ST's ultra-low-cost 8-bit microcontroller, an STM8 core in TSSOP-20 that shows up in countless price-sensitive gadgets. It runs at up to 16 MHz with 8 KB of flash, 1 KB of RAM, and 128 bytes of true data EEPROM (rated 100k write cycles), and brings out 16 GPIO plus a 10-bit ADC, three timers, and hardware UART, SPI, and I2C. It needs no external crystal (internal oscillator) and programs over the single-wire SWIM interface (pin 18). Pin-compatible with the STM8S103F3, it is the default choice when a design needs a real MCU for pennies. Datasheet-verified 20-pin pinout.

Pinout

Pin	Name	Type	Function
1	PD4/BEEP/TIM2_CH1/UART1_CK	bidirectional	Port D bit 4. High-sink GPIO (fast/slow programmable, floating after reset). Muxed default alternate functions: TIM2 channel 1, BEEP output, and UART1 synchronous clock.
2	PD5/AIN5/UART1_TX	bidirectional	Port D bit 5. High-sink GPIO. Muxed with ADC analog input 5 and UART1 data transmit.
3	PD6/AIN6/UART1_RX	bidirectional	Port D bit 6. High-sink GPIO. Muxed with ADC analog input 6 and UART1 data receive.
4	NRST	bidirectional	Active-low bidirectional reset with integrated weak pull-up. Asserted low by the internal POR/BOR, watchdog or an external reset source; can also be driven low externally to reset the device.
5	PA1/OSCIN	bidirectional	Port A bit 1 (slow O1 output, floating after reset). Doubles as the HSE crystal/resonator input (OSCIN) or external clock input.
6	PA2/OSCOUT	bidirectional	Port A bit 2 (slow O1 output, floating after reset). Doubles as the HSE crystal/resonator output (OSCOUT).
7	VSS	power_in	Digital ground return for VDD.
8	VCAP	passive	Output of the internal 1.8 V core regulator. Requires an external stabilization capacitor to VSS (see Section 9.3.1 VCAP external capacitor). Do not drive externally.
9	VDD	power_in	Main digital power supply, 2.95 V to 5.5 V operating range. Decouple locally to VSS.
10	PA3/TIM2_CH3/[SPI_NSS]	bidirectional	Port A bit 3. High-sink GPIO. Default alternate function: TIM2 channel 3. When alternate-function remap bit AFR1 is programmed the pin instead becomes the SPI master/slave-select input (SPI_NSS).
11	PB5/I2C_SDA/[TIM1_BKIN]	bidirectional	Port B bit 5. True open-drain output (no P-buffer, no weak pull-up, no protection diode to VDD); an external pull-up is required. Default alternate function: I2C SDA data line. When AFR4 is programmed the pin becomes TIM1 break input (TIM1_BKIN).
12	PB4/I2C_SCL/[ADC_ETR]	bidirectional	Port B bit 4. True open-drain output (no P-buffer, no weak pull-up, no protection diode to VDD); external pull-up required. Default alternate function: I2C SCL clock. When AFR4 is programmed the pin becomes ADC external trigger (ADC_ETR).
13	PC3/TIM1_CH3/[TLI]/[TIM1_CH1N]	bidirectional	Port C bit 3. High-sink GPIO. Default alternate function: TIM1 channel 3. Remap options: top-level interrupt (TLI) via AFR3, or TIM1 complementary channel 1 (TIM1_CH1N) via AFR7.
14	PC4/CLK_CCO/TIM1_CH4/AIN2/[TIM1_CH2N]	bidirectional	Port C bit 4. High-sink GPIO. Default alternate functions: configurable clock output (CCO), TIM1 channel 4, and ADC analog input 2. When AFR7 is programmed the pin becomes TIM1 complementary channel 2 (TIM1_CH2N).
15	PC5/SPI_SCK/[TIM2_CH1]	bidirectional	Port C bit 5. High-sink GPIO. Default alternate function: SPI clock (SCK). When AFR0 is programmed the pin becomes TIM2 channel 1.
16	PC6/SPI_MOSI/[TIM1_CH1]	bidirectional	Port C bit 6. High-sink GPIO. Default alternate function: SPI master-out/slave-in (MOSI). When AFR0 is programmed the pin becomes TIM1 channel 1.
17	PC7/SPI_MISO/[TIM1_CH2]	bidirectional	Port C bit 7. High-sink GPIO. Default alternate function: SPI master-in/slave-out (MISO). When AFR0 is programmed the pin becomes TIM1 channel 2.
18	PD1/SWIM	bidirectional	Port D bit 1. High-sink GPIO with fast/slow programmability (O4). SWIM single-wire debug and in-circuit programming data pin. In input pull-up during reset phase and after internal reset release to allow SWIM entry.
19	PD2/AIN3/[TIM2_CH3]	bidirectional	Port D bit 2. High-sink GPIO. Default alternate function: ADC analog input 3. When AFR1 is programmed the pin becomes TIM2 channel 3.
20	PD3/AIN4/TIM2_CH2/ADC_ETR	bidirectional	Port D bit 3. High-sink GPIO. Default alternate functions: ADC analog input 4, TIM2 channel 2, and ADC external trigger (ADC_ETR).

Key specifications

Parameter	Value
Core	8-bit STM8
Max clock	16 MHz
Flash	8 KB
RAM	1 KB
EEPROM	128 B (100k cycles)
GPIO	16
ADC	10-bit (5 ch)
Serial	UART + SPI + I2C
Supply voltage	2.95-5.5 V
Package	TSSOP-20

Ordering variants

Same part, different packaging or reel option.

STM8S003F3P6 (TSSOP-20)
STM8S003F3U6 (UFQFPN-20)

CAD (KiCad official)

Symbol: MCU_ST_STM8:STM8S003F3P
Footprint: Package_SO:TSSOP-20_4.4x6.5mm_P0.65mm
3D model: TSSOP-20 (KiCad packages3D)

Provenance

Data extracted from the manufacturer datasheet, not a distributor listing:

Datasheet: STMicroelectronics datasheet (103 pp, sha256 1a0775fefcda1553, retrieved 2026-08-17, tier: manufacturer)
Extractor: ds2sf (Claude). Every pin and dimension is cited to a datasheet page; it refuses to fabricate.
CAD: KiCad official libraries (CC-BY-SA 4.0).

Datasheet PDF