STM8 Low Density Devices
STM8 Low Density devices are among the cheapest microcontrollers available that are powerful enough to host an interactive programming environment. They can be divided in Standard (S) and Low Power (L) devices, and both families have at least two major variants.
In the STM8S family there are STM8Sx03 "low density", STM8Sx05 "medium density", and STM8Sx07 "high density" devices which are presented as product lines:
- x=0: "Value Line"
- x=1: "Access Line"
- x=2: "Performance Line"
- x=9: "Application Specific Line"
The datasheets of the "Value Line" devices sometimes hide the fact that they are rebranded "low density", "medium density" or "high density" chips.
STM8S "low density" devices in the appear to consist of two consist mainly of two different families:
| Family | Other labels |
|---|---|
| STM8S103 | STM8S003 |
| STM8AF6223 | STMS903, STM8S001J3M3, STM8AF6226 |
Both families have a very similar set of peripherals, but the STM8AF6223 family is more versatile, as we'll see below. Likewise, "medium density" and "high density" devices have industrial and automotive variants that sometime have hidden features worth exploring.
STM8S103 is an industrial (i.e. not automotive) STM8S "low density" device.
A comparison of datasheets between the "Value Line" STM8S003F3P6 and the "Access Line" STM8S103F3P6 suggests that it's basically the same silicon with a different specification (e.g. number of Flash write cycles), and with a specified EEPROM size (Value Line devices specify 128 bytes but tests show that up to 640 bytes may be available).
A new STM8S low-density errata sheet was published, the "STM8S001J3, STM8S003xx, STM8S103xx and STM8S903xx device limitations", which means that all these chips are at least "very similar".
In the STM8S001J3 datasheet there is a bit of information that's not in the STM8S003 datasheet:
Note: The PA2, PB0, PB1, PB2, PB3, PB6, PB7, PC1, PC2, PC7, PD0, PD2, PD4, PD7, PE5 and PF4 GPIOs should be configured after device reset in output push-pull mode with output low-state to reduce the device’s consumption and to improve its EMC immunity. The GPIOs mentioned above are not connected to pins, and they are in input-floating mode after a device reset.
Only 32 pin devices provide the GPIOs mentioned in the note (e.g. LQFP32 STM8S003K3). The advice is thus also applicable to STM8S003F3 (i.e. PB1, PB2, PB3, PB6, PB7, PC1, PC2, PD0, PD7, PE5 and PF4).
The latest member of the STM8S00x "Value Line" is the STM8S001J3 in a SO8N package, which has about the same dimensions as the TSSOP20 but, due to the reduced pin count, is very easy to solder.
A note in the STM8S001J3 datasheet states that the STM8S001 is a member of the "low density family", and it's not a special "stripped down" silicon with (curiously) more alternative pin functions:
Note: As several pins provide a connection to multiple GPIOs, the mode selection for any of those GPIOs impacts all the other GPIOs connected to the same pin. The user is responsible for the proper setting of the GPIO modes in order to avoid conflicts between GPIOs bonded to the same pin (including their alternate functions). For example, pull-up enabled on PD1 is also seen on PC6, PD3 and PD5. Push-pull configuration of PC3 is also seen on PC4 and PC5, etc.
This means that the SO8N package contains the same chip as a device with more pins (i.e. the "Application Specific Line" device STM8S903F3 in a TSSOP20 package). It should be possible to combine up to 4 GPIOs (pin 8) for push-pull of up to 80mA load within the specification (pin 8).
The following GPIOs/features are available:
| Pin | GPIO | Features |
|---|---|---|
| 1 | PA1 | OSC_IN |
| 1 | PD6 | UART1_RX, AIN6 |
| 5 | PA3 | TIM2_CH3, [SPI_NSS], [UART1_TX] |
| 5 | PB5 | I2C_SDA |
| 6 | PB4 | I2C_SCL, [ADC_ETR] |
| 7 | PC3 | TIM1_CH3, [TLI], [TIM1_CH1N] |
| 7 | PC4 | TIM1_CH4, CLK_CCO, AIN2, [TIM1_CH3] |
| 7 | PC5 | SPI_SCK, [TIM2_CH1] |
| 8 | PC6 | SPI_MOSI, [TIM1_CH1] |
| 8 | PD1 | SWIM |
| 8 | PD3 | TIM2_CH2, TIM2_CH2, ADC_ETR |
| 8 | PD5 | UART1_TX, AIN5 |
The Alternate Function options if the STM8S001J3 are identical with the STM8S903. Compared to the STM8S003 low density devices there are the following additional features:
- UART1_TX can be mapped to GPIO PA3
- Timer synchronization/chaining features (TIM5 and TIM6 instead of TIM2 and TIM4)
- AIN6 at GPIO PD6 (the STM8S903 has 6 analog inputs)
- AIN7 is connected to an internal bandgap reference
The timer and the bandgap reference features are undocumented. A test (7 ADC! ADC@ .) with a production sample results in 400 at 3.3V supply.
When comparing the description of UART1 LIN features between the STM8AF6223 and Table 52 of the STM8S Reference Manual RM0016 it's a curious fact that "NA" doesn't mean "Not Available" but "Not Applicable". It's likely that there are two styles of UART1, and that STM8S903 and STM8S001 devices indeed support LIN Slave Mode (i.e. automatic synchronization and baudrate measurement).
Whoever assigned the GPIOs to SO8 pins must have a particular set of requirements. A final assessment if the solution we now see should also look at constraints, like the position of bond pads on the die that may have caused some "non features" and restrictions of usability:
- when UART1_TX is mapped to GPIO PA3, UART1_RX is hidden (mapped to GPIO PF4, not connected)
- high risk of lock-out when using a full-duplex UART (why share PD1 pin with PD5, not PD6?)
- no SPI MISO - the SPI is limited to the rarely used half-duplex mode
STM8 Low Density devices consist of two distinct families:
- STM8L101, including STM8L001J3M3 (SO8)
- STM8L05x, including STM8L050J3M3 (SO8)
The STM8L101 family has the lowest power consumption, and 1.5K instead of only 1K RAM but it lacks RTC and ADC and there are important differences and limitations with respect to non-volatile memory (e.g. Flash and EEPROM are unified) and IAP (in application programming). It certainly is the right device for power critical applications that don't Right now no STM8 eForth support is planned.
The STM8L051 family behaves much more similar to STM8S devices. While it's often superior (e.g. RTC, 12 Bit ADC, Vcap not required), the peripherals sometimes show important changes (GPIO interrupts) and it's absurdly difficult to spot differences and commonalities (one should think that it's easy for a company like ST to provide a comparison...).