Input and Output Ports

1Purpose and structure of the course6 min

The chapter follows, in turn, the hardware structure of a pin, the software registers that control it, the path of the signal from outside toward the core, the problems that arise along this path and the external circuits commonly connected to ports. The reference microcontroller remains the ATmega328P, with the advantage of a particularly clear organization: three registers per port and a direct correspondence between bit and pin.

• Distinguish DDRx, PORTx and PINx by role, and explain why three registers are needed
• Explain the four electrical regimes of a pin (DDRx/PORTx) and the consequences of each
• Explain metastability and the role of the two-stage synchronizer
• Size a pull-up or limiting resistor for an LED
• Recognize a multi-function conflict on a pin, and filter the bouncing of a button
• Respect the per-pin, per-group and total current limits of the ATmega328P

2Input-output ports and the digital port10 min

Input and output (I/O) ports are the hardware interfaces through which the microcontroller communicates with external peripherals. A port contains no actual signal processing - it is where a logic value in memory becomes a voltage on a wire, or the other way around.

A pin is considered an I/O pin if the direction of transfer can be changed by the program, with no intervention on the physical circuit - the trait that distinguishes it from a fixed-function pin (power, reset). Pins are not treated individually, but grouped, usually in eights, forming a port: eight bits is exactly the width of a register on an 8-bit machine, so the entire port is read or written through a single instruction. The ATmega328P has three ports - B, C, D - but the pin count in the data sheet and the number actually usable on a real board do not coincide: port B has two pins taken up by the 16 MHz crystal, port C has its last pin taken up by the reset input.

Logic thresholds

The digital port classifies any applied voltage into one of two states. For an AVR powered at 5 V: any voltage below 0.3·VCC (~1.5 V) is safely read as logic 0; any voltage above 0.6·VCC (~3 V) is safely read as logic 1. Between the two thresholds there remains a forbidden band, for which nothing is guaranteed - the result can differ from one chip to another or from one reading to the next.

3The architecture of a port: pin structure and the three registers10 min

Communication is bidirectional, and bidirectionality must be built in hardware. Each pin contains an output buffer (drives the voltage), an input buffer (reads the voltage without changing it) and an enable line (decides which buffer is actually connected) - controlled by a bit in a configuration register. When the output buffer is disabled, the pin goes into high impedance (HiZ), almost like a broken wire.

The five blocks of a pin

The configuration flip-flops (direction + output value, static storage). The push-pull output stage: two complementary transistors, never conducting at the same time - the top one supplies current (source), the bottom one absorbs current (sink). The switchable pull-up resistor (20-50 kΩ on AVR). The protection diodes (toward VCC and toward ground, reverse-biased during normal operation). The input chain (Schmitt trigger + synchronizer, discussed separately).

The three registers of a port

For port B: DDRB, PORTB, PINB (similarly for C, D).

4Configuring ports: through registers and through Arduino12 min

Configuring the direction is the first thing done in a microcontroller program, before the main loop - everything else depends on it, including the electrical integrity of the setup (a wrong direction can put two voltage sources face to face).

#include <avr/io.h>

int main(void)
{
    DDRB |= (1 << PB5);             /* PB5: LED, output */
    DDRD &= ~(1 << PD2);            /* PD2: button, input */
    PORTD |= (1 << PD2);            /* internal pull-up enabled */

    while (1) {
        if ((PIND & (1 << PD2)) == 0) {   /* pressed = tied to ground */
            PORTB |= (1 << PB5);
        } else {
            PORTB &= ~(1 << PB5);
        }
    }
}

Configuration through the Arduino functions

const int LED = 13, BUTTON = 2;
void setup() {
    pinMode(LED, OUTPUT);
    pinMode(BUTTON, INPUT_PULLUP);
}
void loop() {
    digitalWrite(LED, digitalRead(BUTTON) == LOW ? HIGH : LOW);
}

INPUT_PULLUP does, in a single call, exactly what the two lines with DDRx and PORTx did. PORTB |= (1<<PB5) compiles into a single instruction, one cycle. digitalWrite translates the pin number into port+bit through two tables in Flash, checks and stops any active PWM channel, disables and re-enables interrupts - dozens of cycles. For an LED controlled by a button, the difference is irrelevant; for a precise edge or an interrupt that must finish quickly, it becomes decisive.

5Data transfer: sampling the signal9 min

The voltage applied on a pin does not reach the program instantly - the path is sliced by the system clock. A latch-type flip-flop, driven by the clock, captures the value on the pin only once per cycle, at a precise edge. Consequence: the pin's state is not read continuously, but sampled - the program sees the last snapshot taken of the pin.

6Metastability9 min

Metastability is an unstable state in which the output of a flip-flop sits in a transition zone between the two logic levels, being neither 0 nor 1, for an unpredictable duration. It happens when the input signal changes right at the moment of sampling.

The flip-flop ends up there if, at the clock edge, the input voltage is not far enough from the switching threshold - the condition expressed in data sheets through the setup time and the hold time around the edge.

The causes: the lack of correlation between the external signal and the internal clock (a button pressed by a person follows no synchronization rule with a 16 MHz clock) and slow edges (a signal that travels a long cable spends a lot of time in the intermediate zone, directly raising the probability of sampling it there).

7The synchronizer circuit8 min

The solution for the asynchronous nature of external signals is the synchronizer: it does not eliminate metastability, but gives it time to settle before the value reaches the program. Two flip-flops are placed in series, driven by the same clock, and the value used further on is the one at the output of the second one.

What the synchronizer does not guarantee is which value will be propagated if the change landed exactly on the edge - old or new, which just means the transition was seen one cycle earlier or later, almost always acceptable. What it guarantees is that the program will never see a value that is neither 0 nor 1.

The total input delay: d_in = d_latch + d_sync, where d_sync is the number of propagation cycles through the synchronizer. The more stages it has, the lower the probability of error, but the higher d_sync - two stages are the usual compromise for ordinary microcontrollers; very fast or safety-critical systems may use three or more.

8Pull-up and pull-down resistors10 min

A digital input not connected to any defined signal source sits in high impedance (HiZ) - counterintuitive for a beginner, for whom "an unconnected wire is at zero". It is not: the CMOS input has a resistance on the order of hundreds of megaohms, and the potential of a floating pin is set by whatever else is around (capacitive coupling, the 50 Hz mains hum, the electrostatic charge of a nearby hand) - the readings look random.

The fix is a pull-up resistor (between the line and VCC, fixes the rest state at logic 1) or a pull-down resistor (between the line and ground, fixes the rest state at logic 0). The pull-up-with-button-to-ground setup is the most common, producing negative logic: rest = 1, pressed = 0 - preferred because AVR has an internal pull-up, but not an internal pull-down, and the ground trace is usually easier to route on a board than the supply trace.

9Pins with multiple functions7 min

The number of pins on a package is limited, while the number of peripherals on a chip keeps growing - the fix is function multiplexing: the same pin can be used, at different times, by different peripherals. In its ordinary mode, as a plain digital pin, it is called GPIO (general purpose input/output).

Configuring these functions is not done from the port registers, but from the SFRs of the respective peripheral (e.g. UCSR0B for UART, SPCR for SPI). As long as an alternative function is active, the pin can no longer function as GPIO - writes to PORTx no longer have effect, reads from PINx may no longer reflect what is expected.

10Protecting digital pins12 min

Pins are points of contact between a few-nanometer silicon structure and an outside world full of parasitic voltages. Manufacturers provide internal protections, but with well-defined limits - exceeding them destroys the chip.

Internal protections

ESD (electrostatic discharge): the diodes from each pin toward VCC and ground divert the pulse - protection for short, rare events, not for sustained overvoltage. Overvoltage: the same diodes, in clamping mode - work only if a small current (milliamps) flows through the diode; exceeding it destroys the diode and can raise the whole supply of the chip.

Maximum current - three simultaneous limits

11Related circuits: buttons, relays, LEDs, optocouplers10 min

Ports are rarely connected directly to what needs to be read or commanded - between them sit circuits that adapt levels, isolate, filter or amplify.

Buttons and switches: contact bounce

Metal contacts do not stick together instantly - on closing they bounce elastically, touching and separating several times before settling into firm contact (bouncing). For a microcontroller reading the pin a million times a second, a single press becomes a burst of dozens of transitions.

if (reading != previousState) lastChange = millis();
if ((millis() - lastChange) > INTERVAL) {
    if (reading != stableState) {
        stableState = reading;
        /* handle the press here, exactly once */
    }
}
previousState = reading;

Relays

A relay is never driven directly from a pin: the coil draws 30-100 mA (above the absolute maximum of a pin), and when the current is interrupted, the energy stored in the magnetic field produces a reverse-voltage spike of hundreds of volts.

LEDs, multiplexed displays, optocouplers

Wiring several LEDs in parallel on the same resistor is a mistake: LEDs do not have exactly the same V_F, the one with the lower V_F takes a disproportionate share of the current, heats up, which lowers its V_F even further - a process that destroys it. Each LED needs its own resistor.

On multi-digit displays, matching segments are wired together, with the common cathodes driven individually - the program cycles fast enough (above ~50 Hz/digit) that retinal persistence makes all digits appear lit at once. The price: the average brightness drops proportionally to the number of digits, compensated by raising the instantaneous current above the allowed continuous value.

The optocoupler (an LED plus a phototransistor, optically isolated) is used for galvanic isolation - it protects the microcontroller from dangerous voltages on the power side and breaks ground loops. On the input side it is treated like an ordinary LED; on the output side, the phototransistor behaves like a switch to ground, naturally used with a pull-up, just like a button.

12Frequent mistakes4 min

• "I read a button's state from PORTD, it is simpler." False - PORTD holds what the program wrote (the command or the pull-up bit), not the real state on the pin. The program will report unwaveringly what it wrote there, no matter what the button does. Any reading of an input is done from PINx, never from PORTx.
• "An unused pin, left unconnected, causes no harm at all." False - a floating pin has an undefined potential, set by the surrounding noise, with seemingly random readings and extra consumption through the partial conduction of both input transistors. Explicitly configure any unused pin as an input with pull-up, or as an output at a fixed value.
• "A relay is driven directly from a pin, like an LED." False twice over - the coil exceeds the maximum current on a pin, and on interruption it generates a spike of reverse voltage of hundreds of volts, which can destroy the driving transistor and reach, through the protection diodes, the microcontroller's supply. Always use a switching transistor plus a flyback diode in parallel with the coil.

13Summary and glossary5 min

A port groups eight pins to allow access through a single instruction. Each pin has three registers - DDRx for direction, PORTx for command (or pull-up, on input), PINx for the real state - and they play an independent logical role: they cannot be merged into a single one without losing information. The input signal is sampled, not read continuously, which introduces a small delay and can lose pulses shorter than one clock cycle. Sampling an asynchronous signal right at the edge can produce metastability - solved not by elimination, but by a two-stage synchronizer, which makes it improbable. A floating pin is undefined, not zero; a pull-up or pull-down resistor gives it a rest state. Function multiplexing takes a pin away from the GPIO role while the associated peripheral is active. The current limits - per pin, per group, total - are not negotiable, and any load exceeding a few milliamps needs an intermediate power element.

14Self-check questions7 min

1. Explain why an AVR port needs three separate registers and describe a situation where the value in PORTx differs from the one read from PINx for the same pin.
2. A pin has DDRD bit 4 at 0 and PORTD bit 4 at 1. What regime is it in, what level is read at rest, and what happens if an outside contact ties it to ground?
3. Why is reading a pin delayed relative to the physical change of the voltage, and how long is this delay on an ATmega328P at 16 MHz?
4. What is metastability, what internal mechanism explains it, and why can it not be eliminated completely, only made improbable?
5. Compare the role of the Schmitt trigger with the role of the two-stage synchronizer - which cause of metastability does each solve?
6. List three situations where the AVR's internal pull-up is not enough and explain why, for each one.
7. A green LED (V_F=2.1 V) must be driven at 12 mA from a 5 V pin. Calculate the limiting resistor, choose the standard value and determine the power dissipated.

15Directions for further study2 min

The next lecture moves from manually polling a pin to the hardware mechanism that solves exactly the problem of missed short pulses: the interrupt system - vectors, priorities, latency and the pitfalls of writing a correct handler routine.

The registers, configuration examples and circuits from this lecture become a real setup in Laboratory 02.