I wont have the shift registers for about a week. Here is a test circuit which serves as an example key interface. Note the two connections named Clock/Data and Shift/Load. These connect directly to the microcontroller's IO pins. All we need here are two pins. One of which is output only and the other I/O. Pin 10 of the shift register is connected to 5vdc. This effectively shifts logical ones through the circuit. This can help in the debug of the driver since the default state of the switch data is logical 1 via the pull up resistor network. Keep in mind that the device is negative true. That means that a zero bit indicates a button is being pressed. As mention earlier, this can be corrected by simply complimenting the value within the driver routine. This circuit can be easily expanded by wiring Qh of a lessor stage to SER of the greater stage. The resistor between Qh and the clock input must also be used on the last stage. If i recall correctly the highest bit is shifted out first.
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