PCF8574, keypad, and Arduino connected together<\/figcaption><\/figure>\nThen the last step, the code:<\/p>\n
\r\n#include <Arduino.h>\r\n#include <Wire.h>\r\n \r\n\/\/ This is the address for the 'A' variant with all address pins not connected (high).\r\n\/\/ Check the datasheet for other addresses. http:\/\/www.ti.com\/lit\/ml\/scyb031\/scyb031.pdf\r\n#define PCF_ADDR (0x27)\r\n#define INTERRUPT_PIN (2)\r\n#define DEBOUNCE_DELAY (200) \/\/ milliseconds\r\n \r\nbool keyPressed = false;\r\nint32_t lastPress = 0;\r\n \r\n\/\/ Interrupt called when the PCF interrupt is fired.\r\nvoid keypress() {\r\n if (millis() - lastPress > DEBOUNCE_DELAY) {\r\n keyPressed = true;\r\n }\r\n}\r\n \r\n\/\/ The pins for each of the columns in the matrix below. P7 to P0.\r\nuint8_t kColumns[] = { B00000100, B00000001, B00010000 };\r\nuint8_t kRows[] = { B00000010, B01000000, B00100000, B00001000 };\r\nchar kMap[][3] = {\r\n { '1', '2', '3'},\r\n { '4', '5', '6'},\r\n { '7', '8', '9'},\r\n { '*', '0', '#'}\r\n};\r\n\/\/ This is the value which is sent to the chip to listen for all\r\n\/\/ key presses without checking each column.\r\nuint8_t intMask = 0;\r\n \r\n\/\/ The PCF chip only has one I2C address, writing a byte to it sets the state of the\r\n\/\/ outputs. If 1 is written then they're pulled up with a weak resistor, if 0\r\n\/\/ is written then a transistor strongly pulls it down. This means if connecting an LED\r\n\/\/ it's best to have the chip as the ground terminal.\r\n\/\/ If passing a byte value to this, eg B11111111 the left most bit is P7, the right\r\n\/\/ most P0.\r\nvoid write8(uint8_t val) {\r\n int error = 0;\r\n \/\/ This will block indefinitely until it works. This would be better done with a\r\n \/\/ timeout, but this is simpler. If the I2C bus isn't noisy then this isn't necessary.\r\n do {\r\n Wire.beginTransmission(PCF_ADDR);\r\n Wire.write(val);\r\n error = Wire.endTransmission();\r\n } while(error);\r\n}\r\n \r\nuint8_t read8() {\r\n uint8_t read = 0;\r\n do {\r\n read = Wire.requestFrom(PCF_ADDR, 1);\r\n if (read == 1) {\r\n return Wire.read();\r\n }\r\n } while (read != 1);\r\n}\r\n \r\nvoid setup() {\r\n Serial.begin(115200);\r\n \r\n Wire.begin();\r\n \/\/ When a pin state changed on the PCF it pulls its interrupt pin low.\r\n \/\/ It's an open drain so an input pullup is necessary.\r\n pinMode(INTERRUPT_PIN, INPUT_PULLUP);\r\n attachInterrupt(digitalPinToInterrupt(INTERRUPT_PIN), keypress, FALLING);\r\n \r\n \/\/ Calculate the interrupt mask described above.\r\n for (uint8_t c = 0; c < sizeof(kColumns) \/ sizeof(uint8_t); c++) {\r\n intMask |= kColumns;\r\n }\r\n intMask = ~intMask;\r\n write8(intMask);\r\n}\r\n \r\n\/\/ This goes through each column and checks to see if a row is pulled low.\r\nchar getKey() {\r\n for (uint8_t c = 0; c < sizeof(kColumns) \/ sizeof(uint8_t); c++) {\r\n \/\/ Write everything high except the current column, pull that low.\r\n write8(~kColumns);\r\n uint8_t val = read8();\r\n \/\/ Check if any of the columns have been pulled low.\r\n for (uint8_t r = 0; r < sizeof(kRows) \/ sizeof(uint8_t); r++) {\r\n if (~val & kRows[r]) {\r\n return kMap[r];\r\n }\r\n }\r\n }\r\n return '\0';\r\n}\r\n \r\nvoid loop() {\r\n if (keyPressed) {\r\n char key = getKey();\r\n \/\/ Key may not be read correctly when the key is released so check the value.\r\n if (key != '\0') {\r\n Serial.println(key);\r\n lastPress = millis();\r\n }\r\n keyPressed = false;\r\n }\r\n \/\/ After getKey this needs to be called to reset the pins to input pullups.\r\n write8(intMask);\r\n}\r\n<\/pre>\nWhen uploaded and everything wired together the sketch will output the pressed key to the serial console.<\/p>\n
<\/p>\n
Edited 3\/12\/2018: Updated CPP code to compile properly.<\/p>\n","protected":false},"excerpt":{"rendered":"
Interfacing with a keypad takes a lot of IO doesn’t it? There isn’t really space to do much else on an Arduino of ESP8266 after having 7 or 8 pins taken for a keypad. Luckily there’s a cheap solution for this one, using an I2C I\/O expander. In this case the PCF8574AP\u00b7 The ‘A’ being […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[1],"tags":[],"_links":{"self":[{"href":"https:\/\/tstableford.co.uk\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/314"}],"collection":[{"href":"https:\/\/tstableford.co.uk\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/tstableford.co.uk\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/tstableford.co.uk\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/tstableford.co.uk\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=314"}],"version-history":[{"count":4,"href":"https:\/\/tstableford.co.uk\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/314\/revisions"}],"predecessor-version":[{"id":350,"href":"https:\/\/tstableford.co.uk\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/314\/revisions\/350"}],"wp:attachment":[{"href":"https:\/\/tstableford.co.uk\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=314"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/tstableford.co.uk\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=314"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/tstableford.co.uk\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=314"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
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<\/a><\/p>\n![\"PCF8574](\"https:\/\/tstableford.co.uk\/downloads\/PCF8574-pins.gif\")
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