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YamBMS - M5Stack AtomS3

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Important

GPIO Limitation: AtomS3 provides very few GPIOs. This solution works best for:

  • Supervising multiple RS485 BMS
  • Limited number of UART BMS (3 maximum)
  • Applications where compact size is priority

Overview

The M5Stack AtomS3 combined with the Atomic CAN Base provides a compact and powerful solution for YamBMS applications. This combination offers CAN bus communication with isolation, a built-in display, and expandability through additional M5Stack units.

Key Features:

  • Compact design (24×24×13mm AtomS3 footprint)
  • ESP32-S3 with 8MB Flash
  • Built-in 0.85" IPS display (128×128 pixels)
  • Isolated CAN bus communication (up to 1Mbps)
  • 6-axis IMU (MPU6886)
  • Expandable with M5Stack Units (RS485, CAN, etc.)
  • Programmable button
  • Multiple development platforms supported

Board Variants & Components

AtomS3 Controllers

Choose one AtomS3 controller to clip onto the Atomic CAN Base:

AtomS3 Lite AtomS3 AtomS3R (8MB PSRAM)
Model Flash PSRAM Display IMU Best For
AtomS3 Lite (C124) 8MB No 0.85" IPS MPU6886 Budget projects
AtomS3 (C123) 8MB No 0.85" IPS MPU6886 Standard applications
AtomS3R (C126) 8MB 8MB 0.85" IPS MPU6886 Recommended (extra RAM)

Recommendation: Choose AtomS3R (C126) if using Bluetooth BLE for BMS communication, as the extra 8MB PSRAM helps prevent crashes when the BLE stack consumes significant RAM.

Base & Communication Units

Atomic CAN Base CAN Unit RS485 Unit
Component Type Features Use Case
Atomic CAN Base (A103) CAN Base Isolated CAN, up to 1Mbps, 110 nodes Recommended for inverter
CAN Unit (U085) HY2.0 Unit Isolated CAN via Grove port Alternative CAN solution
RS485-ISO Unit (U094) HY2.0 Unit Isolated RS485, up to 500kbps, 256 nodes For RS485 BMS or multi-node

Interfaces Support Capacity

Based on hardware limitations and testing, this board supports:

Interface Maximum Notes
CAN 1x bus CAN bus to BMS or Inverter (after adding a CAN base/unit)
RS485 1x bus Best option for monitoring many BMS of the same brand (after adding a RS485 base/unit)
UART 3× devices¹ CAN be connected to the HY2.0 interface or Atomic Base GPIOs header (shared with RS485)
Bluetooth (BLE) 3× devices¹ BLE stack consumes significant RAM, may cause crashes/reboots

¹ BMS / Balancer / Shunt

Note: These are theoretical limits. Not all combinations have been tested. For large installations with many BMS units, consider boards with more GPIOs like the LilyGo T-Connect or Waveshare ESP32-S3-RS485-CAN.

AtomS3 Specifications

Hardware Specifications

Specification Details
Microcontroller ESP32-S3FN8
Processor Xtensa 32-bit LX7 dual-core @ up to 240MHz
Flash 8MB
PSRAM None (AtomS3 Lite/AtomS3) / 8MB Octal SPI (AtomS3R)
Display 0.85" IPS LCD, 128×128 resolution
UART 1x HY2.0 interface (not isolated)
IMU MPU6886 (6-axis gyro + accel, I2C 0x68)
Button 1× Programmable button
Infrared IR transmitter built-in
USB USB Type-C (power + programming)
DC-DC SY8089 (5V to 3.3V)
Power Input 5V via USB Type-C
Operating Temp 0°C ~ 40°C
Dimensions 24.0 × 24.0 × 12.9mm
Weight 6.9g

GPIO Pinout

Available GPIOs (Bottom Pins):

  • G5 / G6 / G7 / G8 / G38 / G39
  • Power: 5V, 3.3V, GND

HY2.0-4P Expansion Port:

Pin Color Function
1 Black GND
2 Red 5V
3 Yellow G2
4 White G1

Internal Connections:

Peripheral GPIO Pins Notes
LCD Display MOSI: G21, SCK: G17, CS: G15, RS: G33, RST: G34, BL: G16 SPI interface
MPU6886 (IMU) SDA: G38, SCL: G39 I2C address 0x68
IR Transmitter Built-in For infrared control

Atomic CAN Base Specifications

Hardware Specifications

Specification Details
CAN Transceiver CA-IS3050G (isolated)
Maximum Rate 1 Mbps
Supported Nodes Up to 110 nodes
Loop Delay 150ns (low)
Common-mode Voltage -12V to +12V
Isolation Built-in isolated DC-DC
Protection Signal protection + electrical line protection
External Port VH3.96-4P terminal block
Operating Temp 0°C ~ 40°C
Dimensions 24 × 48 × 18mm
Weight 11.1g

Atomic CAN Base Pinout

The Atomic CAN Base connects to AtomS3 through the bottom pins:

Pin LEFT RIGHT Pin
1 3.3V
2 3 UART_TX (CAN_TX)
4 5 UART_RX (CAN_RX)
5V 6 7
GND 8 9

CAN Terminal Block (VH3.96-4P):

  • CAN H (CAN High)
  • CAN L (CAN Low)
  • GND (Ground)

Note: The exact UART pins used depend on configuration. Check the schematic for GPIO assignment.

RS485-ISO Unit Specifications

If you need RS485 communication (e.g., for RS485 BMS, multi-node), add the RS485-ISO Unit (U094):

Specification Details
Isolation Chip CA-IS3082W
Maximum Rate 500 kbps
Supported Nodes Up to 256 nodes
Isolation Voltage 1000V RMS
Protection Fail-safe, Overcurrent, Thermal shutdown, ±15kV ESD
Connection HY2.0-4P Grove cable to AtomS3
Terminal VH-3.96 4P (A, B, GND, +V)
Dimensions 56.0 × 24.0 × 10.2mm
Weight 9.1g

RS485-ISO Unit Pinout:

HY2.0-4P Black Red Yellow White
PORT.C GND 5V UART_RX UART_TX

Termination Resistor: The RS485-ISO Unit includes a 120Ω termination resistor (provided separately) that can be connected when the unit is at an endpoint of the RS485 bus.

Wiring Instructions

Connecting CAN Bus (Atomic CAN Base)

The Atomic CAN Base provides isolated CAN communication via a VH3.96-4P terminal block.

CAN bus Terminal Block (VH3.96-4P)
CAN H H (CAN High)
CAN L L (CAN Low)
GND GND (optional)

Wiring Steps:

  1. Clip the AtomS3 onto the Atomic CAN Base (ensure proper alignment)
  2. Identify CAN H and CAN L wires from your inverter or CAN device
  3. Connect CAN H to H (CAN H terminal)
  4. Connect CAN L to L (CAN L terminal)
  5. Connect GND to GND for common reference (optional)

CAN Bus Best Practices:

  • The Atomic CAN Base features built-in isolation for protection
  • CAN bus operates at up to 1 Mbps data rate
  • Supports up to 110 nodes on the bus
  • Use twisted pair cable (120Ω impedance recommended)
  • Termination: Add 120Ω resistors at both ends of the CAN bus
    • If the AtomS3 is at an endpoint, connect a termination resistor
    • Check if your inverter has built-in termination before adding external resistors

Connecting RS485 (RS485-ISO Unit)

If using the RS485-ISO Unit for RS485 devices:

RS485 Terminal Block (VH3.96-4P)
A + A (Data+ non-inverting)
B - B (Data- inverting)
GND GND (Ground reference)

Wiring Steps:

  1. Connect the RS485-ISO Unit to the AtomS3 via the HY2.0-4P Grove cable (Yellow=RX, White=TX)
  2. Identify A and B wires from your RS485 device (often labeled D+/D- or A/B)
  3. Connect RS485 A (D+) to Pin 1 (A terminal)
  4. Connect RS485 B (D-) to Pin 2 (B terminal)
  5. Connect GND to Pin 3 for common reference (recommended)

RS485 Best Practices:

  • The RS485-ISO Unit features 1000V isolation for protection
  • RS485 operates at up to 500 kbps data rate
  • Supports up to 256 nodes on the bus
  • Use twisted pair cable for noise immunity
  • Termination: Connect the included 120Ω resistor between A and B if the unit is at an endpoint
  • The unit includes fail-safe protection (outputs high if input is open or shorted)

Power Supply

AtomS3 Power Options

USB Type-C Power:

  • Connect a USB Type-C cable for 5V power
  • Recommended for programming, debugging, and testing
  • Built-in SY8089 DC-DC converter steps down 5V to 3.3V for ESP32-S3
  • Typical power consumption: 150-300mA (idle to active WiFi)

External 5V via HY2.0-4P Grove port:

  • Useful when powering the entire assembly from an external 5V supply

Programming & Development

Entering Download Mode

If you have trouble uploading firmware to the AtomS3:

Manual Download Mode:

  1. Press and hold the RESET button (about 2 seconds)
  2. Wait until the internal green LED lights up
  3. Release the button
  4. The device is now in download mode and ready for flashing

Download Mode Animation

LCD Backlight Control

Important: The PWM signal frequency for the LCD backlight driver should be 500Hz. A frequency that is too high may cause abnormal brightness adjustment functionality.

Backlight GPIO: G16

Display Features

Screen Specifications

Specification Details
Size 0.85 inch IPS LCD
Resolution 128 × 128 pixels
Color Depth 65K colors
Driver IC GC9107
Interface SPI
Backlight PWM controlled (GPIO16)

Display Control Parameters

The AtomS3's display can be controlled with the following parameters (useful for YamBMS display management):

Backlight Settings:

  • Backlight control: Manual brightness control (0-100%)
  • Backlight max level: Limit the maximum brightness in general
  • Backlight auto-dim level: Desired brightness when dimmed down (set to 0 to switch off screen completely)
  • Backlight auto-dim time: Time in minutes after which the screen will dim down (set to 0 to disable auto-dimming)

Behavior Notes:

  • Device restores last screen brightness after boot
  • Auto-dim timer does not start automatically - it gets initially started by pressing the screen button
  • Button press bumps brightness to 100% until the timer dims down again
  • This provides a lightweight solution without complex automation

IMU (MPU6886)

The AtomS3 includes a built-in MPU6886 6-axis IMU:

Sensor Details
Model MPU6886
Interface I2C (address 0x68)
Pins SDA: GPIO38, SCL: GPIO39
Features 3-axis gyroscope + 3-axis accelerometer

Use Cases for YamBMS:

  • Orientation detection
  • Vibration monitoring
  • Installation angle verification
  • Advanced motion-based alerts (optional)

Note: The I2C pins on the back of the AtomS3 share the same bus with the MPU6886.

Typical YamBMS Configuration

Configuration 1: AtomS3 + Atomic CAN Base + RS485-ISO Unit (Full Setup)

Best for: Multiple RS485 BMS + inverter via CAN

Components:

  • AtomS3R (recommended)
  • Atomic CAN Base (A103)
  • RS485-ISO Unit (U094)

Connections:

  1. Clip AtomS3R onto Atomic CAN Base
  2. Connect inverter to CAN terminal (CAN H/L)
  3. Connect RS485-ISO Unit to AtomS3 via Grove cable (HY2.0-4P)
  4. Connect BMS RS485 bus to RS485 terminals (A/B)
  5. Power via USB Type-C
  6. Add termination resistors at bus endpoints

GPIO Usage:

  • CAN: Uses UART pins via Atomic CAN Base
  • RS485: Uses G1/G2 via HY2.0-4P Grove port
  • Display: Internal SPI (pre-configured)

Configuration 2: BLE + CAN (Compact Wireless BMS)

Best for: 2-3 BLE devices + inverter via CAN

Components:

  • AtomS3R (8MB PSRAM required for stable BLE)
  • Atomic CAN Base (A103)

Connections:

  1. Clip AtomS3R onto Atomic CAN Base
  2. Connect inverter to CAN terminal
  3. Configure BMS via Bluetooth (up to 3× BMS maximum)
  4. Power via USB Type-C

Limitations:

  • BLE stack consumes significant RAM
  • May cause crashes/reboots with >2 BMS
  • No physical RS485 connection

Troubleshooting

AtomS3 Not Entering Download Mode

  • Solution: Press and hold RESET button (~2 seconds) until green LED lights up, then release
  • Try different USB cable (must support data transfer)
  • Check USB drivers are installed

CAN Bus Communication Issues

  • Verify CAN H and CAN L are correctly connected (not swapped)
  • Check that CAN bus has exactly two 120Ω termination resistors (one at each end)
  • Verify CAN bus speed matches inverter (typically 500kbps)
  • Measure resistance between CAN H and CAN L (should be ~60Ω with proper termination)
  • Check isolation: the Atomic CAN Base has built-in isolation, but verify connections

RS485 Communication Issues

  • Verify A and B wiring (try swapping if communication fails)
  • Check baud rate matches all RS485 devices
  • Ensure ground (GND) is connected for common reference
  • Add 120Ω termination resistor at RS485-ISO Unit if at bus endpoint
  • The RS485-ISO Unit has fail-safe protection - if input is open/shorted, receiver outputs high

BLE Connection Issues (Bluetooth BMS)

  • AtomS3R (8MB PSRAM) strongly recommended for BLE
  • BLE stack consumes significant RAM - limit to 3 BMS maximum
  • May cause crashes/reboots if too many BLE connections
  • Consider switching to RS485 for multiple BMS units

Display Issues

  • Backlight frequency: Ensure PWM frequency is 500Hz (not higher)
  • Brightness control: Use manual control via button or software
  • Auto-dim not working: Timer requires manual activation by pressing button after boot

GPIO Limitation Issues

  • Problem: Not enough GPIOs for all sensors/devices
  • Solution: Use I2C-based sensors (shares I2C bus with IMU)
  • Alternative: Consider boards with more GPIOs (LilyGo T-Connect, Waveshare ESP32-S3-RS485-CAN)

Additional Resources

Official M5Stack Documentation

Schematics & Datasheets

Software Libraries

Where to Buy

Product Comparison

AtomS3 vs Other YamBMS Boards

Feature AtomS3 + Atomic CAN LilyGo T-Connect Waveshare ESP32-S3-RS485-CAN
Size Very compact (24×24mm) Medium (94×83mm) Medium with DIN rail case
Display ✅ 0.85" IPS (128×128) ❌ No ❌ No
RS485 Ports 1 (via Unit) Up to 3 1 (isolated)
CAN Ports 1 (isolated) 1 1 (isolated)
Isolation ✅ Yes (CAN + RS485 Unit) ✅ Yes (both) ✅ Yes (both)
Termination External External ✅ 120Ω jumper-selectable
GPIOs ⚠️ Very limited Moderate Moderate
PSRAM Optional (AtomS3R: 8MB) 8MB 8MB
Best For Compact installations, display needed, RS485 BMS Multiple RS485 ports Industrial, built-in termination
Price Moderate (modular) Moderate Moderate

When to Choose AtomS3:

  • ✅ Compact size is critical
  • ✅ Built-in display desired
  • ✅ Modular expansion preferred
  • ✅ BMS via RS485 (multiple units)
  • ❌ Need many GPIOs
  • ❌ Need multiple RS485 ports simultaneously

FAQ

Q: Which AtomS3 variant should I choose?
A: For YamBMS, we recommend AtomS3R (C126) with 8MB PSRAM if using Bluetooth BLE, as it provides extra RAM to prevent crashes. For RS485-only applications, AtomS3 or AtomS3 Lite is sufficient.

Q: Can I use multiple RS485 and CAN simultaneously?
A: Yes, but with limitations. AtomS3 has very few GPIOs. You can use Atomic CAN Base (CAN) + RS485-ISO Unit (RS485) simultaneously, but additional expansion is limited.

Q: How many BMS can I monitor via UART?
A: Maximum 3× BMS via UART. The 2nd and 3rd UARTs requires soldering connections on the Atomic CAN Base.

Q: Does the Atomic CAN Base require external termination resistors?
A: The Atomic CAN Base does not have built-in switchable termination. You must add external 120Ω resistors at both ends of the CAN bus.

Q: Why does my BLE connection crash after adding the 3rd BMS?
A: The BLE stack consumes significant RAM. Use AtomS3R (8MB PSRAM) and limit to 3 BMS maximum.

Q: Can I power the AtomS3 from the CAN bus?
A: No, you should power the AtomS3 via USB Type-C (5V). The CAN bus voltage is not suitable for directly powering the device.

Q: How do I control the screen backlight?
A: Use PWM on GPIO16 at 500Hz frequency. Brightness can be controlled manually, and auto-dim requires button activation to start the timer.

License: This documentation is part of the YamBMS project and follows the same GPLv3 license.

Acknowledgments: Based on official M5Stack documentation and community contributions.