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Revision 1 Reworks
| Rework | Description |
|---|---|
| #1. I2C MUX Reset and Channel Change | I2C MUXes nRST was not connected, and address prevented connection of BME280 sensors, which are commonly used. This address is changed to one less often used. |
| #2. SPI Level Shifter Direction Swap | SPI level shifters for application support peripherals had direction pins wired the wrong way around, meaning no temperature monitor or DAC control. |
| #3. USB Power Supply (5v) Feedback Resistor Correction |
The I2C MUXes each have an active low reset line. Although these were connected to each other, the net was not connected to the global reset signal, and therefore the I2C MUXes do not work by default.
In addition, it was found that the address of one MUX, U8, is incompatible with many external boards and also the LOKI board itself due to a conflict with the 0x77 address of a BME280. Although it is possible to force the BME280 to a different address, this is not a requirement that many would expect, and many existing boards already have such a sensor with the default address. It therefore makes more sense to change the address of this MUX to prevent the conflict.
To change U8's address from 0x77 to 0x72, the address pins A2:A0 must must be changed from 0b111 to 0b010. This new address is not shared with anything else on the board, and in particular is not shared with the other MUX on the same bus, U7, which is on address 0x73.
- Connect the nRST pin (pin 24) on either U7 or U8 to the bottom right leg of the reset button next to them
- Disconnect pin A2 (pin 18) from its via and connect to GND (there is a test point nearby)
- Disconnect pin A0 (pin 22) from its via and connect to GND (there is a test point nearby)
Rework completed: #todo insert image here
The peripheral support devices on LOKI that are on the SPI bus not routed off board are inaccessible because the level shifters used (one in each direction) have the direction pins switched. Each device has four channels split into pairs, with each pair having its own direction pin. There are three such pins that require a direction swap.
See LOKI - Initial Testing 27-08-2023#Error in Level Shifter Direction
- Disconnect U22 1DIR (pin 4) from its via and connect to ground, easiest on the right of capacitor C71
- Disconnect U23 1DIR (pin 4) and 2DIR (pin 5) from their vias and connect to 1v8, easiest to just bridge across to pin 3 (VCCA) with a wire.
Instructional image: !20230822084643 LOKIrev1 levelshiftreworkinstruct.png
Example completed rework: !20230823085820 LOKIrev1 levelshiftreworkdone.png
The USB power supply is at the wrong voltage because the feedback resistors on the power supply IC (AD5593R).
See LOKI - Initial Testing 27-08-2023#Debugging the low 5v rail.
Swapping the resistors would fix the issue, for an easier rework it is possible to only adapt one. This will not achieve the noise performance modelled with the original resistances however. R84 is much easier to get to.
Keeping one resistance the same, you can change the other as follows:
| R84 | R12 | Resulting voltage | Parallel Version? |
|---|---|---|---|
| 13.3k | 69.8k | ||
| 365k (E48) | 69.8k | No | |
| 360k | 69.8k | No | |
| 13.3k | 2.5333k (ideal) | ||
| 13.3k | 2.49k (E48) |
So, options are:
- Swap the two resistors, which is more work but requires no extra parts
- Change R84 to around 365k, which is an E48 resistor.
- Change R12 to 2.49k, also an E48 resistor