STM32L0LoraTx

Firmware and PCB for a device that sends data with a STM32L0/RFM95, battery or solar powered, to a LoraGW ESP8266/RFM95 node

Build

• Download and install STM32CubeIDE (as of now version 1.5.1)
• Import this repo
• Open .ioc file and select "Project/Generate Code" (should download needed "firmware" files for the chip from ST)
• Build Release or Debug

Wiring

connect Gnd of all components

STM32L011

Not checked without, but generally recommended to use a small ceramic (~10nF) and big electrolyt condensator (~10µF) between + and - near STM pins

• Look at the ioc file with CubeMX pinout view.
• Vdd and Vdda to 1.8-3.3V
• Vss to Gnd
• Boot0 pulled to Gnd (~10k) to safely boot from flash
• STLink for flashing and debug: SWD, SWC, Gnd, NRst (also can provide Vdd via JP1 and STLink-Rx to L0-Tx for serial out)

BME280

• SDA=MOSI, SDC=MISO, others should be obvious

RFM95W

• Ant=~8cm wire (vary length and position to get best average RSSI as reported by LoraRx).
• NSS=CS, others should be obvious

Serial

Rx of a usb to serial converter sends at 115200. I use this command (serial port name ttyUSB_ser_red is defined in my /etc/udev/rules.d) pio device monitor -p /dev/ttyUSB_ser_red -b 115200

LED

can be used without resistor. I use 1kOhm to save battery

References

Good resource for STM32 is Stefan Frings He has links to reference, datasheet, errata and more. E.g.:

• STM32L011 Datasheet
• STM32L011 Reference
• STM32L011 Errata
• Power consumption

Power Info Bits

• Examples for LL are here: STM32Cube/Repository/STM32Cube_FW_L0_V1.11.2/Projects/NUCLEO-L073RZ/Examples_LL/RTC/RTC_ExitStandbyWithWakeUpTimer

• Periodic wakeup timer is in Reference 22.4.6 (Periodic auto-wakeup)

• 1s to 18h periodic wakeup with ck_spre = 1s (see 22.4.3) and WUCKSEL [2:1] = 10

• Enable by WUTE bit in RTC_CR register

• Once setup it runs in normal and low power.

• Sets WUTF flag in RTC_ISR if elapsed and reloads RTC_WUTR (timer counter). User resets WUTF

• Set WUTIE bit in RTC_CR to generate interrupt to wake up from low power modes

• System reset and Sleep, Stop, Standby have no influence on the WUT (wakeup timer)

• Save Lora frame counter in backup register: Reference 22.7.20 RTC backup registers (RTC_BKPxR)

• I couldnt make it work with LL lib, had to use big fat HAL for RCC and RTC. What is missing?

    LL_RTC_DisableWriteProtection(RTC);
    LL_RTC_WAKEUP_Disable(RTC);
    while( !LL_RTC_IsActiveFlag_WUTW(RTC) );
    LL_RTC_WAKEUP_SetClock(RTC, LL_RTC_WAKEUPCLOCK_CKSPRE);
    LL_RTC_WAKEUP_SetAutoReload(RTC, 3); // wake up every 3+1 s
    LL_RTC_EnableIT_WUT(RTC);
    LL_RTC_WAKEUP_Enable(RTC);
    LL_RTC_EnableWriteProtection(RTC);
    while( LL_RTC_IsActiveFlag_WUTW(RTC) );
    LL_RTC_ClearFlag_WUT(RTC);
    NVIC_ClearPendingIRQ(RTC_IRQn);
  

Status

Working and sending LoRa packets to my LoraGW ESP8266 gateway (battery voltages and bme280 weather data)

Two devices operating with 2 AA 1.5V batteries (so far using 0.2-0.4 V per year, so >5 years between battery swaps is more than realistic). They send battery voltage data from within a car trunc - not ideal for RF :) - to a nearby house with no signal problems.

One device runs "forever" with a 3.7 Lipo ~1.6Ah, a ~3x8 cm² 5V solar cell and a small solar charger board (reducing vcc out a bit with a diode). No capacity problems during winter.

Todo

• Provide more sensor data (probably more ADC stuff and more generic)
• Move GPIO pins (CS, LED) to non-ADC pins to have them available for voltage measurements