I2CKeyPad8x8

Arduino library for 8x8 or smaller KeyPad connected to an I2C PCF8575.

Description

Experimental (first tests ==> OK)

The I2CKeyPad8x8 library implements the reading of a 8x8 keypad by means of a PCF8575. Smaller keypads, meaning less columns or rows (e.g. 5x4) can be read with it too.

Breaking change

Since 0.3.0 the library can set a debounce threshold. If this is set (> 0) the getKey() and getChar() functions can return I2C_KEYPAD_THRESHOLD (255).

Related

Relates strongly to https://github.com/RobTillaart/I2CKeyPad. which is an 4x4 version using PCF8574.

• https://github.com/RobTillaart/PCF8575
• https://github.com/RobTillaart/AnalogKeypad
• https://github.com/RobTillaart/I2CKeyPad
• https://github.com/RobTillaart/I2CKeyPad8x8

Connection

The PCF8575 is connected between the processor and the (default) 8x8 keypad. See the conceptual schema below. It might take some trying to get the correct pins connected.

          PROC              PCF8575              KEYPAD
        +--------+        +---------+          +---------+
        |        |        |       0 |<-------->| R       |
        |    SDA |<------>|       . |<-------->| O       |
        |    SCL |<------>|       . |<-------->| W       |
        |        |        |       7 |<-------->| S       |
        |        |        |         |          |         |
        |        |        |       8 |<-------->| C       |
        |        |        |       . |<-------->| O       |
        |        |        |       . |<-------->| L       |
        |        |        |      15 |<-------->| S       |
        +--------+        +---------+          +---------+ 

I2C

I2C addresses

This library uses a PCF8575 chip.

Be careful to select an unique I2C address for every device on the bus.

I2C multiplexing

Sometimes you need to control more devices than possible with the default address range the device provides. This is possible with an I2C multiplexer e.g. TCA9548 which creates up to eight channels (think of it as I2C subnets) which can use the complete address range of the device.

Drawback of using a multiplexer is that it takes more administration in your code e.g. which device is on which channel. This will slow down the access, which must be taken into account when deciding which devices are on which channel. Also note that switching between channels will slow down other devices too if they are behind the multiplexer.

• https://github.com/RobTillaart/TCA9548

Interface

#include "I2CKeyPad8x8.h"

Base

• I2CKeyPad8x8(const uint8_t deviceAddress, TwoWire *wire = &Wire) The constructor sets the device address and optionally allows to selects the I2C bus to use.
• bool begin() The return value shows if the PCF8575 with the given address is connected properly. Call wire.begin() first!
• bool isConnected() returns false if the PCF8575 cannot be connected to.

getKey

• uint8_t getKey() Returns default 0..63 for regular keys, Returns I2C_KEYPAD8X8_NOKEY (64) if no key is pressed and and I2C_KEYPAD8X8_FAIL (65) in case of an error, e.g. multiple keys pressed. If a debounce delay is set (see below), it will return I2C_KEYPAD8X8_THRESHOLD (255) if the function is called too fast.
• uint8_t getLastKey() Returns the last valid key pressed 0..63, or I2C_KEYPAD8X8_NOKEY (64) which is also the initial value. This function does not "cache" failed keys.
• bool isPressed() Returns true if one or more keys of the keyPad are pressed. Note there is no check if multiple keys are pressed or just one. Returns false if no key is pressed or when there is a communication error. So checking getKey() yourself will give more information, but is slightly slower.

KeyMap functions

Note: loadKeyMap() must be called before getChar() and getLastChar() can be used!

• char getChar() returns the char corresponding to mapped key pressed. The function returns I2C_KEYPAD_THRESHOLD (255) if called too fast and a debounce threshold is set.
• char getLastChar() returns the last char pressed. This function is not affected by the debounce threshold.
• bool loadKeyMap(char * keyMap) keyMap should point to a (global) char array of length 67. This array maps index 0..63 on a character and index [64] maps to I2CKeyPad8x8_NOKEY (typical 'N') and index [65] maps I2CKeyPad8x8_FAIL (typical 'F'). Index 66 is the null char. This allows to define a keymap as a null terminated char array, e.g.

char keymap = "1234567890...NF";  //  ... stands for 50+ more chars.

kp8.loadKeyMap(keymap);

WARNING

If there is no key map loaded the user should NOT call getChar() or getLastChar() as these would return meaningless bytes.

Note: a keyMap char array may be longer than 66 characters, but only the first 66 are used. The length is NOT checked upon loading (as it may contain a NULL char).

See also future section below.

Debouncing threshold

Experimental

Since version 0.3.0, the library implements an experimental debounce threshold which is non-blocking.

If a key bounces, it can trigger multiple interrupts, while the purpose is to act like only one keypress. The debounce threshold results in a fast return of getKey() (with I2C_KEYPAD8x8_THRESHOLD) if called too fast.

The default value of the debounce threshold is zero to be backwards compatible. The value is set in milliseconds, with a maximum of 65535 ==> about 65 seconds or 1 minute. A value of 1 still allows ~1000 getKey() calls per second (in theory). A value of 65535 can be used e.g. for a delay after entering a wrong key code / password. Setting a high value might result in missed keypresses so use with care.

The default value of the debounce threshold is zero to be backwards compatible.

• void setDebounceThreshold(uint16_t value = 0) set the debounce threshold, value in milliseconds, max 65535. The default value is zero, to reset its value.
• uint16_t getDebounceThreshold() returns the set debounce threshold.
• uint32_t getLastTimeRead() returns the time stamp of the last valid read key (or NOKEY). This variable is used for the debounce, and may be used for other purposes too. E.g. track time between keypresses.

If a debounce threshold is set, and getKey() or getChar() is called too fast, these functions will return I2C_KEYPAD8x8_THRESHOLD (255).

Feedback welcome!

Basic working

After the keypad.begin() the sketch calls the keyPad.getKey() to read values from the keypad.

• If no key is pressed I2C_KEYPAD8x8_NOKEY code (64) is returned.
• If the read value is not valid, e.g. two keys pressed, I2C_KEYPAD8x8_FAIL code (65) is returned.
• If a debounce threshold is set, I2C_KEYPAD8x8_THRESHOLD (255) might be returned. See section above.
• Otherwise a number 0..63 is returned.

Note I2C_KEYPAD8x8_NOKEY, I2C_KEYPAD8x8_FAIL and I2C_KEYPAD8x8_THRESHOLD, all are 64 or beyond, all valid keys are below 64. This allows easy and fast checking for validity of keys.

Only if a key map is loaded, the user can call getChar() and getLastChar() to get mapped keys.

Interrupts

The library enables the PCF8575 to generate interrupts on the PCF8575 when a key is pressed. This makes checking the keypad far more efficient as one does not need to poll the device over I2C. See examples (TODO).

Smaller keypads

If one wants to connect a smaller keyPad e.g. a 4x4 to the PCF8575, one need to be sure to have the rows on P00..P07 and the columns on P10..P17 (or P08..P15).

This library does not support the usage of the "not used" pins, when connecting a smaller keypad than 8x8.

In issue #7 an idea is proposed to use the https://github.com/RobTillaart/I2CKeyPad with a PCF8575. It proposes to connect a 4x4 keypad to the P00..P07 pins. Warning: this idea is not confirmed to work yet, feedback is welcome.

Future

Must

• update documentation
• keep in sync with I2CKeyPad as much as possible.

Should

• test extensively
  • interrupts
  • keymapping
  • performance
• improve error handling
  • I2C_KEYPAD_ERR_MODE
  • I2C_KEYPAD_ERR_COMM (66?) or map to I2C_KEYPAD_FAIL?

Could

• add examples
  • from https://github.com/RobTillaart/I2CKeyPad?
• KeyMap
  • checking if NULL? ==> FAIL, how?
  • checking length of keymap during load.
  • default ASCII map (32..96?)
  • in PROGMEM?
• add uint8_t getAddress()

Wont

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Thank you,