The shift registers on Led Labs can be operated manually by switches as well as program code. As the 3 inputs are left floating random data can be seen on the 24 led propagation fountain unless a program is running, or the inputs are tied to a logic level. To operate manually with switches with help, say from the 555 timer providing the clock input, (or a switch could also be used here for single stepping to clock data in), this is where the switches on Led Labs come in useful as some are wired as pull-up or pull-down configurations. These configurations when connected to the inputs stop their logic levels from floating, meaning the pin is not subjected to noise or external influences causing spurious oscillations of the input pins. Normally this could be controlled by simply using a pull-up or pull-down resistor to tie it to a logic level. When using a switch it means that the user needs to either make the pin active low or active high whilst still stopping pin oscillations ‘floating’, so the pull-up pull-down resistor has a switch to force the pins state from a logic 0 to a logic 1 or vice versa. The button command 10k pulled down keeps the pins logic level at 0 until pressed forcing it to logic 1 or high, and the button command 10k pulled up keeps the logic level at 1 until pressed forcing it to logic 0 or low. When looking at data sheets for various IC’s and pins have a bar or line above the function of what it does, this means it is active low, so this can be tied to a logic level to either enable or disable the function of that pin, or connect to a switch to choose when to activate it.
To experiment with the shift registers, without code, manual switches can be used to see how data is clocked in, if a clock signal from the 555 timer is connected to the clock input of the shift register, set to about the max speed of the first dip switch setting of the 555 timer, and the data input to the shift registers were connected to the switch 10k pulled down on led labs. When this pulled down resistor and switch is rapidly pressed data represented by the leds can be seen propagating across the leds, If the switch is held down and let go, all data is clocked in (all leds lit) and then clears, the position of this switch determines if a 1 or 0 gets clocked in. The speed of the 555 timer setting can be adjusted to show this well, data is clocked in on the rising edge of the clock pulse. If the strobe is connected to the switch command 10k pulled down on led labs, as well; it means these two pins, strobe and data are in a low state, to send data in to the storage register for a 0 hold the strobe down and and release it, to shift a 1 in press the data switch then press down on the strobe switch, this can be done many times until the pattern required is represented by leds at the output latch. The 555 timer speed is best set for about 1-2 hz or quite slow to catch the data visually.
Some of the later revision boards have two buttons with leds attached near the shift register inputs, these can be used, however these are not tied to a level like the button command switches are. These tend to be ‘floating’ when just connected with a wire, until the switch is pressed.