• Two Wires required: TX (transmit) and RX (receive).
• Ground required: It needs ground as a voltage reference.
• Layer 1: It specifies more control signals.
• It can transmit arbitrary binary data, that is application-specific.

UART (part 2)

• Identical configuration required: A sender and receiver must be configured with identical communication parameters.
  • Baud rate: between 50 bps and 3 Mbps.
  • Common baud rates: 9600 bps and 115200 bps.
  • Data bits: 5 to 9, typically 8, LSB first.
  • Parity: none, even, or odd, used for error detection.
  • Stop bits: usually 1 or 2, which is used for pause between messages.
• USART: Optional synchronous signal, but it is not commonly used today.

UART Usage

• Common use: Observing the debug output of a microcontroller.
• Alternative use: Bidirectional communication between a microcontroller and PC.
• Use by Linux: Embedded Linux devices can use UART for console input/output.
• Serial console: UART is called a serial console or TTY (teletype) for input/output.
• MCU to Peripheral IC communication: Peripheral IC must have a UART interface.
• Limitation: Not well suited for connecting many peripherals to an MCU, two dedicated pins per peripheral is required.

USB UART Adapters

• Usage: Connects a UART device to a PC.
• Linux interfaces: Known as /dev/ttyUSB<N> not /dev/ttyS<N>.
• Integration: It can be integrated into USB cables or on a PCB.
• FTDI Chips: Adapters are from FTDI (Future Technology Devices International).
• PC Usability: It can be connected to a PC via USB and used as a UART device.

JTAG (Joint Test Action Group)

• Standard for testing integrated circuits.
• It has a daisy chain topology, the signal is passed through all devices and components.
• It checks PCBs, by checking pins on a chip.
• Uses access to the internal signals of a chip.
• Serial Wire Debug (SWD) interface replaces in ARM Cortex-M MCUs.

JTAG - Usage

• The internal state of a chip can be read and modified via JTAG.
• Access an MCU's flash memory or an FPGA's SRAM configuration memory.
• Debug the program running on a microprocessor.
• Read and write memory and registers, set breakpoints, and step through instructions.
• The internal design is required, making vendor-specific hardware (adapters) necessary.
• Software can be vendor-specific or open source, such as OpenOCD.

JTAG - Security Considerations

• Common in development: It is exposed on development boards.
• More or less accessible: It is sold to customers.
• Security Risk: It's a security risk because code can be read, modified, and security features disabled.
• JTAG is often disabled on chips by the manufacturer.
• Chips can be protected at the board level (by not soldering JTAG pins) or at the chip level.
• Code can bypass special registers, the fuse can be burned, which is irreversible.

Inter-Integrated Circuits I2C

• Logical Topology: Star - one controller device controls communication, and multiple target devices can be connected.
• Physical Topology: Bus - multiple devices can be connected using the same pair of wires for SCL (clock) and SDA (data).
• Signal management: Wires are pulled up to a positive voltage by default. Devices pull the wire to ground to transmit a low bit, using an open-drain circuit.
• Limiting Factor: Pull-up resistors and parasitic capacitance act as an RC low-pass filter, which limits speed.
• Frequencies: predefined: 100 kHz, 400 kHz, 1 MHz, and 3.4 MHz.

Inter-Integrated Circuits I2C - protocol

• Device Addressing: Uses a 7-bit address.
  • Usually fixed for the product.
  • Some devices set bits of the address.
  • Allows two identical devices on the same bus.
• Communication start: Initiated by the controller sending a START condition (S), which is a high-to-low transition of SDA while SCL is high.
• Data Transfer: In 8-bit bytes, MSB first, with SDA transitions when SCL is low.
• Acknowledgment: Each byte is acknowledged by the receiver which pulls SDA low during the 9th clock pulse.
• Communication end: Ends with the controller sending a STOP condition (P).

I2C - Usage

• Common devices used: sensors and actuators.
• Small storage devices, like EEPROMs.
• System Management Bus (SMBus): A stricter subset of I²C for power supplies, batteries, temperature sensors, and part of PCI and PCIe interface definition.
• Two-wire interface (TWI): Incomplete implementation of I²C, used by Atmel AVR MCUs, compatible with I²C devices.

Serial Peripheral Interface (SPI)

• Logical Topology: Star (one main device, multiple subnode devices).
• Alternate Names: The main/subnode can be referred to by other names (e.g. controller/peripheral, master/slave).
• Signal Names: SCLK (clock), MOSI (main out, sub in), MISO (main in, sub out), CS (chip select).
• Bus on MISO/MOSI and SCLK, Star on CS: same data signal for all devices, one CS per device.
• Daisy chain on MISO/MOSI, Bus on SCLK and CS (exotic): data signals are passed through all devices, same CS for all devices.

SPI - Protocol

• Layer-2 protocol: It Does not specify voltage levels and data contents.
• Bidirectional: Data transmits bidirectionally while a chip select (CS) line is pulled low.
• Subnode selection: Selected by CS, it sends data to the main node on MISO.
• Main device: It generates the clock signal on SCLK while a subnode is selected.
• Configuration: Polarity of SCLK and phase shift between SCLK and MISO/MOSI are configured by setting CPOL (0 or 1) and CPHA (0 or 1).
• Transfer: Numbers of bits per transfer is not specified.
• Parallel: Parallel data transmission using multiple MOSI/MISO lines, like Quad SPI (QSPI).

SPI - Usage

• It connects Sensors and actuators.
• Connects Storage devices like EEPROMs flash (often QSPI).
• Other interfaces: Implicitly used by some other interfaces, like SD cards.