Hardware Circuits and the TTL Bus
Learning objectives: build MicroDuck's electrical system — power, servo bus communication, and IMU integration — and master the most common troubleshooting (servo packet loss, communication timeouts, over-current protection).
6.1 System Electrical Architecture
lithium battery → BMS → power distribution
├─ RK3566 mainboard (5V/3.3V logic)
├─ IMU (I2C/SPI)
└─ servo bus (TTL, 15 HD1910 daisy-chained)
Key point: servo power and mainboard logic power must be separated. When 15 servos move simultaneously the current is large; if they share a power supply with the mainboard (even with a common ground), the mainboard voltage drops and it resets.
6.2 Power Scheme and BMS
- Battery: 2S (7.4 V) or 3S (11.1 V) lithium; servos are rated 4.8–8.4 V, so 2S is safer;
- BMS: must include low-voltage protection and over-current protection. At low voltage, servo torque drops sharply and position readback drifts, destabilizing the policy; over-current burns the servo drivers;
- Consider adding power soft-start (inrush limiting) to avoid the inrush of charging capacitors at power-on disturbing the bus.
6.3 The Feetech HD1910 TTL Bus Topology
Fifteen servos are daisy-chained on the TTL bus, each with a unique ID:
mainboard UART(TTL) ── HD1910 ID=1 ── ID=2 ── ... ── ID=15
- Baud rate: up to 1 Mbps; start debugging at a lower rate (e.g., 500 kbps);
- Half-duplex TTL: transmit and receive share a line, requiring direction control on the mainboard side;
- Each servo must be configured with its ID and baud rate first; ID conflicts scramble the entire bus.
6.4 IMU Integration and Mounting
- Models: MPU6050 / ICM20948;
- Interface: I2C or SPI; I2C bandwidth is limited, so prefer SPI for higher sample rates;
- Mounting position: as close to the machine's center of mass as possible, on a rigid part of the torso;
- Mounting tilt: the IMU's mounting attitude enters the attitude-estimation initial value. If it is mounted with a tilt, it must be compensated in firmware; otherwise the attitude angle drifts directly and Sim2Real fails.
6.5 Bus Troubleshooting
| Symptom | Likely cause | Handling |
|---|---|---|
| A servo does not respond | ID conflict / wrong address | reconfigure the ID |
| Random packet loss | long bus, signal interference | lower the baud rate, add bus isolation |
| Bus freezes after power-on | insufficient power / a shorted servo | power up one servo at a time |
| Position readback drift | supply voltage sag | check BMS and wire gauge |
⚠️ Long-line interference: with 15 daisy-chained servos and long cables, TTL signals are easily affected by EMI. Use shielded cable or add bus-isolation chips. This is the most easily overlooked pitfall from simulation to hardware.
6.6 Wiring Convention for Power/Signal Separation
- Servo power wires (thick) come directly from the BMS, not through the mainboard;
- The mainboard and servos share only a signal ground; logic levels are driven by the mainboard;
- Place a current-limiting resistor/fuse in series at the servo-bus end to protect the mainboard UART.
6.7 Engineering Pitfall Log
- Shared power causing mainboard reset: current spikes during multi-servo emergency stops pull down the mainboard voltage and reboot the system. Fixed by separating the power supplies.
- ID conflict paralyzing the whole bus: two servos configured with the same ID scramble every command. Recovered by powering each servo individually and reconfiguring its ID.
6.8 Chapter Summary
You should be able to build an electrical system with separated power, a correct bus, and the IMU integrated, and to troubleshoot servo packet loss and power faults.
The next chapter brings the system up: flash the RK3566 image, deploy the Rust firmware, and understand the 50 Hz control loop.