Lesson 4. A decoder in a package: 74HC138 and a running light

After this lesson the library has the 74HC138 — a 3-to-8 decoder — and the canvas has 16 lamps and a running light: a counter chooses the number, two 74HC138 put out the right lamp.

4.1. The idea: number → one wire

Three wires A0 A1 A2 are a number 0–7. The decoder chooses the one output of eight with this number. That is how a processor chooses a memory chip or a device by its address: the top address bits go to a decoder, its outputs go to the "select" inputs of the chips.

4.2. The idea: active 0

The chosen 74HC138 output is 0, the others are 1. In the datasheet Y0…Y7 have a bar. It is more convenient this way: the "select" input of memories and devices also acts on a zero, so the wire goes straight. The enable inputs differ: E1, E2 act on a zero, E3 on a one. That is done on purpose.

4.3. The idea: 4 → 16 from two 3 → 8

The top bit of the number goes to E1 of one chip and to E3 of the other: at 0 the first works, at 1 the second. No extra gates — that is why the 74HC138 has three enable inputs. Decoders with 32 and 64 outputs are built the same way.

4.4. Building

The same project as in lesson 3: the library has the gates and the chips of lessons 1–3.

Step 1. Chip 74HC138 — 3-to-8 decoder

Package pins from the datasheet: 1 A0, 2 A1, 3 A2, 4 E1, 5 E2, 6 E3, 7 Y7, 9 Y6, 10 Y5, 11 Y4, 12 Y3, 13 Y2, 14 Y1, 15 Y0; 8 — GND, 16 — VCC. A0 A1 A2 is a number (0–7); the output with this number becomes 0, the others are 1 (the outputs are active 0). Enable: E1, E2 — active 0, E3 — active 1; when not enabled, all outputs are 1. Inside — packages from lessons 1–2: 74HC04 (inverters), two 74HC08 (combinations and enable), two 74HC00 (outputs).

Ctrl+G → name 74HC138. Legs: inputs A0, A1, A2, E1, E2, E3; outputs Y7, Y6, Y5, Y4, Y3, Y2, Y1, Y0.

Step 2. Clock and reset

Clock — 2Hz. The Reset button is CLR, through a 74HC04: the 74HC74 reset acts on a zero.

Step 3. Counter 0–15

Two 74HC74 — the same divider as in lesson 2: Q3 Q2 Q1 Q0 count 0–15.

Step 4. Two 74HC138

The three low bits are the number within a group of eight, on A0 A1 A2 of both. The top bit Q3 chooses which one works: on the first it goes to E1 (works at 0), on the second to E3 (works at 1). That is what the enable inputs are for: two 3-to-8 decoders make a 4-to-16 without a single extra gate.

Step 5. 16 lamps

Lamps L0–L15 go straight to the outputs. Put them side by side, in order.

4.5. Check

1. Clock — 2Hz. Press Reset: all lamps are lit except L0.
2. The dark lamp runs: L1, L2 … L15, then L0 again — one per CLK edge.
3. At L7 → L8 the shadow jumps to the second 74HC138: Q3 became 1.

Why a shadow runs and not a light: the 74HC138 output is active 0, the chosen output goes out. On a real board an LED is connected to VCC through a resistor — and it lights exactly on a zero. There a light would run.

Lesson 4 paste (Cmd+I)

On top of lesson 3.

# Lesson 4 — 74HC138: a 3-to-8 decoder; a running light on 16 lamps (two 74HC138, a counter on two 74HC74).# Paste: Cmd+I on top of lesson 3. chip 74HC138 {  portIn(A0)  portIn(A1)  portIn(A2)  portIn(E1)  portIn(E2)  portIn(E3)  portOut(Y7)  portOut(Y6)  portOut(Y5)  portOut(Y4)  portOut(Y3)  portOut(Y2)  portOut(Y1)  portOut(Y0)  74HC04(1A=A0, 1Y=nA0, 2A=A1, 2Y=nA1, 3A=A2, 3Y=nA2, 4Y=nE1, 4A=E1, 5Y=nE2, 5A=E2, 6A=GND)  74HC08(1A=nA1, 1B=nA0, 1Y=t0, 2A=nA1, 2B=A0, 2Y=t1, 3Y=t2, 3A=A1, 3B=nA0, 4Y=t3, 4A=A1, 4B=A0)  74HC08(1A=nE1, 1B=nE2, 1Y=g, 2A=g, 2B=E3, 2Y=G, 3Y=G0, 3A=G, 3B=nA2, 4Y=G1, 4A=G, 4B=A2)  74HC00(1A=G0, 1B=t0, 1Y=Y0, 2A=G0, 2B=t1, 2Y=Y1, 3Y=Y2, 3A=G0, 3B=t2, 4Y=Y3, 4A=G0, 4B=t3)  74HC00(1A=G1, 1B=t0, 1Y=Y4, 2A=G1, 2B=t1, 2Y=Y5, 3Y=Y6, 3A=G1, 3B=t2, 4Y=Y7, 4A=G1, 4B=t3)} clock(CLK)button{label="Reset"}(CLR)74HC04(1A=CLR, 1Y=nCLR, 2A=GND, 3A=GND, 4A=GND, 5A=GND, 6A=GND)74HC74(1RD=nCLR, 1D=Q0N, 1CP=CLK, 1SD=VCC, 1Q=Q0, 1QN=Q0N, 2QN=Q1N, 2Q=Q1, 2SD=VCC, 2CP=Q0N, 2D=Q1N, 2RD=nCLR)74HC74(1RD=nCLR, 1D=Q2N, 1CP=Q1N, 1SD=VCC, 1Q=Q2, 1QN=Q2N, 2QN=Q3N, 2Q=Q3, 2SD=VCC, 2CP=Q2N, 2D=Q3N, 2RD=nCLR)74HC138(A0=Q0, A1=Q1, A2=Q2, E1=Q3, E2=GND, E3=VCC, Y0=L0, Y1=L1, Y2=L2, Y3=L3, Y4=L4, Y5=L5, Y6=L6, Y7=L7)74HC138(A0=Q0, A1=Q1, A2=Q2, E1=GND, E2=GND, E3=Q3, Y0=L8, Y1=L9, Y2=L10, Y3=L11, Y4=L12, Y5=L13, Y6=L14, Y7=L15)led(L0)led(L1)led(L2)led(L3)led(L4)led(L5)led(L6)led(L7)led(L8)led(L9)led(L10)led(L11)led(L12)led(L13)led(L14)led(L15)