Introduction

An alternative firmware for 433 MHz radio to wifi bridges, targetting these boards/microcontrollers:

These microcontrollers were originally intended for radio decoding but lacked support for additional protocols.
As a consequence many people just bypass the microcontroller with a hardware modification:
https://github.com/arendst/Tasmota/discussions/13283

Decoding may then be done directly on the ESP8265 (e.g., ESPHome/Tasmota/ESPurna typically used for this purpose).

The intent here is to avoid the need to perform hardware modification.
This can be accomplished by either on of two ways:

1) Mirroring the voltage levels on the radio data pins over to uart pins (used as gpio) already connected to ESP8265.
(this uses the microcontroller as a passthrough but using software instead of hardware modification)

2) Decoding/encoding radio packets on the microcontroller and sending formatted to ESP8265 over uart/serial
(similar to factory firmware but with ability for source code protocols to be added, behavior modified, etc.)

A benefit to keeping the microcontroller involved is that the radio LED and buzzer may still be controlled.
The downside is the effort required to develop firmware and flash by the end user.

The firmware radio decoding is inspired by 1) RF-Bridge-EFM8BB1 (Portisch) and 2) rc-switch projects.

Status

THIS IS A WORK IN PROGRESS and requires multiple flashing steps.

For many users it is probably easiest to flash the passthrough mode.
Then perform any actual decoding/encoding on the ESP8265 using Tasmota/ESPHome.

Building the firmware

if you dont wish to use the pre-build from releases, Install SDCC compiler for your platform:
https://sdcc.sourceforge.net/

On command line run make.
For now modify Makefile to select desired target.
Built firmware placed in build directory.
See Flasher section below.

Flashing

OB38S003 reprogamming requires erasing the radio chip because the stock firmware is protected.
The stock firmware cannot be recovered because it has not been read out.

Steps overview:

1. Erase the RFbridge oboard ESP8265 (to ensure no interference from the ESP8265 with radio chip's serial lines). Important if you're coming from stock RFbridge (no tasmota/ESPHome).
2. Have an external flasher board prepared (see below). Could be a D1 Mini, NodeMCU, or an arduino, or an official flasher.
3. Connect the external flasher pins to the OB38S003 pins on the bridge (SCL<->SCL, SDA<->SDA, GND<->GND, 3.3V<->3.3V). See ESP8266 pinout and OBS38S003 pinout
4. Download the firmware wanted and place in the same directory as flashscript.py
5. Run FlashScript.py which will erase the OB38S003 and write the firmware you choose.
6. Flash ESPhome/Tasmota to the RFbridge's internal ESP8265.

Note: Some users (especially when flashing passthrough firmware) has experienced inteference between the onboard ESP8265 and OB38S003 while others did not and were able to erase and update the RF chip many times without erasing and having to reflash the ESP8265 with Tasmota/ESPhome. So rules of thumb are 1) Coming from stock: Erase ESP8265 first, flash OB38S003, then flash ESP8265. 2) Coming from esphome/tasmota: flash OB38S003 only. No need to reflash the ESP8265 3) Flashing passthrough firmware: Erase ESP8265 first, flash OB38S003, then flash ESP8265.

Detailed flashing guide

Flasher (official)

An official MSM9066 programmer or open source flasher (see below) can be used.

EFM8BB1 reprogramming can also be done with Tasmota using the RFbridge's own internal ESP8265
The module must be set as Sonoff Bridge (25) to allow flashing:
https://tasmota.github.io/docs/Modules/
Follow the instructions for flashing Portisch but using this firmware:
https://tasmota.github.io/docs/devices/Sonoff-RF-Bridge-433/

Flasher (open source) (Recommended)

An Arduino based flasher written for the built in ESP8265/ESP8266/ESP32 is available:
https://github.com/mightymos/OnbrightFlasher

Logic analyzer decodings of several flasher operations/programming cycles had been captured:
https://github.com/mightymos/msm9066_capture

While cumbersome to use, several people have successfully flashed hex files.

Tasmota/ESPHome

On Sonoff RF bridges, the radio chip is flashed independently of the ESP8256 chip. Flashing the ESP8256 with Tasmota/ESPHome can be done before or after flashing the radio chip, although some users were only successful by erasing the ESP8266 first, flashing the radio chip, then flashing ESPHome/Tasmota on to the ESP8256.

The configuration of ESPHome/Tasmota depends on whether passthrough or RCswitch/Portisch firmwares are used

Using passthrough hex file:

The following pins can be used in ESPHome / Tasmota:
-flash tasmota-sensors.bin to ESP on RFbridge
-configure module as e.g. Generic(0))
-save

now set pins to:
TX pin to GPIO1 (RFSend)
RX pin to GPIO3 (RFRecv)
WIFI LED pin to GPIO13 (LedLink)

Using RCswitch or Portisch file:

See example YAML configs (https://github.com/mightymos/RF-Bridge-OB38S003/tree/main/example_esphome_yaml)

Previous Work

A successful attempt was made to compile "Portisch" with the open source SDCC compiler.
It is difficult to fit all decoding protocols and sniffing features in code and ram spaces.
This effort allowed porting to the R2 V2.2 Sonoff (white box).
https://github.com/mightymos/SonOfPortisch

There are at least three versions of rcswitch.
We attempt to use the simplest and most understandable for now (from sui77).
https://github.com/sui77/rc-switch
https://github.com/1technophile/rc-switch
https://github.com/arendst/Tasmota/tree/development/lib/lib_rf/rc-switch

Special Thanks

We thank Vincent Defert for the use of modified makefiles and familiarity with 8051/8052 based microcontrollers:
https://github.com/area-8051/uni-STC