Three INA219 chips are used to monitor power supply currents for +12V (nominal) busses on Marble. Full-scale currents are programmable via the PG bits of the INA219 config register as shown in this table:
| INA219 config[12:11] | FMC1 | FMC2 | main | |
|---|---|---|---|---|
| I2C address | 0x80 | 0x82 | 0x84 | |
| INA219 refid | U17 | U32 | U57 | |
| shunt value | 0.082 Ohm | 0.082 Ohm | 0.0273 Ohm | |
| 00 ( ± 40mV) | 0.488 A | 0.488 A | 1.46 A | |
| 01 ( ± 80mV) | 0.976 A | 0.976 A | 2.93 A | |
| 10 ( ± 160mV) | 1.951 A | 1.951 A | 5.85 A | |
| (default) | 11 ( ± 320mV) | 3.902 A | 3.902 A | 11.71 A |
Note that the first-run boards so far have 0.02 Ohm shunt resistors for U17 and U32, which give pretty ridiculous values for the current (full-scale) that the FMC spec says should be limited to 1A. Also, the shunt for U57 there was 0.0227 Ohms.
Considerations that led to those resistor values were:
- General operations should use PG bits config[12:11] = 01
- Can turn range up or down by a factor of two for special use cases
- Full-scale 160 mV should not burn up the 1206 0.5W shunt resistor
Final answer, as shown above, and also used on the second batch of Marble-Mini: 82 mOhm Ohmite KDV12DR082ET (tripled for the main input shunt).