Shift, then latch

Two registers, not one

The chip on the board holds a byte twice over.

The first copy is the shift register: eight cells in a row, each handing its bit to the next one every time SRCLK goes high. Bits enter at one end on SER and walk along. After eight pulses the whole byte is in.

The second copy is the output register, and it is what the eight segment pins actually show. It does not change while the shift register is filling. It changes once, completely, when RCLK goes high.

Run it and watch which of the two rows moves:

Why the second register exists

Without it, every clock pulse would be visible on the pins. Sending a 3 after a 7 would put four or five wrong shapes on the glass on the way, and at the rate a clock updates you would see it as a permanent flicker.

With it, the display goes from one digit straight to the next. Nothing in between is ever shown, because nothing in between ever reaches the pins.

In code that is three lines, and the order is not negotiable:

digitalWrite(PIN_RCLK, LOW);                     // hold the outputs still
shiftOut(PIN_SER, PIN_SRCLK, MSBFIRST, value);   // eight bits, eight clocks
digitalWrite(PIN_RCLK, HIGH);                    // now show them, all at once

shiftOut is an Arduino function, not a feature of the chip. It wiggles the data pin and the clock pin eight times and then returns. The latch is yours.

MSBFIRST decides which bit lands where

The third argument to shiftOut says which end of the byte goes down the wire first. MSBFIRST sends bit 7 first, and because each pulse pushes everything one place further along, the bit sent first ends up furthest from the input — on the chip's last output.

So with MSBFIRST, bit 7 lands on the output the board labels H and bit 0 on the one labelled A. That is the mapping every segment table on the internet assumes, and it is why one byte draws one digit without any rearranging.

Send LSBFIRST by mistake and every byte arrives mirrored. Nothing is damaged and nothing is miswired — you simply get a different, recognisable, wrong shape, which is the easiest fault in this book to identify.