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keybrd/IOE_scan_development/openDrain_activeLow/openDrain_activeLow.ino

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Arduino
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/* simulate one scan of a key martix, on open-drain active-low I/O expander
BREADBOARD SETUP *******************************
Teensy LC connected to MCP23018 I/O expander via I2C
4.7k Ohm pullup resistors on SDA and SCL
use volt meter to measure pin voltages
MCP23018 has open-drain outputs (open-drain can only sink current)
MCP23018 PIN DIAGRAM ***************************
write port B read port A
GND VSS 1 28 NC
NC 2 27 GPA7
jumper0 GPB0 3 26 GPA6
GPB1 4 25 GPA5
GPB2 5 24 GPA4 jumper4
GPB3 6 23 GPA3
jumper4 GPB4 7 22 GPA2
GPB5 8 21 GPA1
GPB6 9 20 GPA0 jumper0
GPB7 10 19 INTA
power VDD 11 18 INTB
SCL SCL 12 17 NC
SDA SDA 13 16 RESET power
NC 14 15 ADDR GND
*/
#include "Wire.h"
const uint8_t ADDR = 0x20; //MCP23018 I2C address with ADDR pin grounded
void setup()
{
delay(1000); //time for Serial print to work
// ================= configure ================
Serial.print("config ");
Wire.begin();
Wire.beginTransmission(ADDR);
Wire.write(0x01); //IODIRB Configure
Wire.write(0); //as output
Wire.endTransmission();
Wire.beginTransmission(ADDR);
Wire.write(0x00); //IODIRA Configuration
Wire.write(~0); //as input
Wire.endTransmission();
Wire.beginTransmission(ADDR);
Wire.write(0x0C); //GPPUA pull-up
Wire.write(~0); //pull-up enabled
Wire.endTransmission();
// =================== scan ===================
Serial.println("scan");
Wire.beginTransmission(ADDR);
Wire.write(0x13); //GPIOB output
Wire.write(B00001111); //pins 0-3 off, pins 4-7 sink on (strobe, LED on)
Wire.endTransmission();
Wire.beginTransmission(ADDR);
Wire.write(0x12); //GPIOA (immediately before requestFrom)
Wire.endTransmission();
Wire.requestFrom(ADDR, static_cast<uint8_t>(1)); //request one byte from GPIOA read
Serial.print("portA=");
Serial.println(Wire.read(), BIN); //prints portA=11101111
}
void loop() { }