Using the MCP4131 digital pot safely

[joan]

I've been playing with the SPI digital pot MCP4131 datasheet.

Pinout

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CS    1 o  o 8  5V
SCLK  2 o  o 7  B
MIMO  3 o  o 6  Wiper
GND   4 o  o 5  A

Because of the limited number of pins MISO/MOSI is multiplexed on the MIMO pin.

Commands are 8 or 16 bits. After the first 6 bits the MCP4131 starts driving MIMO as an output.

I've just connected the Pi's MOSI gpio to MIMO. This works and lets me output commands to the device but I was wondering if this was safe.

[bbodin]

Since the SDI/SDO pin of the MCP4131 has an open-drain drive, it is protected from a drive conflict. To protect the Pi's MOSI gpio pin, ensure that it is configured as an open-drain output. If that is not possible, add a series resistor (about 200 ohm) between the Pi's MOSI and MCP4131's SDI/SDO to limit the current when the Pi's pin is driven high and the other side is driven low.

[joan]

Since the SDI/SDO pin of the MCP4131 has an open-drain drive, it is protected from a drive conflict. To protect the Pi's MOSI gpio pin, ensure that it is configured as an open-drain output. If that is not possible, add a series resistor (about 200 ohm) between the Pi's MOSI and MCP4131's SDI/SDO to limit the current when the Pi's pin is driven high and the other side is driven low.

Thanks.

I assume that was what diagram 6.1 Typical MCP41X1 SPI Interface Connections (Host Controller Hardware SPI) was trying to tell me.

Supplementary. Given I have a standard 3 wire SPI on the other channel does it matter if I have the resistor at the MIMO pin rather than the Pi end?

Is this OK?

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Pi MOSI --------+---------------- DIN ADC
                |
                |
                +----RESISTOR---- MIMO POT

Rather than?

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Pi MOSI ----RESISTOR----+-------- DIN ADC
                        |
                        |
                        +-------- MIMO POT

[bbodin]

Supplementary. Given I have a standard 3 wire SPI on the other channel does it matter if I have the resistor at the MIMO pin rather than the Pi end?

Is this OK?

Code: Select all

Pi MOSI --------+---------------- DIN ADC
                |
                |
                +----RESISTOR---- MIMO POT

Rather than?

Code: Select all

Pi MOSI ----RESISTOR----+-------- DIN ADC
                        |
                        |
                        +-------- MIMO POT

They are both OK but I'd go with the first configuration so that the resistor doesn't slow down the connection between Pi MOSI and DIN ADC.

[joan]

...
They are both OK but I'd go with the first configuration so that the resistor doesn't slow down the connection between Pi MOSI and DIN ADC.

Thanks. I have added a 470 ohm resistor as per the first configuration.