Shift Registers

This demonstration introduces a time or history element. The initial application uses a key press to send a lit LED down a shift register. Because a manual key press can be long in duration, a pulse generator is used to limit a derived pulse to no more time than until the next incr, when action is taken. Shifting is only enabled by incr is high in order to slow the light changes down to be visible.

The extension is more challenging. This uses a second button (button 3) to send a lit LED the opposite way down the shift register, and hence change the shifting direction. Both buttons require pulse generators. Care has been taken to ensure the first press causes a LED to light, not just change shifting direction. There are two ways to code this, a single all in one if clause which is complicated, or breaking the logic out into two if clauses, one for the general shifting and a second for poking the ‘1’ into the correct end of the shift register. This second formulation relies on understanding how VHDL signals simulate; that the assignments actually occur on the clock edge not before, hence a later assignment can override an earlier one.