Hardware: M5Stack Dial v1 (ESP32-S3) + M5Stack H-Bridge Unit v1.1 (STM32F030 + RZ7899, FW v2)
Arduino Core: ESP32 3.3.7 | M5Unified: 0.2.15 | M5GFX: 0.2.21 | M5UnitHbridge: 1.0.0

Overview

We spent several sessions trying to get the H-Bridge Unit v1.1 working reliably with the M5Stack Dial v1. What started as a simple "connect and drive a motor" task turned into a systematic investigation of all possible port, voltage, and I2C combinations.

This post documents the full test matrix with serial output and current measurements. We observed three things that surprised us — please read carefully and let us know if we misunderstood something or if there is a known solution we missed:

1. The official M5UnitHbridge library does not seem to work with this hardware — we believe it is due to a missing Repeated Start condition in its I2C read implementation, but we may be wrong.
2. Wire.begin() must be called after M5Dial.begin() — contrary to what you might expect, this re-configures the hardware I2C peripheral to Port A and works correctly. Is this intended behavior?
3. HPWR mode does not behave as expected — forward direction produces no output, backward direction appears limited to ~9% effective duty cycle regardless of speed value. We tried PWM frequency changes, common GND, different ports and I2C methods — all without improvement. Could this be a firmware issue, or are we missing a required initialization step?

We hope this is useful to others, and we very much welcome corrections or explanations from M5Stack or the community.

TL;DR

Getting the H-Bridge Unit v1.1 working with M5Dial is not straightforward. This post documents a complete test matrix covering all port/voltage/I2C combinations. The short version:

• 5V mode + Hardware I2C (Wire) + correct registers = works, but USB power limits motor load to ~500mA
• M5UnitHbridge library = broken for this hardware (wrong I2C read protocol, motor does not respond)
• HPWR mode appears non-functional in FW v2: forward channel dead, backward works at fixed ~9% duty cycle regardless of speed value — firmware bug suspected

Hardware Setup

Key Discovery 1: Wire.begin() After M5Dial.begin()

M5Dial.begin() initializes the internal I2C bus (touch controller) on G11/G12.

• Wire after M5Dial.begin() → internal bus, not Port A
• Wire1.begin(13, 15) → G15 held LOW by touch controller → SCL blocked
• ✅ Solution: Call Wire.begin(13, 15) after M5Dial.begin() — this re-configures the hardware I2C peripheral to Port A pins and works correctly

auto cfg = M5.config();
M5Dial.begin(cfg, false, false);
// ...
Wire.begin(13, 15);
Wire.setClock(100000);

Key Discovery 2: Correct Register Map

The H-Bridge register layout differs from various online sources. Verified by M5UnitHbridge library source:

Key Discovery 3: M5UnitHbridge Library Broken for This Hardware

The official M5UnitHbridge library uses endTransmission() (with STOP condition) before requestFrom():

// Library readBytes() — BROKEN for H-Bridge v1.1
_wire->beginTransmission(addr);
_wire->write(reg);
_wire->endTransmission();        // ← sends STOP
_wire->requestFrom(addr, length); // ← new START (not Repeated Start)

The STM32F030 in the H-Bridge requires Repeated Start between write and read. Without it:

• getFirmwareVersion() returns 255 instead of 2
• Write operations appear to succeed but the motor does not respond

✅ Solution: Use Wire directly with endTransmission(false) for reads:

Wire.beginTransmission(HBRIDGE_ADDR);
Wire.write(reg);
Wire.endTransmission(false);  // Repeated Start — NO stop condition
Wire.requestFrom(HBRIDGE_ADDR, len);

Complete Test Matrix

Serial Console Excerpts

Test 1a — 5V, Hardware Wire, correct registers (motor runs, brownout at ~78%)

FW-Version: 2
[1] Vorwaerts: Rampe 0->255 (100ms/Schritt)
  write reg=0x00 val=1 ... err=0
  write reg=0x01 val=0 ... err=0
  spd=  0
  ...
  write reg=0x01 val=198 ... err=0
  spd=198
E BOD: Brownout detector was triggered

Test 1b — 5V, M5UnitHbridge library (motor does not respond)

begin: OK
FW-Version: 255   ← should be 2 — Repeated Start missing in library
[1] Vorwaerts: Rampe 0->255 (100ms/Schritt)
  spd=  0 ... spd=255
[2] Vorwaerts: halte 100% fuer 3s
...
=== Fertig ===
← no brownout, no motor current, motor never moved

Test 4 — HPWR, Bit-Banging, with register readback (motor does not respond)

FW-Version: 2  (read OK)
[1] Readback-Test: dir=1 spd=128
  Readback: dir=1 (soll=1)  spd=128 (soll=128)   ← registers confirmed correct
[2] Vorwaerts: Rampe 0->255 (100ms/Schritt)
  spd=0 ... spd=255
[3] Vorwaerts: halte 100% fuer 3s
Motor dreht nicht
Messungen: 13.5V confirmed, 79mA from supply, 4mV at motor terminals

Test 3b — HPWR, Bit-Banging, COM A (asymmetric: forward dead, backward works)

FW-Version: 2
[1] Vorwaerts: Rampe 0->255    → Motor does NOT turn, ~4mV at terminals
[5] Rueckwaerts: Rampe 0->255  → Motor RUNS, -1.2V at terminals, 128mA from supply
                                  (~9% duty cycle at 13.5V, regardless of speed value)
[7] Rueckwaerts: Rampe 255->0  → Motor slows cleanly and stops at spd=0

Test 5 — HPWR, register snapshot (default PWM frequency)

[R] Register-Snapshot (Firmware-Defaults):
  0x00 DIR   = 0  (0=STOP,1=FWD,2=BWD)
  0x01 SPD8  = 0
  0x02 SPD16 = 0
  0x04 FREQ  = 1000 Hz    ← default is audible range — explains motor beeping

HPWR Mode — Appears Non-Functional in Firmware v2

After extensive testing across all port/I2C combinations, our conclusion is that HPWR mode is functionally broken in firmware v2.

Additional tests (Tests 5 & 6)

• Default PWM frequency: 1000 Hz (register 0x04) — audible range, explains the motor "beeping"
• Setting frequency to 10000 Hz: no improvement, forward direction still inactive
• Connecting external supply GND to Grove GND: no improvement

HPWR test results across all combinations

Asymmetric behavior detail (Test 3b)

Forward (dir=1):

• ~4mV at motor terminals regardless of speed value
• Brief 120mA spike from supply at spd=0–10, drops to 80µA by spd=40
• Motor does not turn

Backward (dir=2):

• -1.2V at motor terminals — only ≈9% effective duty cycle at 13.5V supply, regardless of speed value (0–255)
• 128mA from external supply
• Motor runs through full ramp 0→255→0 and stops correctly
• Requires a manual push past static friction at low speeds

Evidence pointing to firmware bug

1. I2C communication confirmed working (FW=2 readable, register readback correct)
2. Speed value has no effect on output voltage in HPWR mode (9% fixed regardless of spd=1 or spd=255)
3. PWM frequency change has no effect
4. GND connection has no effect
5. Issue present on both COM A and COM B
6. Issue present with both Hardware I2C and Bit-Banging
7. 5V mode works correctly on the same hardware

Our interpretation: The STM32F030 firmware v2 may not properly implement HPWR mode — the DIP switch position appears to be detected but the RZ7899 drive logic may not be correctly configured for external supply operation. However, we acknowledge we could be missing a required initialization step or configuration. We would greatly appreciate clarification from M5Stack or anyone who has successfully used HPWR mode.

Working Configuration (Minimal Code)

#include <M5Dial.h>

#define HBRIDGE_ADDR  0x20
#define REG_DIR       0x00
#define REG_SPEED8    0x01

bool hb_write(uint8_t reg, uint8_t val) {
    Wire.beginTransmission(HBRIDGE_ADDR);
    Wire.write(reg);
    Wire.write(val);
    return Wire.endTransmission() == 0;
}

bool hb_read(uint8_t reg, uint8_t* out) {
    Wire.beginTransmission(HBRIDGE_ADDR);
    Wire.write(reg);
    Wire.endTransmission(false);  // Repeated Start
    return Wire.requestFrom((uint8_t)HBRIDGE_ADDR, (uint8_t)1) == 1
        && (*out = Wire.read(), true);
}

void setup() {
    auto cfg = M5.config();
    M5Dial.begin(cfg, false, false);
    Wire.begin(13, 15);           // COM A, after M5Dial.begin()
    Wire.setClock(100000);
    // DIP switch: 5V mode — HPWR forward channel does not work
    hb_write(REG_DIR, 1);         // FORWARD
    hb_write(REG_SPEED8, 128);    // 50%
}

⚠️ Safety Note: H-Bridge Runs Independently After ESP32 Reset

The STM32F030 inside the H-Bridge has a lower minimum operating voltage than the ESP32-S3. This means:

Scenario 1 — Brownout:
The ESP32-S3 crashes (brownout at ~700mA from Grove 5V). The STM32F030 keeps running and holds the last direction + speed. The motor continues turning until power is physically removed.

Scenario 2 — Reset button:
Pressing the M5Dial reset button resets the ESP32-S3, but the STM32F030 is unaffected. Motor keeps running.

Scenario 3 — Software crash:
Any ESP32-S3 crash leaves the H-Bridge in its last state.

Safe practice — always send STOP as the first action in setup():

Wire.begin(13, 15);
Wire.setClock(100000);
Wire.beginTransmission(HBRIDGE_ADDR);
Wire.write(0x00); Wire.write(0);   // DIR = STOP
Wire.endTransmission();
Wire.beginTransmission(HBRIDGE_ADDR);
Wire.write(0x01); Wire.write(0);   // SPEED = 0
Wire.endTransmission();
// ... rest of setup

Current Measurements (5V Mode, Grove Side)

Note: USB 2.0 limit is 500mA — sustained motor load exceeds this. Use a powered USB hub or a 5V supply with higher current rating. Do not rely on HPWR mode — it is non-functional in FW v2.

Summary

Tested May 2026 on M5Stack Dial v1 (ESP32-S3) + H-Bridge Unit v1.1 (STM32F030 + RZ7899, FW v2) / Arduino IDE 2.3.8 / ESP32 Core 3.3.7 / M5Unified 0.2.15

Appendix: Full Serial Output per Test

Test 1a — COM A | 5V | Hardware Wire | correct registers

=== Test 1a: Wire direkt, korrekte Register ===
FW-Version: 2
[1] Vorwaerts: Rampe 0->255 (100ms/Schritt)
  write reg=0x00 val=1 ... err=0
  write reg=0x01 val=0 ... err=0
  spd=  0
  
  write reg=0x01 val=198 ... err=0
  spd=198
E BOD: Brownout detector was triggered

Lauf mit Ammeter (Grove-Seite):
  write reg=0x01 val=52 ... err=0
  spd= 52
E BOD: Brownout detector was triggered
Strom: ~300mA Ruhestrom, ~700mA beim Laufen (nach Anschieben), 3.3V an Motor

Test 1b — COM A | 5V | M5UnitHbridge Library

=== Test 1b: M5UnitHbridge Library ===
begin: OK
FW-Version: 255   ← should be 2
[1] Vorwaerts: Rampe 0->255 (100ms/Schritt)
  spd=  0 ... spd=255
[2] Vorwaerts: halte 100% fuer 3s
[3] Vorwaerts: Rampe 255->0
[4] STOP ... [8] STOP
=== Fertig ===
← no brownout, no motor current, motor never moved

Test 2a — COM A | 5V | Bit-Banging

=== Test 2a: COM A | 5V | Bit-Banging ===
[0] I2C Scan...  Gefunden: 0x20 <- H-Bridge
FW-Version: 2
[1] Vorwaerts: Rampe 0->255 (100ms/Schritt)
  spd=  0 ... spd= 62
E BOD: Brownout detector was triggered
← Motor dreht. Brownout bei spd=62 (~24%). GPIO-Umschalten erhoht Strombedarf.

Test 2b — COM B | 5V | Bit-Banging

=== Test 2b: COM B | 5V | Bit-Banging ===
    SDA=G2 (gelb), SCL=G1 (weiss)
[0] I2C Scan...  Gefunden: 0x20 <- H-Bridge
FW-Version: 2  (read OK)
[1] Vorwaerts: Rampe 0->255 (100ms/Schritt)
  spd=  0 ... spd= 63
E BOD: Brownout detector was triggered
← Motor hat gezuckt, Spannung messbar. Brownout bei spd=63, identisch mit 2a.

Test 3a — COM A | HPWR | Hardware Wire

=== Test 3a: COM A | HPWR | Wire direkt ===
    SDA=G13, SCL=G15
FW-Version: 2
[1] Vorwaerts: Rampe 0->255 (100ms/Schritt)
  write reg=0x00 val=1 ... err=0
  write reg=0x01 val=0 ... err=0
  spd=  0 ... spd=255
[2] Vorwaerts: halte 100% fuer 3s
[3] Vorwaerts: Rampe 255->0
=== Fertig ===
Messungen: 13.5V OK, 0.083mA aus Netzteil, Motor dreht nicht.

Test 3b — COM A | HPWR | Bit-Banging

=== Test 3b: COM A | HPWR | Bit-Banging ===
    SDA=G13, SCL=G15
[0] I2C Scan...  Gefunden: 0x20 <- H-Bridge
FW-Version: 2
[1] Vorwaerts: Rampe 0->255   → Motor dreht NICHT, ~4mV, 120mA-Spike bei spd=0..10
[5] Rueckwaerts: Rampe 0->255 → Motor dreht ✓, -1.2V, 128mA aus Netzteil
[7] Rueckwaerts: Rampe 255->0 → Motor haelt sauber bei spd=0
=== Fertig ===
Asymmetrie: Vorwaerts tot, Rueckwaerts ~9% eff. Duty Cycle bei 13.5V

Test 4 — COM B | HPWR | Bit-Banging

=== Test 4: COM B | HPWR | Bit-Banging ===
    SDA=G2 (gelb), SCL=G1 (weiss)
[0] I2C Scan...  Gefunden: 0x20 <- H-Bridge
FW-Version: 2  (read OK)
[1] Readback-Test: dir=1 spd=128
  Readback: dir=1 (soll=1)  spd=128 (soll=128)   ← Register korrekt geschrieben
[2] Vorwaerts: Rampe 0->255 ... Motor dreht nicht
Messungen: 13.5V OK, 79mA aus Netzteil, 4mV an Motor.

Test 5 — COM A | HPWR | Bit-Banging | PWM-Frequenz auslesen

=== Test 5: COM A | HPWR | Bit-Banging | FreqRead ===
    SDA=G13, SCL=G15
[0] I2C Scan...  Gefunden: 0x20 <- H-Bridge
FW-Version: 2
[R] Register-Snapshot (Firmware-Defaults):
  0x00 DIR   = 0  (0=STOP,1=FWD,2=BWD)
  0x01 SPD8  = 0
  0x02 SPD16 = 0
  0x04 FREQ  = 1000 Hz   ← default PWM frequency
Motor dreht nicht.

Test 6 — COM A | HPWR | Bit-Banging | PWM-Frequenz auf 10kHz gesetzt

=== Test 6: COM A | HPWR | Bit-Banging | FreqSet 10kHz ===
    SDA=G13, SCL=G15
[F] Setze PWM-Frequenz auf 10000 Hz...
    FREQ nach Set = 10000 Hz   ← write accepted
[R] Register-Snapshot:
  0x04 FREQ  = 10000 Hz
[1] Vorwaerts: Rampe 0->255   → Motor dreht nicht. Frequenz hat keinen Einfluss.