Calling keyscan() returns a uint8_t, whose value is 0xff if no keys are being pressed at the time. The TM1637 can only see one key press at a time, and there is no "rollover". If a key is pressed, then the values are as follows:
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keyscan results are reversed left for right from the data sheet.
To modify a "generic" TM1637 board for use with a keyboard, you must add connections to either or both of pins 19 and 20 (these are the "row" selects) and then to as many of pins 2 through 9 (the "columns") as needed. It is easiest to connect to the "column pins" (2-9) by picking them up where they connect to the LED displays (see second photo). Generic keyboards that are a 4x4 matrix won't work; the TM1637 can only scan a 2x8 matrix. Of course, fewer keys are acceptable; I use a 1x4 keyboard in my projects.
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Further, the TM1637 chip needs a fairly hefty pull-up on the DIO pin for the keyscan() routine to work. There is no pull-up in the TM1637 itself, and the clone boards don't seem to have one either, despite the data sheet calling for 10K ohms pull-ups on DIO and CLOCK. 10K is too weak anyway. The slow rise-time of the DIO signal means that the "true/high" value isn't reached fast enough and reliably enough for the processor to read it correctly. The new pull-up reduces the rise time of the signal, so that true/high values are achieved in a few microseconds. I find that a 1K (1000) ohm resistor from DIO to 3.3 v works well. This is perfect with a 3.3 volt processor like the ESP8266 or ESP32, and a 5V Atmega 328 ("Arduino UNO") family processor is happy with that as well.
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The TM1637 boards want to be run off of 5 volts, regardless of what the processor voltage is. Their logic levels are compatible with 3.3 volt processors, and they need 5 volts to make sure the LEDs light up.
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The unmodified generic TM1637 board (front and back).</br>
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The modified generic TM1637 board with connector for 1x4 keyboard. The blue wire is bringing out pin 19 (k1). Four segments/columns are picked up from the LEDs.</br>
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The 4 button keyboard plugged into the TM1637 board.</br>
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Scope photo showing slow rise time of DIO pin (upper trace) on the unmodified TM1637. The lower trace is the CLK. The 8 fast CLK pulses on the left represent the 0x42 command to read keyboard being sent to the TM1637.</br>
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Scope photo showing faster rise time of DIO pin (upper trace) with 1000 ohm pull-up on DIO. In both scope photos, the F5 key is pressed; the bits are least significant bit (LSB) first, so read as 10101111 left to right.</br>
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The scope photos were taken using the TM1637_keyscan_raw example, with the scope trigger hooked to the TRIGGER pin, and the two channel probes hooked to DIO and CLK. Vertical sensitivity is 2v/division, horizontal timebase is 20usec/division.
