Lect# 9 Register (Continued) Register • A register is a sequential circuit that can be set to a specific state and retain that state until externally changed. • State is a combination of bits
Register Design • An S-R latch is a simple 1-bit register. • With (S)et input, latch output can be set to 1 • With (R)eset input, latch output can be set to 0.
Register Design • A D flip-flop can be used as a 1-bit register • Input 0, output is set to 0 when flip-flop is clocked • Input 1, output is set to 1 when flip-flop clocked • Can consider the clock to be a load signal. • On rising edge of load signal, the data on D input line is stored in the flip-flop
d inputs c b a • Da Dd Dc Db Ck Qd Ck Qa Ck Qb Ck Qa load outputs d c b a Multibit Register • Use multiple flip-flops. Each stores a single bit. • Common load signal.
Shift registers Classification, basic function, uses: • Plain shift registers: • Serial-in serial-out – SISO (also first-in first-out – FIFO) • Serial-in parallel-out – SIPO • Parallel-in serial out – PISO • Parallel-in parallel-out – PIPO • Counters based on shift registers • Ring counter • Johnson counter • Shift register and feedback based circuits
Shift registers • Serial-in serial-out – SISO • Serial-in parallel-out – SIPO • Parallel-in serial out – PISO • Parallel-in parallel-out – PIPO
Plain shift registers CLOCK SERIN SEROUT