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How serial actually works

The fixes tell you what to change. These tell you why it works that way. Each one pairs with something in the lab you can go and poke at.

Learn by doingThe Lab →Interactive tools: build a UART frame, see baud error, budget the power.Write-upsThe Blog →Builds, fixes and the stories behind them.

A data line paired with a clock line on the right; on the left the same data line alone, with empty space where the clock would be

Why does serial have no clock line? The wire it saves, and the price you pay

SPI and I2C send a clock. UART does not, and that single missing wire explains baud rates, start bits, and why two devices must agree in advance.

Pairs with What 8N1 actually means

A single serial frame drawn as a square wave, with the opening falling edge and the closing high segment picked out brighter than the data bits between them

What are start and stop bits for? Two bits that carry no data and cannot be removed

The start bit and stop bit carry nothing and cost you 20% of your bandwidth. Here is the job each one does, and why removing either breaks the link.

Pairs with What 8N1 actually means

A descending ladder of line segments, each exactly half the length of the one above it

Why is 9600 the default baud rate? A crystal, a teleprinter, and a lot of doubling

9600 is not a round number and not a fast one. It survives because of one crystal frequency that divides exactly into it, and a chain of doublings from 1920.

Pairs with Baud mismatch playground

A square wave with sample ticks beneath it, correctly centred at the left and drifting progressively onto the pulse edges toward the right

How much baud rate error is too much? The real number is 5.26%, not 2%

The famous 2% baud tolerance is a convention, not a specification. Here is where the real ceiling comes from, why it is 5.26%, and why the stop bit breaks first.

Pairs with How close is close enough?

A single narrow pipe feeding several outlets, with the pipe's capacity marked and the outlets' combined demand exceeding it

How much current can an Arduino pin actually supply? The number that decides most Bluetooth failures

The Uno's 3.3V pin gives 50 mA in total, not per device. That one number, and the word total, explain most modules that never appear in a phone's Bluetooth list.

Pairs with Will it brown out?

A voltage line dipping below a marked threshold and recovering, repeatedly, forming a sawtooth

What a brown-out actually is, and why it looks exactly like a dead component

A supply sagging below what a part needs, so it resets. No smoke, no heat, no error — which is why a brown-out gets diagnosed as a dead module.

Pairs with Will it brown out?

A small radio module mounted on a larger breakout board, with the board's regulator highlighted between the input pin and the module

Why your Bluetooth breakout has its own regulator — and why 5V is not overvolting it

The bare radio runs at 3.3V, but the board it is mounted on does not. That distinction is why feeding an HC-05 5V is using it as designed, not abusing it.

Pairs with Will it brown out?

Two square waves carrying the same data, the upper one perfectly even and unbroken, the lower one with uneven pulse widths and short gaps

SoftwareSerial vs hardware serial — one is a peripheral, the other is your CPU pretending

SoftwareSerial bit-bangs a UART on the CPU, blocks interrupts while it sends, and only one instance can listen. Here is when that matters and what to use instead.

Three waveforms with the same rhythm: a small square wave, a taller inverted one, and a smooth twisted pair

TTL vs RS-232 vs USB serial — same bytes, three completely different wires

All three are called serial and only one is a UART. The framing is identical, the voltages are not, and connecting the wrong two can destroy a pin.

A table of five board families with the pins a Bluetooth module goes on: Uno and Nano on 10 and 11 with SoftwareSerial, Mega on 19 and 18 with Serial1, Leonardo, Micro, Uno R4 and Nano Every on 0 and 1 with Serial1, ESP32 on any free GPIO with Serial2

Which pins for a Bluetooth module on each board — Uno, Nano, Mega, Leonardo, Uno R4 and ESP32

Where an HC-05, HC-06 or HM-10 goes on an Uno, Nano, Mega, Leonardo, Uno R4, Nano Every or ESP32, which serial port to use, and which pins cannot receive.

Stuck on a specific symptom right now? The fixes index maps it to the answer. These pages are for afterwards, when you want to know why.