[STM32 + Arduino] Serial Link
[STM32 + Arduino] Serial Link is a runnable microcontroller simulation example on SensBench — open the wired circuit, inspect the code, and compile real firmware in your browser, no install needed. Last updated Aug 14, 2026.
FREE SIMULATOR EXAMPLE · INTERMEDIATE
[STM32 + Arduino] Serial Link
This example sends PING n messages from an STM32 Blue Pill’s USART1 (PA9 TX) to an Arduino Uno’s serial input (pin 0 RX) at 9600 baud — a two-board serial link over one wire plus ground. It is the minimal cross-board UART demo.
| Board | STM32 Blue Pill (STM32F103C8, Cortex-M3, 72 MHz) |
|---|---|
| Difficulty | intermediate |
| Category | communication |
| Libraries | None — just the Arduino core |
| Runs in | Sensbench browser simulator — free, no account |
The exact circuit you get when you open this example — already wired, ready to run.
The sketch
// STM32 Blue Pill — sends PING over USART1 (PA9 = TX).
// Wiring: STM32 PA9 -> Uno pin 0 (RX), STM32 GND -> Uno GND.
unsigned long n = 0;
void setup() {
Serial.begin(9600); // USART1
pinMode(PC13, OUTPUT);
}
void loop() {
n++;
Serial.print("PING ");
Serial.println(n);
digitalWrite(PC13, n % 2 ? HIGH : LOW);
delay(1000);
}How it works
The Blue Pill’s USART1 TX lives on PA9, and this sketch sends PING n every second at 9600 baud — deliberately slower than the 115200 used elsewhere because the 5 V-logic Uno receiver and breadboard wiring favor the forgiving rate. The PC13 LED toggles with the counter as a heartbeat.
The wiring is STM32 PA9 (TX) to Uno pin 0 (RX), with both grounds tied. Serial is point-to-point here: TX on one board, RX on the other, no sharing. The 1-second cadence keeps the link readable and leaves the CPU nearly idle — a pattern that scales to longer telemetry payloads.
Before you build it for real
Serial links have three classic failure modes, all hidden by the simulator: miswired TX/RX (TX must go to RX, never TX-to-TX), missing common ground (the level has no reference), and baud mismatch (both ends must agree exactly — 9600 vs 115200 looks like garbage). The 3.3 V TX from the STM32 reads as a valid HIGH on the Uno’s 5 V UART receiver, which usually works, but a true bidirectional link needs level shifting. And Serial.print() at 9600 baud is slow: each character takes ~1 ms on the wire, so a long burst can stall the loop if you forget the pacing.
Get the parts
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FAQ
Why 9600 baud instead of 115200?
The Uno’s hardware serial is reliable at 9600 baud on its 16 MHz clock; 115200 works on AVR but leaves less timing margin, and cross-board links over breadboard wires prefer the slower, more forgiving speed. Both boards must agree exactly — a one-baud mismatch corrupts every byte.
What does the Uno do with the PING messages?
The Uno side in this example is the receiver: its pin 0 (RX) listens to the STM32’s USART1 TX. The data crosses the wire at 9600 baud and appears in whatever sketch the Uno runs — the demo’s point is the link itself, not the payload.