MCP23018

16-Bit I/O Expander with Open-Drain Outputs

The MCP23018 is very easy to interface with the Raspberry Pi through the I²C bus. It has 5.5V tolerant inputs, making it possible to read TTL inputs without doing any level conversion. If you want TTL level outputs, you only need to level change the two I² lines and run the device at 5V.

The MCP23018 does, however, have an unusual way to set its address.

TL;DR

I²C bus addressing

On an I²C bus, each device has a unique address, typically programmable by hardware address lines. When a message is received on the I²C bus with a matching the slave addres, the device will respond to commands on the bus.

The slave address doesn't change as the device is operating, so using address pins uses a lot of space for very little functionality. To reduce the number of pins used, the MCP23018 has a single analogue input to set the address.

The voltage presented on the ADDR pin is read at power-on and at reset, and is decoded by comparing it against several threshold voltages in a 3 bit flash-ADC. This address is then remembered until the next reset.

The voltage space is divided up into 8 equal chunks from V_SS to V_DD, allowing for 8 of these devices on the bus, numbered 0x20 to 0x27. The ideal voltage to present to the ADDR pin is mid-way through the range, ie. 1/16, 3/16, 5/16, … 15/16 of V_DD.

Potential Divider

A simple potential divider can do the job well, provided that the tolerance of the resistors doesn't permit the voltage to stray outside of the permitted region. The permitted region for each address is where the ADC will definitely decode the presented voltage to the correct address, even if there is noise, drift in the reference voltages etc.

The permitted region is relatively small, less than half of the space available. The exact formula is ±20% of 1/8 of the supply voltage.

It can be quite tricky to find appropriate resistor values to set the address. If you only have two devices to attach, I suggest using addresses 0x20 and 0x27, for which any resistor will work.

Assumptions

When generating the tables below, I have assumed 5% tolerance resistors throughout. Apart from the top and bottom addresses, you cannot guarantee correct operation with 10% tolerance resistors. You can, of course, use resistors with better tolerance, but it is not necessary.

I have planned for a current flow of 1mA through the the potential divider with V_DD at 5V. I can't find documentation on the current flow through the the ADDR pin, but it is should be several orders of magnitude less so it won't affect the resistor calculations.

About the tables

Only the relative voltages are important, so apart from the current flow in the final column, all the voltage levels are normalised to the range 0…1. Multiply them by V_DD if you want to compare them with measurements.

The Actual level column gives the lowest and highest levels that will be seen on the ADDR pin should the resistors be at the very limit of their allowed tolerance. In all cases, these are within the permitted range.

Picking the resistors

Look up the address, and use the R₁ the R₂ values from any of the tables below. Check that the resistor tolerance is at least as good as 5%.

E24 series

E12 series

This tables allows you to use more common resistors, but they must still be no more than 5% tolerance.

E6 series

This tables allows you to use even more common resistors. Again, they must be no more than 5% tolerance.

Peter Benie <peterb@chiark.greenend.org.uk>
Electronics