Shift, then latch

Three pins, eight outputs

The chip between the display and the board, U1, is a 74HC595. It is why the module needs three signal wires and not eight. Inside it are two registers, one behind the other:

• a shift register, eight stages long, which takes one bit at a time;
• an output register, eight bits wide, which is what the eight outputs, and so the segments, actually follow.

Each has its own clock. CLOCK (SRCLK) moves the bit on DATAIN (SER) into the shift register. LATCH (RCLK) copies all eight stages into the output register at once, on its rising edge.

With the latch at the end, the top row fills one stage at a time while the bottom row, the one the segments follow, does not move. Eight ticks, one latch, and the digit changes in a single step.

Set the latch to fire after every bit and the digit shows the byte arriving: one half-finished pattern after another. Nothing is loose. The register is doing what the sketch told it to.

In code

digitalWrite(LAT, LOW);                  // freeze the segments
shiftOut(SER, CLK, MSBFIRST, pattern);   // eight bits, one at a time
digitalWrite(LAT, HIGH);                 // all eight appear together

shiftOut() is a loop in the Arduino library: for each bit it sets the data pin, raises the clock and lowers it. The two digitalWrite calls around it are the part worth understanding, because their position is the difference between a clean update and a sweep.

The segments still need a supply before any of this shows. A byte that is latched but dark means the 12 V or the dimmer is missing, not that the byte is wrong: the register holds its byte whether or not the digit is lit.

Two pins tied on the board

On this board U1's output-enable pin is tied to GND, so the outputs are always on, and its clear pin is tied to the logic rail, so nothing can clear the register. The only way to blank the digit by software is to shift in zeros and latch them. The only other way is the PWM pin, which switches the segments' supply and leaves the register alone.

Anything clocked in beyond eight bits comes out of the register's last stage, which the board wires to the DATAOUT pin. That is what the chain pages build on.