Lesson 5. A shift register: 74HC595 and 8 lamps over three wires

After this lesson the library has the 74HC595 — the very chip an Arduino uses to light eight LEDs over three wires — and the canvas has eight lamps that you fill bit by bit and show at once.

5.1. The idea: bits over one wire

A shift register is eight flip-flops in a chain: on every SHCP edge each one takes its neighbour's value, and the first takes the bit from DS. Eight edges — and the register holds a whole byte that came over one wire. The last flip-flop comes out on the Q7S pin: feed it to DS of the next 74HC595 — and you get 16, 24, 100 outputs over the same three wires.

5.2. The idea: a latch shows everything at once

While a byte is shifting in, its bits run through all eight positions. If the lamps sat right on the shift register, they would flicker. So the 74HC595 has a second row of eight flip-flops — the latch: on the STCP edge it copies the whole byte at once, and the lamps change only once.

5.3. The idea: the third state

The output of an ordinary gate is always 0 or 1. The 74HC595 output can be disconnected (OE = 1): the wire belongs to nobody. That is needed when several chips sit on the same wires — only one talks. A disconnected wire is neither 0 nor 1 by itself, so a resistor to ground is put on the board. Inside, the switch is a Transmission gate: at OE = 0 it connects the latch to the pin, at OE = 1 it opens.

5.4. Building

The same project as in lesson 4: the library has the 74HC04 and the 74HC74.

Step 1. Chip 74HC595 — shift register with a latch

Package pins from the datasheet: 1 Q1, 2 Q2, 3 Q3, 4 Q4, 5 Q5, 6 Q6, 7 Q7, 9 Q7S, 10 MR, 11 SHCP, 12 STCP, 13 OE, 14 DS, 15 Q0; 8 — GND, 16 — VCC. DS — data, SHCP — shift (edge), STCP — latch (edge), MR — clears the shift register (active 0), OE — enables the outputs (active 0), Q7S — serial output (for a chain of 74HC595). Inside: 16 flip-flops — eight 74HC74 (8 for shifting, 8 for the latch), a 74HC04 and eight Transmission gate switches — the third state: at OE = 1 the outputs are disconnected from the latch.

Ctrl+G → name 74HC595. Legs: inputs MR, SHCP, STCP, OE, DS; outputs Q1, Q2, Q3, Q4, Q5, Q6, Q7, Q0.

Step 2. Three wires and buttons

Switch DATA is the bit we shift in. The Shift button is SH, Latch — ST, Clear — CLRB. Switch OFF turns the outputs off (OE).

Step 3. Clear through a 74HC04

The 74HC595 MR is active 0, the button gives 1: we invert it.

Step 4. 74HC595

Outputs Q0–Q7 go to lamps L0–L7, Q7S to lamp SO.

Step 5. Resistors to ground

When the outputs are off (OE = 1), the wire from the 74HC595 is connected to nothing — the third state. By itself it is neither 0 nor 1, so on a real board you put a resistor to ground: it pulls the disconnected wire to 0. Resistor from the palette — between L<n> and GND.

Step 6. Lamps

Eight lamps in a row, L7 on the left — like a byte: the top bit on the left.

5.5. Check

1. Press Clear, then Latch: all lamps go out.
2. Shift in the byte 1011 0010 — top bit first: for each bit set DATA (1, 0, 1, 1, 0, 0, 1, 0) and press Shift. Lamps L0–L7 do not change — the data is still in the shift register. Lamp SO shows the bit coming out at the end.
3. Press Latch: the lamps show 1011 0010 at once (L7 on the left).
4. OFF = 1 — all lamps go out: the outputs are disconnected, the resistors pull the wires to 0. OFF = 0 — 1011 0010 again.

Why it is handy: the byte goes in over one wire, and the lamps change only on Latch — all at once, without flicker. Three wires — DATA, Shift, Latch — instead of eight; that is how an Arduino drives eight LEDs, and a chain of 74HC595 through Q7S — even a hundred.

Lesson 5 paste (Cmd+I)

On top of lesson 4.

# Lesson 5 — 74HC595: a shift register with a latch; 8 lamps over three wires, like Arduino.# Paste: Cmd+I on top of lesson 4. chip 74HC595 {  portIn(MR)  portIn(SHCP)  portIn(STCP)  portIn(OE)  portIn(DS)  portOut(Q1)  portOut(Q2)  portOut(Q3)  portOut(Q4)  portOut(Q5)  portOut(Q6)  portOut(Q7)  portOut(Q0)  portOut{name=Q7S}(s7)  74HC74(1RD=MR, 1D=DS, 1CP=SHCP, 1SD=VCC, 1Q=s0, 1QN=sn0, 2QN=sn1, 2Q=s1, 2SD=VCC, 2CP=SHCP, 2D=s0, 2RD=MR)  74HC74(1RD=MR, 1D=s1, 1CP=SHCP, 1SD=VCC, 1Q=s2, 1QN=sn2, 2QN=sn3, 2Q=s3, 2SD=VCC, 2CP=SHCP, 2D=s2, 2RD=MR)  74HC74(1RD=MR, 1D=s3, 1CP=SHCP, 1SD=VCC, 1Q=s4, 1QN=sn4, 2QN=sn5, 2Q=s5, 2SD=VCC, 2CP=SHCP, 2D=s4, 2RD=MR)  74HC74(1RD=MR, 1D=s5, 1CP=SHCP, 1SD=VCC, 1Q=s6, 1QN=sn6, 2QN=sn7, 2Q=s7, 2SD=VCC, 2CP=SHCP, 2D=s6, 2RD=MR)  74HC74(1RD=VCC, 1D=s0, 1CP=STCP, 1SD=VCC, 1Q=r0, 1QN=rn0, 2QN=rn1, 2Q=r1, 2SD=VCC, 2CP=STCP, 2D=s1, 2RD=VCC)  74HC74(1RD=VCC, 1D=s2, 1CP=STCP, 1SD=VCC, 1Q=r2, 1QN=rn2, 2QN=rn3, 2Q=r3, 2SD=VCC, 2CP=STCP, 2D=s3, 2RD=VCC)  74HC74(1RD=VCC, 1D=s4, 1CP=STCP, 1SD=VCC, 1Q=r4, 1QN=rn4, 2QN=rn5, 2Q=r5, 2SD=VCC, 2CP=STCP, 2D=s5, 2RD=VCC)  74HC74(1RD=VCC, 1D=s6, 1CP=STCP, 1SD=VCC, 1Q=r6, 1QN=rn6, 2QN=rn7, 2Q=r7, 2SD=VCC, 2CP=STCP, 2D=s7, 2RD=VCC)  74HC04(1A=OE, 1Y=en, 2A=GND, 3A=GND, 4A=GND, 5A=GND, 6A=GND)  tgate(r0, en, Q0)  tgate(r1, en, Q1)  tgate(r2, en, Q2)  tgate(r3, en, Q3)  tgate(r4, en, Q4)  tgate(r5, en, Q5)  tgate(r6, en, Q6)  tgate(r7, en, Q7)} input(DATA)button{label="Shift"}(SH)button{label="Latch"}(ST)button{label="Clear"}(CLRB)input(OFF)74HC04(1A=CLRB, 1Y=nCLRB, 2A=GND, 3A=GND, 4A=GND, 5A=GND, 6A=GND)74HC595(DS=DATA, SHCP=SH, STCP=ST, MR=nCLRB, OE=OFF, Q0=L0, Q1=L1, Q2=L2, Q3=L3, Q4=L4, Q5=L5, Q6=L6, Q7=L7, Q7S=SO)resistor(L0, GND)resistor(L1, GND)resistor(L2, GND)resistor(L3, GND)resistor(L4, GND)resistor(L5, GND)resistor(L6, GND)resistor(L7, GND)led(L0)led(L1)led(L2)led(L3)led(L4)led(L5)led(L6)led(L7)led(SO)