Lesson 2. The F/F flip-flop

After this lesson the library has the flip-flop the CD4029 counter is made of: it can load a bit at once, and on the clock — flip or stay. We try it with switches and a generator.

2.1. Idea: a counter is flip-flops that flip

A binary counter is a row of flip-flops. On every clock the lowest bit always flips, the next one — only if the lower was 1 (counting up) or 0 (counting down). So each flip-flop must be able to do three things:

• load — at once, without the clock (to set the counter to any number: PE);
• flip on the clock — when allowed (TE = 0);
• hold — when not allowed (TE = 1).

2.2. Idea: flipping through exclusive OR

Inside is an ordinary D flip-flop: on the clock it remembers what is at input D. Feed D with NOT Q and the flip-flop flips; feed it Q and it stays. XOR chooses: D = NOT(Q XOR TE).

• TE = 0: Q XOR 0 = Q, D = NOT Q — flip;
• TE = 1: Q XOR 1 = NOT Q, D = Q — hold.

2.3. Idea: a NAND with three inputs

A D flip-flop with loading is the classic 7474 circuit: six 3-input NANDs. We build NAND3 straight from transistors — this is how it is made inside a chip.

2.4. Build

The same project. The library needs NOT, NAND, XOR (lesson 0) and CD4011 (lesson 1). The lesson 1 probe can be deleted.

Step 1. Chip NAND3 — NAND with 3 inputs from 6 transistors

If NAND3 is already in the library (for example, from the "74-Series Chips" kit), skip this step.

As inside a CMOS chip: three pmos in parallel from VCC to the output, three nmos in a chain from the output to GND. The output is 0 only when all three inputs are 1. Transistor: gate, drain, source.

Ctrl+G → name NAND3. Legs: inputs A, B, C; outputs Y.

Step 2. Chip FF4029 — the F/F flip-flop of the CD4029 counter

On the TI drawing (Fig. 9) the flip-flop is a box with a table; inside — a D flip-flop (six 3-input NANDs, as in the 7474) and three controls. PE = 0 — load at once (through set sdn / reset rdn). On the falling edge of CL: TE = 0 — flip (D = NOT Q, through XOR and NOT), TE = 1 — hold. What is loaded is NOT J: the CD4029 itself has an inverter in front of J, so the whole chip loads the number as it is.

Ctrl+G → name FF4029. Legs: inputs PE, J, TE, CL; outputs Q, QN.

Step 3. Probe: switches and generator

Clock — click the body up to 1Hz.

Step 4. Flip-flop

Step 5. Lamps

2.5. Check

1. Clock — 1Hz. PE = 0, J = 0: Q is lit — NOT J was loaded. J = 1 — Q goes out.
2. PE = 1 (loading off), TE = 0: Q and QN swap on every falling edge of CL — when the CL lamp goes out. Q blinks half as often as CL: the flip-flop divides by 2.
3. TE = 1 — Q freezes: the clock runs, the flip-flop holds.

After the check delete the probe (frame and Delete) — NAND3 and FF4029 stay in the library.

Lesson 2 paste (Cmd+I)

You need the gates NOT, NAND, XOR (lesson 0).

# Lesson 2 — the F/F flip-flop of the CD4029 counter: NAND3 from transistors, a D flip-flop from six NAND3, load and flip.# Paste: Cmd+I into the same project (NOT, NAND, XOR from lesson 0 needed). chip NAND3 {  portIn(A)  portIn(B)  portIn(C)  portOut(Y)  pmos(A, Y, VCC)  pmos(B, Y, VCC)  pmos(C, Y, VCC)  nmos(A, Y, m1)  nmos(B, m1, m2)  nmos(C, m2, GND)} chip FF4029 {  portIn(PE)  portIn(J)  portIn(TE)  portIn(CL)  portOut(Q)  portOut(QN)  NOT(CL, cp)  NOT(PE, ld)  NOT(J, jn)  NAND(ld, jn, sdn)  NAND(ld, J, rdn)  XOR(Q, TE, x)  NOT(x, d)  NAND3(A=sdn, B=n4, C=n2, Y=n1)  NAND3(A=n1, B=rdn, C=cp, Y=n2)  NAND3(A=n2, B=cp, C=n4, Y=n3)  NAND3(A=n3, B=rdn, C=d, Y=n4)  NAND3(A=sdn, B=n2, C=QN, Y=Q)  NAND3(A=Q, B=n3, C=rdn, Y=QN)} power(VCC)ground(GND)input(PE)input(J)input(TE)clock(CL)FF4029(PE=PE, J=J, TE=TE, CL=CL, Q=Q, QN=QN)led(CL)led(Q)led(QN)