Search › A-Z › S › STM32F103 Blue Pill
STM32F103 Blue Pill
2015 Open source · CC0 On-board only Online
STM32F103 Blue Pill is the ubiquitous $2 hobbyist board — an STMicroelectronics STM32F103C8 (a 72 MHz ARM Cortex-M3, ARMv7-M without an FPU, with 64 KB flash and 20 KB SRAM) on a small blue PCB — emulated from scratch in the browser on this site's shared ARM core. It runs REAL compiled firmware: a flat flash image with the ARM vector table at 0x08000000 boots exactly as on silicon (SP and PC loaded from the table), then drives the real STM32F103 registers. The two on-board peripherals a bare Blue Pill actually has are shown — the on-board LED on PC13 (active-low) and a USART1 serial console — with nothing invented: no breadboard, no fake audio (a bare Blue Pill has no speaker, so it is honestly silent). Because the whole chip is ordinary JavaScript, it plugs into this site's shared debugger: the real Thumb-2 disassembler, live r0-r15 / xPSR / N-Z-C-V, side-effect-free FLASH / SRAM / peripheral memory at the real addresses, single-instruction step, execution breakpoints and write-watchpoints.
STM32F103C8 at STMicroelectronics ↗
Runs on: Web browser
STM32F103 Blue Pill Online Emulator
Play STM32F103 Blue Pill using JavaScript directly in your browser.
Controls
Runs real compiled STM32F103 "Blue Pill" firmware on an in-browser STMicroelectronics STM32F103 (ARM Cortex-M3, 72 MHz, no FPU). The default is "Blink PC13": a bare-metal C program that enables the GPIOC clock in RCC, configures PC13 as an output and blinks the on-board LED (PC13 is active-low, so the LED lights when the pin is driven low). "Timer Blink (TIM2)" blinks the same LED but times it with the TIM2 general-purpose timer instead of a busy loop — it starts TIM2 and polls the update flag (SR.UIF). "USART Hello" brings up USART1 (TX on PA9) and prints a greeting to the serial console while blinking the LED each line; "USART Counter" prints an incrementing counter over USART1. All four are original CC0 programs compiled with clang for thumbv7m. Open the debugger, or add ?debug, to single-STEP real Thumb instructions from flash at 0x08000000, set PC breakpoints and write watchpoints, and read the ARM registers (r0-r15, xPSR, N/Z/C/V) and the real STM32F103 memory map live (FLASH 0x08000000, SRAM 0x20000000, peripherals 0x40000000). A bare Blue Pill has no user input wired to the firmware and no on-board speaker, so it is honestly silent with no input pad.
Runs real compiled STM32F103 "Blue Pill" firmware on an in-browser STMicroelectronics STM32F103 (ARM Cortex-M3, 72 MHz, no FPU). The default is "Blink PC13": a bare-metal C program that enables the GPIOC clock in RCC, configures PC13 as an output and blinks the on-board LED (PC13 is active-low, so the LED lights when the pin is driven low). "Timer Blink (TIM2)" blinks the same LED but times it with the TIM2 general-purpose timer instead of a busy loop — it starts TIM2 and polls the update flag (SR.UIF). "USART Hello" brings up USART1 (TX on PA9) and prints a greeting to the serial console while blinking the LED each line; "USART Counter" prints an incrementing counter over USART1. All four are original CC0 programs compiled with clang for thumbv7m. Open the debugger, or add ?debug, to single-STEP real Thumb instructions from flash at 0x08000000, set PC breakpoints and write watchpoints, and read the ARM registers (r0-r15, xPSR, N/Z/C/V) and the real STM32F103 memory map live (FLASH 0x08000000, SRAM 0x20000000, peripherals 0x40000000). A bare Blue Pill has no user input wired to the firmware and no on-board speaker, so it is honestly silent with no input pad.
Runs real compiled STM32F103 "Blue Pill" firmware on an in-browser STMicroelectronics STM32F103 (ARM Cortex-M3, 72 MHz, no FPU). The default is "Blink PC13": a bare-metal C program that enables the GPIOC clock in RCC, configures PC13 as an output and blinks the on-board LED (PC13 is active-low, so the LED lights when the pin is driven low). "Timer Blink (TIM2)" blinks the same LED but times it with the TIM2 general-purpose timer instead of a busy loop — it starts TIM2 and polls the update flag (SR.UIF). "USART Hello" brings up USART1 (TX on PA9) and prints a greeting to the serial console while blinking the LED each line; "USART Counter" prints an incrementing counter over USART1. All four are original CC0 programs compiled with clang for thumbv7m. Open the debugger, or add ?debug, to single-STEP real Thumb instructions from flash at 0x08000000, set PC breakpoints and write watchpoints, and read the ARM registers (r0-r15, xPSR, N/Z/C/V) and the real STM32F103 memory map live (FLASH 0x08000000, SRAM 0x20000000, peripherals 0x40000000). A bare Blue Pill has no user input wired to the firmware and no on-board speaker, so it is honestly silent with no input pad.
Runs real compiled STM32F103 "Blue Pill" firmware on an in-browser STMicroelectronics STM32F103 (ARM Cortex-M3, 72 MHz, no FPU). The default is "Blink PC13": a bare-metal C program that enables the GPIOC clock in RCC, configures PC13 as an output and blinks the on-board LED (PC13 is active-low, so the LED lights when the pin is driven low). "Timer Blink (TIM2)" blinks the same LED but times it with the TIM2 general-purpose timer instead of a busy loop — it starts TIM2 and polls the update flag (SR.UIF). "USART Hello" brings up USART1 (TX on PA9) and prints a greeting to the serial console while blinking the LED each line; "USART Counter" prints an incrementing counter over USART1. All four are original CC0 programs compiled with clang for thumbv7m. Open the debugger, or add ?debug, to single-STEP real Thumb instructions from flash at 0x08000000, set PC breakpoints and write watchpoints, and read the ARM registers (r0-r15, xPSR, N/Z/C/V) and the real STM32F103 memory map live (FLASH 0x08000000, SRAM 0x20000000, peripherals 0x40000000). A bare Blue Pill has no user input wired to the firmware and no on-board speaker, so it is honestly silent with no input pad.
Configurations
| Configuration | Emulator | Machine | OS | Legal | |
|---|---|---|---|---|---|
| Blink PC13 (on-board LED) | STM32F103 Blue Pill | STM32F103 Blue Pill | open | Open ⛶ | |
| Timer Blink (TIM2) | STM32F103 Blue Pill | STM32F103 Blue Pill | open | Open ⛶ | |
| USART Hello | STM32F103 Blue Pill | STM32F103 Blue Pill | open | Open ⛶ | |
| USART Counter | STM32F103 Blue Pill | STM32F103 Blue Pill | open | Open ⛶ |
Chips
Notes
Embedding
This is a two-layer stack: a real ARM CPU core underneath, and a from-scratch STM32F103 SoC + Blue Pill board on top. It loads real Cortex-M3 flash images and runs them on the actual on-chip peripherals — RCC clock gating, the GPIO ports, USART1 and the TIM2/TIM3 timers — at their real addresses. The firmware is genuine compiled machine code, not a shim.
- The CPU.
cortex-m7.jsexecutes Thumb-2. The real chip is a Cortex-M3 (ARMv7-M without the FPU); this core is ARMv7-M, so it runs the M3 image unchanged — an M3 image simply never emits VFP. - Real reset. The image is a real flash image with the ARM vector table at
0x08000000: on reset the board loadsSPfrom word 0 andPCfrom word 1, exactly like the silicon. - The board, in JavaScript.
bluepill-board.jsmaps FLASH/SRAM/peripherals at their real STM32F103 addresses, answers the RCC clock-ready bits the firmware spins on, folds GPIOBSRR/BRRintoODR, captures every byte written toUSART1->DRinto the serial console, and counts TIM2/TIM3.
| Piece | Kind | What it does |
|---|---|---|
BluePill.create(canvas) | factory | Build the ARM core + the STM32F103 memory map + peripherals. |
load(bytes) / reset() | method | Copy the flat image into FLASH; reset loads SP/PC from the vector table at 0x08000000. |
runFrame() | method | Step ~60k instructions and advance the TIM2/TIM3 timers. |
ledOn() / getSerial() | method | The on-board LED (PC13, active-low) state and the accumulated USART1 output. |
present() | method | Draw the on-board LED indicator and the serial console to the canvas. |
Debugger integration
bluepill-debug.js reads window.EMU_BOOT and hands the shared debugger a genuine ARM Cortex-M3 machine — the same core the firmware runs on, exposed instruction-accurately.
- Registers.
registers()reads r0-r12, SP, LR, PC, xPSR and the APSR flags N/Z/C/V live each refresh; each has aset()that writes straight back into the core's register file. - Disassembly. FLASH is decoded with the shared
cortex-m7decoder (the Thumb-2 disassembler), so the code window shows real ARM mnemonics at the real reset address0x08000000. - Single-step. Step calls
transport.stepInsn, which advances the core exactly one Thumb instruction; PC and the registers update after each step. You can watch the reset handler copy.datato SRAM and fall intomain(). - Breakpoints & watchpoints. Execution breakpoints are a PC set the run-loop checks before each instruction; write watchpoints wrap the board's memory-write path and pause when a watched address is written (e.g.
GPIOC->ODRat0x4001100CorUSART1->DRat0x40013804). Both are real. - Memory map. Three chips read the real address space side-effect-free: FLASH (program, disassembled, 0x08000000), SRAM (0x20000000), and the peripheral block (0x40000000 — RCC, GPIOA-C, USART1, TIM2/3). Reading the dynamic registers (USART1 SR, GPIO IDR) is side-effect-free.
Architecture
The STM32F103 "Blue Pill" is the ubiquitous hobbyist board: a $2 STMicroelectronics STM32F103C8 — a 72 MHz ARM Cortex-M3 (ARMv7-M, no FPU) with 64 KB flash and 20 KB SRAM — on a small blue PCB with an on-board LED and a USB connector. This build models the chip from scratch on the shared ARM core.
- Cortex-M3 CPU — runs the real Thumb-2 image. FLASH is at
0x08000000(the vector table: word0 = initial SP, word1 = reset handler), SRAM at0x20000000, all peripherals at0x40000000. On reset SP and PC are loaded from the vector table, exactly as on silicon. - RCC (Reset & Clock Control,
0x40021000) — the firmware gates each peripheral's clock throughAPB1ENR/APB2ENRbefore touching it, and spins on the clock-ready bits (HSERDY,PLLRDY); the board answers those bits so real clock-setup code proceeds. - GPIO ports A/B/C — the STM32
CRL/CRHpin-config,ODRoutput data and the atomicBSRR/BRRset/reset registers. The on-board LED is on PC13 and is active-low: the board lights it whenGPIOC->ODRbit 13 is driven low. - USART1 (
0x40013800) — the serial port.SRkeepsTXE/TCasserted so polling transmit code runs, and every byte written toDRis captured onto the on-screen serial console (TX on PA9). - TIM2 / TIM3 — general-purpose 16-bit timers with prescaler (
PSC), auto-reload (ARR), counter (CNT) and the update flag (SR.UIF), advanced by executed instructions; the Timer Blink program times the LED off TIM2. SysTick and the NVIC come from the core. - Content. Four original CC0 bare-metal C programs compiled with clang for thumbv7m: Blink PC13, Timer Blink (TIM2), USART Hello and USART Counter. No proprietary firmware is used.
- Honest limits. A bare Blue Pill has no on-board audio (no speaker/DAC), so this emulator is genuinely silent with no sound path. It also has no user-input peripheral wired to the firmware (the LED is an output and the serial console is TX-only), so there is no input pad. USB, ADC, SPI/I2C and the interrupt-driven paths of the untouched peripherals are not modelled — the bundled firmware polls.