Port pinning
| Pin | Function | Pigtail wire color |
|---|---|---|
| 1 | GND | brown |
| 2 | P7 | white |
| 3 | SCL | blue |
| 4 | SDA | black |
| 5 | VIN, 5V or 3.3V | grey |
| 6 | P8 | pink |
P7 & P8 Inputs (I2C Connector)
The P7 and P8 inputs are two protected signal inputs located on the I2C connector.
Both inputs share the same input protection and filtering circuit and are intended for low-speed pulse detection.
These inputs are clamped to the internal 3.3 V domain of the microcontroller.
Supported Signal Levels
- Recommended input voltage range: 0 – 3.3 V
- Absolute maximum: 5 V (current-limited)
- Signals above 3.3 V are safely clamped internally, but should be avoided for continuous operation
⚠ Important:
Unlike other analog inputs, P7 & P8 are not 5 V tolerant for continuous use.
If a signal exceeds 3.3 V, the internal clamp diodes will conduct.
Internal Signal Conditioning
Each input includes:
- Series resistor (1 kΩ)
Limits current into the protection diodes if the input exceeds 3.3 V - Clamp diodes to 3.3 V
Protect the microcontroller from overvoltage - Input capacitor (10 nF)
Provides noise filtering and signal smoothing
This design prioritizes robustness and noise immunity for field-connected signals.
Pulse Input Mode
P7 and P8 as pulse inputs.
Recommended pulse frequency:
| Pulse frequency | Behavior |
|---|---|
| 0 – 1 kHz | Fully reliable |
| 1 – 2 kHz | Typically reliable |
| > 2 kHz | Not recommended |
Signal requirements:
- Push-pull or actively driven source
- Logic high ≤ 3.3 V preferred
- Clean duty cycle for best reliability
I2C Context
A7 and A8 are located on the I2C connector but are not the I2C data lines themselves.
- SCL and SDA are the actual I2C signals
- each have 4.7 kΩ pull-ups to 3.3 V
- I2C devices should run only on 3.3V logic