#340 IxpandO

IxpandO is an input/output expansion board based on Microchip’s MCP23017, BoldportClub Project #16. Demo program covers digital output and interrupt-driven input.

Here’s a quick demo of it in action..

Notes

IxpandO is an input/output expansion board based on Microchip’s MCP23017, BoldportClub Project #16.

The board design leaves the door open for various configurations. I decided to go initially with the default setup of 8 inputs (switches) and 8 outputs (LEDs), with an Arduino sketch to demonstrate digital output and interrupt-driven input.

The MCP23017 is the I²C variant of the chip. See LEAP#313 for a similar demonstration of the MCP23S17 SPI variant.

Parts and Unboxing

Construction

The IxpandO GitHub repository contains the full design of the Boldport board. The following schematic is a transcription that includes:

• optional controller (in this case, an Arduino-compatible Cuttle)
• optional LEDs connected to bank B (GPB)
• optional DIP switches connected to bank A (GPA)

About the MCP23017

The Microchip MCP23017 is a 16-bit port expender:

• 16 GPIO ports
• Configurable interrupt output pins
• High-speed I²C interface at up to 1.7 MHz (MCP23S17 is the SPI variant)
• Three hardware address pins to allow up to eight devices on the bus
• Operating voltage 1.8V to 5.5V
• Maximum source/sink current per pin: 25mA
• VSS max current: 150 mA
• VDD max current: 125 mA

Note the current limit gotcha: 16 x 25ma >> 125mA, so care is required not to have too many pins fully loaded at once, or pins should only be used to switch larger currents (not provide power themselves).

Key Commands

I²C Write

• send control byte including the Start condition and ACK (R/W bit cleared)
• send register address byte
• send at least one data byte
• send stop or restart condition

I²C Read

• send control byte including the Start condition and ACK (R/W bit set)
• send register address byte
• read at least one data byte
• send stop or restart condition

I²C Sequential Write/Read

For sequential read/write, instead of transmitting a Stop or Restart condition, the master clocks the next byte(s) until finally sending a Stop or Restart condition.

The address used for sequential bytes depends on whether Byte and Sequential Mode is enabled.

Byte and Sequential Modes

Byte Mode: internal address counter remains the same after each byte transfer. Useful for polling the GPIO register or for continually writing to the output latches.

Sequential Mode: internal address counter remains the same after each byte transfer. Useful for bulk writes without needing control bytes to set the address for each.

Registers

There are 21 registers. Of these, 10 replicated in two banks, and one IOCON register is shared.

Test Program

The IxpandO.ino sketch demonstrates

• digital output to LEDs on bank B (GPB)
• interrupt-driven input for the DIP switch on bank A (GPA)

It uses the Wire Library to directly control the MCP23017, via standard I²C connections on PC5 (SCL), PC4 (SDA).

Changing any DIP switch position triggers INTA. The sketch attaches a hardware interrupt handler for this input, which sets a flag on change.

When the flag is set, the sketch reads all the switch positions and acts accordingly.

The demo program has a very simple behaviour:

• when all switch positions are off, it runs a demo cycle on the LEDs
• if any switches are on, it lights the corresponding LED for all switch positions.

Credits and References

• LEAP#313 MCP23S17 - similar demonstration of the MCP23S17 SPI variant
• Boldport IxpandO - official product page
• MCP23017T-E/SO info and datasheet
• Wire Library
• ..as mentioned on my blog