Skip to content

UART

Summary

  • UART (Universal Asynchronous Receiver/Transmitter) is a hardware peripheral that converts data between parallel form (inside the MCU) and serial form (on the wire), and is the oldest, simplest serial protocol still in everyday use on embedded boards.
  • "Asynchronous" is the defining trait: there's no shared clock line between sender and receiver — both sides just have to agree in advance on a baud rate and frame format, and resynchronize at the start of every frame.
  • UART itself only defines the electrical framing of each byte. It says nothing about addressing, multiple devices on a bus, or application-layer structure — that's left entirely to whatever protocol runs on top of it (or to point-to-point wiring, which is the common case).

Core Signals

A minimal UART connection needs only two signal wires plus a common ground — no clock, unlike SPI or I²C:1

Signal Direction Purpose
TX (Transmit) Output Carries data out from this device
RX (Receive) Input Carries data in to this device
GND — Common voltage reference, required for the signal levels to mean anything

Table 1. Minimal UART Wiring

TX connects to RX, not TX to TX

When wiring two UART devices together, cross the lines: Device A's TX goes to Device B's RX, and vice versa. Connecting TX-to-TX (a very common first-timer mistake) means neither side ever receives anything.

Frame Format

Each byte is sent as a self-contained frame, bracketed by start and stop bits so the receiver can detect where it begins and ends without a shared clock:2

Field Typical Value Purpose
Start Bit 1 bit, always low Signals the beginning of a frame; this is what lets the receiver resynchronize each time
Data Bits Usually 8 (5–9 possible) The actual payload byte
Parity Bit None / Even / Odd (optional) A simple single-bit error-detection check
Stop Bit(s) 1 or 2 bits, always high Marks the end of the frame and gives the receiver a brief idle period before the next start bit

Table 2. UART Frame Fields

Frame format is usually written in shorthand

A configuration like "8N1" means 8 data bits, No parity, 1 stop bit — by far the most common setting. You'll also see "8E1" (even parity) or "7O2" (7 data bits, odd parity, 2 stop bits) on older or more conservative systems.

Baud Rate

Baud rate is the number of signal symbols (here, bits) transmitted per second, and it's the one parameter both sides absolutely must agree on in advance:3

Common Baud Rate Typical Use
9600 Classic default, very reliable over longer/noisier wiring
19200 / 38400 Common step-up for debug consoles and simple sensors
115200 De facto standard for modern MCU debug/serial consoles
230400 and above High-throughput links (e.g. GPS modules at high update rates, some wireless modems)

Table 3. Common UART Baud Rates

Baud rate mismatch doesn't fail cleanly

If both sides aren't within roughly a few percent of the same baud rate, the receiver samples bits at the wrong moments and produces garbled bytes or framing errors (below) — there's no handshake to catch this automatically, so it's a very common first debugging step with a new UART link.

Oversampling

To correctly detect the start bit and sample each data bit in the middle (where the signal is most stable), most UART hardware oversamples the incoming line, typically at 16x the configured baud rate:4

Oversampling Rate Trade-off
16x Standard choice on most MCU UART peripherals; good noise tolerance
8x Used when a higher baud rate is needed from a limited clock source; less tolerant of clock drift
4x or lower Rare, used only for very constrained timing budgets

Table 4. UART Oversampling Rates

Voltage Levels & Standards

The same frame format is used across several different electrical standards, which are not directly compatible with each other:5

Standard Voltage Levels Typical Use
TTL / CMOS UART 0V = low, 3.3V or 5V = high Chip-to-chip on the same board (MCU ↔ sensor, MCU ↔ MCU)
RS-232 ±3V to ±15V, inverted logic Legacy PC serial ports, some industrial/lab equipment
RS-485 Differential signaling, supports multi-drop Long-distance or noisy industrial links, allows multiple devices on one bus

Table 5. UART Electrical Standards

Never wire RS-232 directly to a 3.3V MCU pin

RS-232's voltage swing and inverted polarity can damage a TTL-level UART pin. A level shifter / converter IC (e.g. MAX3232) is required to bridge the two.

Flow Control

Flow control lets one side signal the other to pause, preventing buffer overruns when one device is slower than the other:6

Method Mechanism
None No flow control; the application must ensure the receiver can keep up
Hardware (RTS/CTS) Two extra wires — Request to Send / Clear to Send — let either side assert "not ready" at the electrical level
Software (XON/XOFF) Special in-band control bytes pause and resume transmission, at the cost of those byte values being unusable as data

Table 6. UART Flow Control Methods

FIFO Buffering

Most modern UART peripherals include a small hardware FIFO (First-In, First-Out) buffer on both TX and RX, reducing how often the CPU needs to service an interrupt for every single byte:7

Without FIFO With FIFO
One interrupt per byte — high CPU overhead at high baud rates One interrupt per several bytes (buffer threshold) — far less CPU overhead
Byte can be lost if the CPU is delayed even briefly Several bytes of headroom before data is lost

Table 7. FIFO vs. No FIFO

Common Error Conditions

Because UART has no shared clock and minimal error checking, a handful of error flags cover most real-world failure modes:2

Error Cause
Framing Error The stop bit wasn't where it was expected — usually a baud rate mismatch or line noise
Parity Error The received parity bit doesn't match the calculated parity of the data bits
Overrun Error A new byte (or FIFO) arrived before the CPU read the previous one, so data was lost
Break Condition The line is held low for longer than a full frame — often used deliberately as a reset/attention signal, but can also indicate a disconnected or misconfigured line

Table 8. Common UART Error Flags

Useful Resources

  1. Texas Instruments. (n.d.). UART Fundamentals. https://www.ti.com/lit/an/slaa066a/slaa066a.pdf ↩

  2. NXP Semiconductors. (n.d.). Understanding UART. https://www.nxp.com/docs/en/application-note/AN10369.pdf ↩↩

  3. Texas Instruments. (n.d.). UART Fundamentals. https://www.ti.com/lit/an/slaa066a/slaa066a.pdf ↩

  4. NXP Semiconductors. (n.d.). Understanding UART. https://www.nxp.com/docs/en/application-note/AN10369.pdf ↩

  5. Texas Instruments. (n.d.). UART Fundamentals. https://www.ti.com/lit/an/slaa066a/slaa066a.pdf ↩

  6. NXP Semiconductors. (n.d.). Understanding UART. https://www.nxp.com/docs/en/application-note/AN10369.pdf ↩

  7. Texas Instruments. (n.d.). UART Fundamentals. https://www.ti.com/lit/an/slaa066a/slaa066a.pdf ↩