SENTToUSB — STM32F042 USB ↔ SENT Bridge

USB CDC (Virtual COM Port) adapter for SAE J2716 SENT sensors. Talks to the host using the SLCAN text protocol over any serial terminal.

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Hardware

MCU	STM32F042G4UX (UFQFPN28, 48 MHz HSI48, no crystal)
PA2	SENT RX — TIM2 CH3 input capture, internal pull-up
PA4	SENT TX — GPIO push-pull driven by TIM14
USB	Full-speed CDC (Virtual COM Port, no driver needed on Win10+)

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How RX works

Sensor ──SENT signal──► PA2 (TIM2 CH3, RISING edge capture)
                              │
                    ISR: timestamp stored in RX HAL ring buffer
                              │
                    Main loop: bridge decodes batch of 10 edges
                              │
                    SLCAN 't' frame → USB → host

1. Host sends O\r — bridge enters RX mode.
2. Every RISING edge on PA2 triggers the TIM2 CH3 capture ISR, which records the raw 16-bit counter value. A separate overflow ISR extends timestamps across the 16-bit rollover (~1.4 ms at 48 MHz).
3. SentApp_Process() (main loop) polls the RX HAL for complete batches of 10 edges: sync + status + 6 data nibbles + CRC + leading edge of next interval.
4. The bridge converts timestamps to µs intervals, extracts nibbles, validates CRC, and produces a CAN frame.
5. The frame is serialised as t<ID><DLC><data>\r and flushed to USB.

Default config (MLX90377): 3 µs/tick, 6 nibbles, DATA_ONLY CRC, seed 0x03, output CAN ID 0x510.

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How TX works

Host ──SLCAN 't' frame──► USB CDC RX callback (ISR context)
                              │
                    bridge → TX HAL queue
                    tim14_kick(): start TIM14 if idle
                              │
              TIM14 ISR fires once per half-interval (3 µs/tick):
                Phase 0: pop next interval → PA4 LOW for 15 µs
                Phase 1: PA4 HIGH for remaining ticks
                Repeat until queue empty → PA4 idle HIGH, TIM14 stops

• TIM14 prescaler 143 → 1 tick = 3 µs (1 SENT tick at standard rate).
• Each SENT interval = 15 µs active-LOW pulse + HIGH for (N − 5) ticks.
• The bridge builds the full interval sequence (sync 56T + status + nibbles + CRC + 12T pause) and pushes it to the TX HAL. The ISR drains it without any main-loop involvement.

Switch to TX mode and send one frame:

O\r                       open SLCAN channel
t600102\r                 start TX mode
t52050100123456\r         transmit: status=1, nibbles [1,2,3,4,5,6]

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SLCAN quick reference

Command	Effect
O\r	Open — start SENT RX
C\r	Close — stop
V\r / v\r	Hardware / firmware version
N\r	Serial number (unique per MCU, XOR-fold of 96-bit UID)
F\r	Status flags
t<ID><DLC><data>\r	Send CAN frame

Control frames (CAN ID 0x600, DLC = 1):

data[0]	Action
01	Start RX
02	Start TX
03	Stop
04	Learn tick period / nibble count / CRC mode from live signal

TX data frame (CAN ID 0x520):

t52050100123456\r
      ↑↑ ↑
      ││ └─ status byte then data nibbles packed (status=0x01, nibbles=0x00,0x12,0x34,0x56)
      │└─── DLC = 5
      └──── CAN ID 0x520

Decoded RX frame (device → host, default CAN ID 0x510):

t5103AABBCC\r    DLC=3, 3 bytes = 6 nibbles packed high-nibble-first

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Python scripts

Script	Purpose
sent_MLX - Copy.py	MLX90377 GUI — live angle gauge, decoded nibbles, frame log. Connect sensor to PA2, open port in the GUI; data appears automatically.
sent_viewer.py	General SLCAN monitor GUI with TX panel and Learn mode
sent_test.py	CLI — open port, apply config, print received frames for N seconds
check_rx.py	Loopback sanity check: pings COM8 (TX), listens on COM9 (RX)
tx_signal.py	Continuous TX every 10 ms for logic-analyzer capture on PA4
verify_sent.py	Logic2 automation: capture PA4 signal and cross-check with USB output

Requirements

pip install pyserial

verify_sent.py additionally needs pip install saleae and Logic2 running with the automation API enabled (port 10430).

MLX90377 quick start

python "sent_MLX - Copy.py"

Select the COM port, click Connect. Firmware auto-starts RX on O\r; angle and magnitude update live.

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Build

STM32CubeIDE manages the Makefile. To build from the command line, set the paths to the bundled tools and run make inside the Debug folder:

set MAKE=C:\ST\STM32CubeIDE_1.19.0\STM32CubeIDE\plugins\com.st.stm32cube.ide.mcu.externaltools.make.win32_2.2.0.202409170845\tools\bin\make.exe
set GCC=C:\ST\STM32CubeIDE_1.19.0\STM32CubeIDE\plugins\com.st.stm32cube.ide.mcu.externaltools.gnu-tools-for-stm32.13.3.rel1.win32_1.0.0.202411081344\tools\bin
set PATH=%GCC%;%PATH%
cd Debug
%MAKE% -j4 all

Output: Debug/SENTToUSB.elf (~28 KB flash, ~6 KB RAM on STM32F042G4 with 32 KB flash / 6 KB SRAM).

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Flash

SWD (ST-Link)

set CLI=C:\ST\STM32CubeIDE_1.19.0\STM32CubeIDE\plugins\com.st.stm32cube.ide.mcu.externaltools.cubeprogrammer.win32_2.2.200.202503041107\tools\bin\STM32_Programmer_CLI.exe
%CLI% -c port=SWD freq=4000 -w Debug/SENTToUSB.elf -v -hardRst

USB DFU (no ST-Link required)

Step 1 — trigger DFU from the running device (replace COM8 with your port):

python -c "import serial,time; s=serial.Serial('COM8',115200,timeout=1); s.write(b'B\r'); time.sleep(0.3); s.close()"

Step 2 — flash (~3 s after step 1, once the device enumerates as DFU):

%CLI% -c port=USB1 -w Debug/SENTToUSB.elf -v -hardRst

The B command writes a magic value (0xDEADBEEF) to a .noinit RAM variable and resets. On the next boot the firmware detects the magic, clears it, and jumps to the STM32F042 ROM bootloader at 0x1FFFC400.