MCP23S17 Class for Arduino

Author:  Cort Buffington
Contact: cort.buffington+arduino@gmail.com
Date:    March 2012

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Introduction:

This class is written to simplify using the Microchip MCP23S17 general purpose I/O expander IC in the Arduino environment. Some understanding of the MCP23S17 is required, so if you are not familiar with it, download the datasheet for it and have a look. The rest of this description will assume a *basic* understanding of the chip.


Download:

Version 0.1:

Get the class here!


Implementation:

The goal of this implementation is to provide a software interface that mimics the existing Arduino I/O functions:

  • pinMode(pin, mode)
  • digitalWrite(pin, value)
  • digitalRead(pin)

The class does include several more methods that can be used to simplify configuration in the same "Arduino-ish" way, methods for writing/reading 8-bit registers (configuration and I/O ports) at once, as well as writing/reading consecutive registers (allowing all 16 bits to be read or written with one method call). The interrupt features of the chip are not directly supported with method for specifically configuring them, however, the byte and word read/write methods may be used to configure and use the interrupt features. These features can get somewhat complicated, and any user to is prepared to use them will likely prefer the more generic methods for controlling them.

Upon initialization of an MCP23S17 as an object, ALL MCP23S17s on the SPI bus (sharing the same slave select) will be placed into hardware addressing mode. This allows up to 8 MCP23S17s to be used with a single slave select.


Methods:

MCP()

Description

Instantiate an MCP23S17 device as an object.

Syntax

MCP object_name(address)

Parameters

object_name: any arbitrary name given to create the object
address: address (0-7) of the device configured with address (pins A0, A1, A2)

Returns

none

Example

 
MCP onechip(1);  // create an object at address 1 called "one chip"
MCP twochip(2);  // create an object at address 2 called "two chip"

pinMode()

Description

Configure pin(s) as either input or output on the selected object (device specified by an address)

Syntax

object.name.pinMode(pin, mode);
or
object.name.pinMode(mode);

Parameters

object_name: the name given when this object was created

pin: the pin number (1-16) on which to set as input or output

mode: if a pin is specified, either a "HIGH" (1) for input (default) or a "LOW" (0) for output. If a pin is not specified, mode should be a word indicating the mode of each of the 16 I/O pins on the chip.

Returns

none

Example

 
void setup() {
  onechip.pinMode(4, HIGH); // sets pin 4 as an input
  onechip.pinMode(16, LOW); // sets pin 16 as an output
  twochip.pinMode(0B0000111100001111); // sets pins 1-4 and 9-12 as input, 5-8 and 13-16 as output
}

pullupMode()

Description

Configure the weak pull-up resistors on pins defined as inputs. This has no effect on pins that are configured as outputs.

Syntax

object_name.pullupMode(pin, mode);
or
object_name.pullupMode(mode);

Parameters

object_name: the name given when this object was created

pin: the pin number (1-16) on which to enable or disable the internal weak pull-up resistor

mode: if a pin is specified, either "HIGH" (1) to enable or "LOW" (0) to disable (default) the weak pull-up resistor on the specified pin. If a pin is not specified, mode should be a word indicating the pull-up mode of each of the 16 pins on the chip. Configuring pull-up has no effect on pins while they are configured as output.

Returns

none

Example

 
void setup() {
  onechip.pullupMode(4, HIGH); // enable the pull-up on pin 4
  twochip.pullupMode(0B0000111100000000); // enable the pull-ups on pins 9-12
}

inputInvert()

Description

Configure inversion on pins configured as inputs.

This will cause an inverted input pin to read as "LOW" (0) when it is actually in a high state, or as "HIGH" (1) when it is actually in a low state. This has no effect on pins that are configured as outputs.

Syntax

object_name.inputInvert(pin, inversion);
or
object_name.inputInvert(inversion);

Parameters

object_name: the name given when this object was created

pin: the pin number (1-16) on which to set or clear inversion

inversion: if a pin is specified, either "HIGH" (1) is specified to enable, or "LOW" (0) to disable (default) inversion on the specified pin. If a pin is not specified, mode should be a word indicating the inversion state of each of the 16 pins on the chip. Configuring inversion has no effect on pins while they are configured as output.

Returns

none

Example

 
void setup() {
  onechip.inputInvert(4, LOW); // disable inversion on pin 4
  twochip.inputInvert(0B0000000000001111); // enable inversion on pins 1-4
}

digitalWrite()

Description

Write a "HIGH" or "LOW" value to a digital I/O pin(s)

Syntax

object_name.digitalWrite(pin, value);
or
object_name.digitalWrite(value);

Parameters

object_name: the name given when this object was created

pin: the pin number (1-16) who's value will be set

value: if a pin is specified, either a "HIGH" (1) or a "LOW" (0) value may be set on the specified pin. If a pin is not specified, value should be a word indicating the output state of all 16 pins on the device. Writing pins configured as inputs has no effect.

Returns

none

Example

 
void loop() {
  onechip.digitalWrite(16, HIGH); // set pin 16 to "HIGH"
  twochip.digitalWrite(0B1100000000110000); // Set 5, 6, 15 & 16 to high, 7,8, 13 & 14 to low - inputs ignored
}

digitalRead()

Description

Reads the value of input pin(s), either "HIGH" ("1") or "LOW" ("0)

Syntax

object_name.digitalRead(pin);
or
object_name.digitalRead();

Parameters

object_name: the name given when this object was created

pin: the pin number (1-16) who's value will be read. If no pin number is supplied, a word will be read containing the input state of all pins. The values for pins configured as output should be disregarded if the "word-mode" version is used.

Returns

"HIGH" (1) or "LOW" (0) if a pin is supplied. a word (16 bits) is returned if no pin argument is given

Example

 
void loop() {
  int onevalue;
  int twovalue;

  onevalue = onechip.digitalRead(4); // assigns the value of pin4 to onevalue
  twovalue = twochip.digitalRead(); // assigns the value of all 16 I/O pins to twovalue
}

wordWrite()

Description

This is an advanced method to write a register pair in the MCP23S17. This class operates the MCP23S17 in "BANK=0" mode. The intention is that a registers for both ports may be written by supplying a single word as an argument. The low byte is written to the register address supplied, and the high byte to the next higher register address.

Syntax

object_name.wordWrite(base register, value);

Parameters

object_name: the name given when this object was created

base register: the beginning register address to write, for example, if 0x02 is given, the low byte of "value" will be written to 0x02 and the high byte of "value" to the register at 0x03

value: a word (unsigned int) that will be broken into two bytes and written to two consecutive registers, starting with the "base register" address

Returns

none

Example

 
void loop() {
  onechip.wordWrite(0x12, 0xFF00); // Set GPIOA to 0x00 and GPIOB to OxFF
}

byteWrite()

Description

This is an advanced method to write any single register in the MCP23S17.

Syntax

object_name.byteWrite(register, value);

Parameters

object_name: the name given when this object was created

register: the register address to write

value: a byte (unsigned char) that will be written to the specified registers

Returns

none

Example

 
void loop() {
  twochip.byteWrite(0x13, 0xF0); // Set GPIOB (portB) to 0xF0 (0B11110000) 
}

byteRead()

Description

This is an advanced method to read any single register in the MCP23S17.

Syntax

object_name.byteRead(register);

Parameters

object_name: the name given when this object was created

register: the register address to be read

Returns

unsigned char (uint8_t)

Example

 
void loop() {
  int twovalue;
  twovalue = twochip.byteRead(0x12); // Read GPIOA (portA)
}


Wiring Example:

Wiring a MCP23S17 can be tricky for beginners, so here is an example for wiring it up

Click here to open the image in full size

The important part besides the obvious VCC and GND connections is, that RESET has to be as well connected to the positive rail. The 23S17 uses (contrary to its sister-IC 23017) SPI instead of I²C, so it got 4 Pins for communication:

  • MISO - Master Input, Slave Output
  • MOSI - Master Output, Slave Input
  • SCK - Serial Clock
  • CS - Chip Select (aka: SS - Slave Select)

To find the correct Pins on your Arduino, look on the Mainpage of your Board (Arduino Uno, Arduino Nano, etc.) and scroll to the Section where the SPI-Pins are declared. In general they are on the following pins:

  • Arduino MISO: Pin 12
  • Arduino MOSI: Pin 11
  • Arduino SCK: Pin 13
  • Arduino CS: Pin 10

The second tricky part for beginners might be the adressing of this slave-device. The 23X17 got 3 Pins for adressing, so there are 8 different adresses possible. The adresses are selected by connecting each pin to either GND or the positive rail. In this manner we can give it the adresses from 0 (000 > A0 to GND, A1 to GND, A1 to GND) to 7 (111 > A1 to +, A2 to +, A3 to +). It might be a good idea to use a DIL-switch for this, as its easier to change adresses in case of conflicts when the circuit is soldered onto a PCB.


Code Example:

 
MCP onechip(1);    // create an object at address 1 called "onechip"
MCP twochip(2); // create an object at address 2 called "twochip"

void setup() {
  onechip.pinMode(4, HIGH); // sets pin 4 as an input
  onechip.pinMode(16, LOW); // sets pin 16 as an output
  twochip.pinMode(0B0000111100001111); // sets pins 1-4 and 9-12 as input, 5-8 and 13-16 as output

  onechip.pullupMode(4, HIGH); // enable the pull-up on pin 4
  twochip.pullupMode(0B0000111100000000); // enable the pull-ups on pins 9-12

  onechip.inputInvert(4, LOW); // disable inversion on pin 4
  twochip.inputInvert(0B0000000000001111); // enable inversion on pins 1-4
}

void loop() {
  int onevalue;
  int twovalue;

  onechip.digitalWrite(16, HIGH); // set pin 16 to "HIGH"
  twochip.digitalWrite(0B1100000000110000); // Set 5, 6, 15 & 16 to high, 7,8, 13 & 14 to low - inputs ignored

  onevalue = onechip.digitalRead(4); // assigns the value of pin4 to onevalue
  twovalue = twochip.digitalRead(); // assigns the value of all 16 I/O pins to twovalue

  /* These are for context only - use them only if you really know what you're doing
  onechip.wordWrite(0x12, 0xFF00); // Set GPIOA to 0x00 and GPIOB to OxFF
  twochip.byteWrite(0x13, 0xF0); // Set GPIOB (portB) to 0xF0 (0B11110000)
  twovalue = twochip.byteRead(0x12); // Read GPIOA (portA)
  */
}


Application Example:

Driving a 101 Segment LED Barmeter

This class makes is very simple to drive a whole 101 Segment LED Barmeter. In this Example we used a BL101-1004FX08 by www.barmeter.com. The 101 Segments are driven by 21 Pins on the Barmeter-Module: 8 of them are Cathodes and 13 are Anodes. With the 16 channels of our MCP23S17, we can drive all the anodes directly and have still 3 pins over. With these 3 Pins, we can control a 4051 as it has a 3-Bit input-adress for his 8 I/O pins. With those 8 additional pins on the other hand, we can now control all the cathodes from the barmeter. While this might not be the best Solution, it sure can solve the problem, and we can drive 101 Segments with just 4 Pins from the Arduino on the SPI-Bus.

Click here to open the image in full size

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