ATMega32 TWI interfaces to MCP23017 I2C I/O expanding

Overview

MCP23017 is a 16-bit microprocessor digital I/O expander, built up with two 8-bit GPIO ports. The inputs/outputs are bi-directional. I/O direction is controlled by its data direction control register that's similar to one's in its PICMicro device.

This device is required in any applications that the MCU has used up its digital I/O. For example, a master MCU need to toggle 16 output relays. The manufacturer has released an example of using this device to create a matrix keypad.

I/O Pins

The I/O expander also support interrupt whenever the logic inputs changed.

MCP23017-E/SP I use for my prototyping

MCP23017 Pin Diagram

I make a summary of this 28-pin device:

• 1 to 8 : 8-bit GPIOB port, ranges from GPB0 to GPB7.
• 9 : VDD - positive supply voltage (2.0 to 5.5 V)
• 10 : VSS - Supply ground
• 11 : NC - Not Connected
• 12 : SCK - Serial Clock
• 13 : SDA - Serial Data
• 14 : NC - Not Connected
• 15 16 17 : A0 A1 A2 - I2C optional address select pins (must be properly wired between GND and VDD)
• 18 : RESET - Active low reset pin
• 19 20 : INTB INTA - Digital input change interrupt trigger pin.
• 21 to 28 : 8-bit GPIOA port, ranges from GPB0 to GPB7.

Two-wire Serial Communication (TWI)

Using I2C (or TWI), this device could clock up to 1700 kHz. It's supplied from 2.0 to 5.5 V, but for most of electronics hobbyist a 5.0 V supply voltage is a preference. 

Just like other TWI devices, MCP23017 read and write operations are similar. This device contain I/O port register, output register, interrupt control registers etc.

MCP23017 Register Addresses

These registers could locate at both banks - bank 0 and bank 1 with different location. By default, without any control register setting, it locate at bank 0.

TWI Writting 

The writing address of MCP23017 is 01000000 in binary. To write to this device,

1. initiate a start condition
2. write 01000000 for MCP23017 slave writing
3. write the address of a specific register we want to send data to
4. write any data to that register
5. send a stop condition

The register we select has its own address list in the table above. For example we want to enable the interrupt on the input port.

TWI Reading

The reading address of MCP23017 is 01000001 in binary. To read to this device,

1. initiate a start condition
2. write 01000001 for MCP23017 slave reading
3. write the address of a specific register we want to read from
4. Read the TWI TWDR register
5. send a stop condition

Interfacing and Programming In Atmel Studio 7

A communication interface from a master MCU to MCP23017 uses only two wires - SCL and SDA. Most of TWI slave device left these two pins in open-drain, requires two external pull up resistor with a resistance between 4.7 kOhm to 10 kOhm.

However, ATMega32 pins can be pulled high via software. Hence, it eliminates the requirement of adding two external resistors for TWI communication.

In this example, I only make a basic digital I/O reading and writing without using any interrupt features.

A running program

GPIOx configured as input while GPIOx configured as output. There direction control registers are IODIRA and IODIRB respectively. Both GPIOx ports have their own internal weak pull up resistors name as GPPUA and GPPUB for both ports. 

Reading from GPIOx to get its inputs status. Writing to OLATx register to send digital data to GPIO output registers.

The sample program as shown above, GPIOA configured as digital inputs. I turn on its internal weak pull up resistor. GPIOB configured as digital output port. The main program just reads digital input from GPIOA, and send the result to GPIOB.

Source Code

/*

 * i2c_mcp23017.c

 *

 * Created: 9/9/2020 6:47:16 PM

 * Author : aki-technical

 */ 

#include <avr/io.h>

#define F_CPU 16000000UL

#include <util/delay.h>

#define mcp23017Read 0b01000001

#define mcp23017Write 0b01000000

void i2cInit(void){

TWSR=0x03; //Bit Rate Pre-scaler Is 1:64

TWBR=0x0F;

TWCR=(1<<TWEN); //Enable The TWI Module

PORTC|=(1<<0);

PORTC|=(1<<1);

}

void i2cStart(void){

TWCR=(1<<TWINT)|(1<<TWSTA)|(1<<TWEN);

while((TWCR&(1<<TWINT))==0);

}

void i2cWrite(unsigned char data){

TWDR=data;

TWCR=(1<<TWINT)|(1<<TWEN);

while((TWCR&(1<<TWINT))==0);

}

unsigned char i2cRead(char ACK){

if(ACK==0)

TWCR=(1<<TWINT)|(1<<TWEN)|(1<<TWEA);

else

TWCR=(1<<TWINT)|(1<<TWEN);

while((TWCR&(1<<TWINT))==0);

return TWDR;

}

void i2cStop(){

TWCR=(1<<TWINT)|(1<<TWEN)|(1<<TWSTO);

}

/*GPIO A and B Direction Control Register*/

const char IODIRA = 0x00;

const char IODIRB = 0x01;

/*GPIOA PULL UP REGISTER CONTROL*/

const char GPPUA  = 0x0C;

/*GPIOA I/O PORT register*/

const char GPIOA  = 0x12;

/*GPIOB Output Register*/

const char OLATB  = 0x15;

/*MCP23017 Configuration Setting*/

void mcp23017Config(char address,char config){

i2cStart();

/*Select the write address*/

i2cWrite(mcp23017Write);

/*Select a register address*/

i2cWrite(address);

/*Send configuration data*/

i2cWrite(config);

i2cStop();

}

/*Read Data From MCP23017*/

unsigned char mcp23017Return(char address){

/*Select a specific address*/

i2cStart();

i2cWrite(mcp23017Write);

i2cWrite(address);

i2cStop();

/*Read data from the given address*/

i2cStart();

i2cWrite(mcp23017Read);

unsigned char i2cData=i2cRead(1);

i2cStop();

return i2cData;

}

int main(void)

{

unsigned char i2cData;

i2cInit();

/*Pull SCL and SDA high*/

PORTC=0x03;

/*Set GPIOA To Input*/

mcp23017Config(IODIRA,0xFF);

/*Set GPIOB To Output*/

mcp23017Config(IODIRB,0x00);

/*Turn On Pull Up Resistors On GPIOB*/

mcp23017Config(GPPUA,0xFF);

_delay_ms(10);

while (1)

{

/*Read digital input from GPIOA*/

i2cData=mcp23017Return(GPIOA);

/*Send it back to GPIOB*/

mcp23017Config(OLATB,i2cData);

_delay_ms(10);

}

}

Schematic Diagram

Schematic Diagram - The master TWI doesn't use any other digital I/O rather than its TWI pins.

Click here to download the zip file of this example project.

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