Lesson 3. A counter in a package: 74HC390 and a stopwatch

After this lesson the library has the 74HC86 (four XORs) and the 74HC390 — two decade counters in one package — and the canvas has a stopwatch: a start/stop switch, a Reset button and two digits, 00–99.

3.1. The idea: ÷2 and ÷5 make ÷10

The decade counter in the 74HC390 is cut into two parts:

• the ÷2 stage — one flip-flop (Q0), clock CP0;
• the ÷5 stage — three flip-flops (Q1 Q2 Q3), clock CP1, counting 0, 1, 2, 3, 4, 0.

Connect Q0 to CP1 and you get 0–9 in binary (Q3 Q2 Q1 Q0). The parts are separate on purpose: the same chip can make ÷2, ÷5 or ÷10 depending on how you connect it.

3.2. The idea: clocking on the fall

The 74HC390 flip-flops act when the clock falls from 1 to 0 (in the datasheet CP0 and CP1 have a bar). That is handy for a chain: the top bit Q3 falls exactly when the counter goes 9 → 0 — and this fall clocks the next counter, the tens. Inside we make the fall from an edge: a 74HC04 inverts the clock, and then it is an edge for the 74HC74.

In fact everything inside the 74HC390 is chips from lessons 1–2: two 74HC74 per counter, one 74HC04 for six inversions, 74HC02, 74HC08 and the new 74HC86 (four XORs) for the ÷5 stage. We build from packages, as on a real board.

3.3. The idea: reset on a one

The 74HC74 reset acted on a zero; the 74HC390 MR acts on a one. That happens: every chip follows its own datasheet. So here the reset button is connected directly, without an inverter.

3.4. Building

The same project as in lesson 2: the library has the gates, 74HC00–74HC32 and 74HC74.

Step 1. Chip 74HC86 — 4 × exclusive OR (XOR)

Package pins from the datasheet: 1 1A, 2 1B, 3 1Y, 4 2A, 5 2B, 6 2Y, 8 3Y, 9 3A, 10 3B, 11 4Y, 12 4A, 13 4B; 7 — GND, 14 — VCC. Like the 74HC00, only with four XORs inside: the output is 1 when the inputs differ. It is needed inside the 74HC390.

Ctrl+G → name 74HC86. Legs: inputs 1A, 1B, 2A, 2B, 3A, 3B, 4A, 4B; outputs 1Y, 2Y, 3Y, 4Y.

Step 2. Chip 74HC390 — 2 × decade counter (÷2 and ÷5)

Package pins from the datasheet: 1 1CP0, 2 1MR, 3 1Q0, 4 1CP1, 5 1Q1, 6 1Q2, 7 1Q3, 9 2Q3, 10 2Q2, 11 2Q1, 12 2CP1, 13 2Q0, 14 2MR, 15 2CP0; 8 — GND, 16 — VCC. CP0, CP1 — clock on the fall (in the datasheet they have a bar), MR — reset, active 1. Inside each counter: a ÷2 stage (Q0) and a ÷5 stage (Q1 Q2 Q3, counting 0–4). Everything inside is chips from lessons 1–2: flip-flops — 74HC74 (two per counter); six inverters — one 74HC04 (a falling clock = a rising inverted clock, reset 1 → 0 for the 74HC74); NOR — 74HC02, AND — 74HC08, exclusive OR — 74HC86 (one gate per counter, the free ones to GND).

Ctrl+G → name 74HC390. Legs: inputs 1CP0, 1MR, 1CP1, 2CP1, 2MR, 2CP0; outputs 1Q0, 1Q1, 1Q2, 1Q3, 2Q3, 2Q2, 2Q1, 2Q0.

Step 3. Clock, switch and button

Clock — 1Hz: the stopwatch counts seconds. Switch RUN — start/stop, the Reset button is RST.

Step 4. Start / stop — 74HC08

TICK = CLK AND RUN: while RUN = 0, the clock does not reach the counter.

Step 5. Units and tens — one 74HC390

The first counter is the units: the clock goes to 1CP0 (÷2), its 1Q0 to 1CP1 (÷5): together 0–9. The second is the tens: its clock is the units 1Q3, which falls from 1 to 0 exactly when the units go 9 → 0. MR of both is the reset button, directly: the 74HC390 reset acts on a one.

Step 6. Display

Two Display (bus) ⚙: width 4, DEC, buses T (tens) and U (units). The TICK lamp blinks with every cycle.

3.5. Check

1. Clock — 1Hz. Press Reset: the display shows 00.
2. RUN = 1: the stopwatch counts 01, 02 … 09, 10, 11 … — one per second. Want it faster? Clock 10Hz.
3. At 99 → 00 the tens and units go to zero together.
4. RUN = 0 — counting stops, the TICK lamp goes out. RUN = 1 — it continues from the same place.
5. Reset in the middle of counting — at once 00.

Lesson 3 paste (Cmd+I)

On top of lesson 2.

# Lesson 3 — 74HC390: two decade counters in a package; a stopwatch 00–99.# Paste: Cmd+I on top of lesson 2. chip 74HC86 {  portIn(1A)  portIn(1B)  portIn(2A)  portIn(2B)  portIn(3A)  portIn(3B)  portIn(4A)  portIn(4B)  portOut(1Y)  portOut(2Y)  portOut(3Y)  portOut(4Y)  XOR(1A, 1B, 1Y)  XOR(2A, 2B, 2Y)  XOR(3A, 3B, 3Y)  XOR(4A, 4B, 4Y)} chip 74HC390 {  portIn(1CP0)  portIn(1MR)  portIn(1CP1)  portIn(2CP1)  portIn(2MR)  portIn(2CP0)  portOut(1Q0)  portOut(1Q1)  portOut(1Q2)  portOut(1Q3)  portOut(2Q3)  portOut(2Q2)  portOut(2Q1)  portOut(2Q0)  74HC04(1A=1CP0, 1Y=1c0, 2A=1CP1, 2Y=1c1, 3A=1MR, 3Y=1nMR, 4Y=2c0, 4A=2CP0, 5Y=2c1, 5A=2CP1, 6Y=2nMR, 6A=2MR)  74HC02(1Y=1d1, 1A=1Q1, 1B=1Q3, 2Y=2d1, 2A=2Q1, 2B=2Q3, 3A=GND, 3B=GND, 4A=GND, 4B=GND)  74HC08(1A=1Q1, 1B=1Q2, 1Y=1d3, 2A=2Q1, 2B=2Q2, 2Y=2d3, 3A=GND, 3B=GND, 4A=GND, 4B=GND)  74HC86(1A=1Q1, 1B=1Q2, 1Y=1d2, 2A=2Q1, 2B=2Q2, 2Y=2d2, 3A=GND, 3B=GND, 4A=GND, 4B=GND)  74HC74(1RD=1nMR, 1D=1Q0N, 1CP=1c0, 1SD=VCC, 1Q=1Q0, 1QN=1Q0N, 2QN=1Q1N, 2Q=1Q1, 2SD=VCC, 2CP=1c1, 2D=1d1, 2RD=1nMR)  74HC74(1RD=1nMR, 1D=1d2, 1CP=1c1, 1SD=VCC, 1Q=1Q2, 1QN=1Q2N, 2QN=1Q3N, 2Q=1Q3, 2SD=VCC, 2CP=1c1, 2D=1d3, 2RD=1nMR)  74HC74(1RD=2nMR, 1D=2Q0N, 1CP=2c0, 1SD=VCC, 1Q=2Q0, 1QN=2Q0N, 2QN=2Q1N, 2Q=2Q1, 2SD=VCC, 2CP=2c1, 2D=2d1, 2RD=2nMR)  74HC74(1RD=2nMR, 1D=2d2, 1CP=2c1, 1SD=VCC, 1Q=2Q2, 1QN=2Q2N, 2QN=2Q3N, 2Q=2Q3, 2SD=VCC, 2CP=2c1, 2D=2d3, 2RD=2nMR)} clock(CLK)input(RUN)button{label="Reset"}(RST)74HC08(1A=CLK, 1B=RUN, 1Y=TICK, 2A=GND, 2B=GND, 3A=GND, 3B=GND, 4A=GND, 4B=GND)74HC390(1CP0=TICK, 1MR=RST, 1Q0=U0, 1CP1=U0, 1Q1=U1, 1Q2=U2, 1Q3=U3, 2Q3=T3, 2Q2=T2, 2Q1=T1, 2CP1=T0, 2Q0=T0, 2MR=RST, 2CP0=U3)display{bits=4, base="DEC", busSource="T"}()display{bits=4, base="DEC", busSource="U"}()led(TICK)