201305.pdf 43279KB May 08 2013 11:07:04 PM

designideasConnect a 4×3 matrix keyboardto a microcontroller using two I/O pinsAruna Prabath Rubasinghe, University of Moratuwa, Moratuwa, Sri Lanka↘Matrix keyboards are commonas an input device in microcontroller-basedprojects. A conventionalway of connecting a matrix keyboardto a microcontroller is to use multipleI/O pins of the MCU. The MCU thenuses a scanning algorithm to identifywhich keys are pressed. A drawback ofthis method is that it requires a largenumber of the MCU’s I/O pins to connectthe keyboard. For example, toconnect a 4×3 keyboard requires sevendigital I/O pins. This becomes a problemwhen the project is based on alow-pin-count MCU or when theMCU being used does not have enoughfree I/O pins.Two solutions for this issue areavailable: Use readily available I/Oexpanders, or assign a unique voltageto each key using a resistor networkand then use an analog pin to readthe voltage and determine which keyis pressed. Each solution has its owndisadvantages.Since most of the time I/O expandersrequire a special communicationprotocol (I 2 C or SPI, for example) toread and write data, the MCU shouldhave built-in communication modules,or the user has to implement therelevant communication-protocol softwarewisely, which adds significantlyto the overhead of the MCU. On theD 21N4148D 11N4148D 31N4148D 51N4148D 41N4148CLOCKR 310k1413 ECLKIC 14017Q0 3Q1 2Q2 4Q3 7Q4 10Q5 1Q6 5Q7 6Q8 9Q9 11D 7D 8D 9R 11kR 41kR 51kR 61kABCD1 2 31 2 34 5 67 8 90 #D 61N4148R 747kINTERRUPTD 10NOTE: DIODES D 7 , D 8 , D 9 , AND D 10 ARE EACH A 1N4148.15MR12CO1413 ECLKIC 24017Q0 3Q1 2Q2 4Q3 7Q4 10Q5 1Q6 5Q7 6Q8 9Q9 1115MR12COFigure 1 This circuit for a 4×3 keyboard shows a more efficient architecture using two CD4017 Johnson counters with only twoI/O pins.52 EDN | MAY 2013 [ www.edn.com]