I have used Nick Gammon's (MAX7219) and (bitBangedSPI) libraries to drive SPI 7-segment 8 digit led modules successfully from the Arduino Uno. I would like to extend the number of Slave Select (CS) choices by using the MCP23S17 chip. The Mcp23s17 libraries to use might be https://github.com/dreamcat4/Mcp23s17 or https://github.com/MajenkoLibraries/MCP23S17. I have found the stackExchange entry How do you use SPI on an Arduino? to be helpful and this entry to be helpful http://www.ermicro.com/blog/?p=1050, but all the examples just turn on LED lights, not digits of 7-segment led modules.
I realize I might have to modify the MAX7219 and bitBangedSPI libraries where the digitalWrite command is used. However, I'm not sure how to modify or drive these 7-segement modules using the MCP23S17 chip.
A skeleton script to display PI selected on pin 6 on the Uno and address 0x0 of the MCP23S17 is shown below. I would like to choose which pinout to utilize as well but am not sure how this would be coded.
#include <SPI.h>
#include <bitBangedSPI.h>
#include <MAX7219.h>
// Mcp23s17 library available from https://github.com/dreamcat4/Mcp23s17
#include <Mcp23s17.h>
// Wire up the SPI Interface common lines:
// #define SPI_MOSI 7 //arduino <-> SPI Master Out Slave In -> SI (Pin 13 on MCP23S17 DIP)
// #define SPI_MISO not used 12 //arduino <-> SPI Master In Slave Out -> SO (Pin 14 on MCP23S17 DIP)
// #define SPI_CLOCK 8 //arduino <-> SPI Slave Clock Input -> SCK (Pin 12 on MCP23S17 DIP)
// Then choose any other free pin as the Slave Select (pin 10 if the default but doesn't have to be)
#define MCP23S17_SLAVE_SELECT_PIN 6 //arduino <-> SPI Slave Select -> CS (Pin 11 on MCP23S17 DIP)
// MULTIPLE DEVICES
// Up to 8 MCP23S17 devices can share the same SPI bus and slave select pins.
// Assign each chip a unique 3-bit device address (by setting the A2,A1,A0 pins)
// Then below, device address is optional 2nd parameter to the constructor fn...
// MCP23S17 Mcp23s17_0 = MCP23S17(MCP23S17_SLAVE_SELECT_PIN,0x0);
// ...
// MCP23S17 Mcp23s17_7 = MCP23S17(MCP23S17_SLAVE_SELECT_PIN,0x7);
// SINGLE DEVICE
// Instantiate a single Mcp23s17 object
MCP23S17 Mcp23s17_0 = MCP23S17(MCP23S17_SLAVE_SELECT_PIN,0x0);
const byte chips = 1;
static char outputBuffer[20]; // outputBuffer[] likely 2x the size it needs to be.
// 1 chip, bit banged SPI on pins 6, 7, 8
MAX7219 display(chips, 6, 7, 8); // Chips / LOAD / DIN / CLK
void setup()
{
// Set all pins to be outputs (by default they are all inputs)
Mcp23s17_0.pinMode(OUTPUT);
display.begin();
display.setIntensity(1);
float pi = PI;
dtostrf(pi, 7, 7, outputBuffer);
display.sendString(outputBuffer);
} // end of setup
void loop() { }
An example of the 8-digit 7-segment display. They are available on Amazon, Ebay, AliExpress etc. This one has 0.33 inch digits; other modules are 0.56 inch digits.
