Outline Registers Counters 5/11/2019

Registers Definition: Array of flip-flops (and possibly added combinational gates) to store binary data Each flip-flop stores one bit Register with n flip-flop stores n bits There are many types of registers, each providing a certain form of storing data In many registers, a clock (or a combination of clock and data) controls the timing of register; i.e. clock says when to update the data stored at register Registers are heavily used in computers 5/11/2019

Registers (continued) A simple register: What data to be stored: D1, D2, D3 and D4 Where to store data: Q1, Q2, Q3 and Q4 When to load data: Clock How to reset (clear) the register: Clear Symbol: 5/11/2019

Registers (continued) Disadvantage: We may not want to update data on each clock, i.e. we want loading at some particular times and not all clock pulses Solution 1: Clock gating Clock is gated with an input called “load” that tells the circuit when we want to load data If Load = 1, C input = Clock (updating happens on each positive edge of Clock) If Load = 0, C input = 1 (no edge of Clock will be observed and therefore no updating) Disadvantage: Clock skew 5/11/2019

Registers (continued) Solution 2: Control loading via flip-flop inputs Load input is applied in forming data lines If Load = 1, input data reach the flip-flip inputs If Load = 0, the present data at the output (Q) will be fed back and reloaded., i.e. new input data is ignored Clock skew is avoided 5/11/2019

Registers (continued) Shift registers: A simple shift register: Circuit Serial Input Serial Output Clock: connected to all flip-flops Symbol: Clock can be gated (similar to other registers) Gating clock creates clock skew 5/11/2019

Registers (continued) Serial transfer between shift registers: Clock is gated Registers with more bits can be formed similarly Timing diagram 5/11/2019

Registers (continued) Table for serial transfer: Serial addition: Using shift registers to perform addition Suitable for cases where bits of numbers to be added are received via a serial line 5/11/2019

Registers (continued) Shift register with parallel load: Feature: shift register is load in parallel Circuit If Load = 1, register is loaded in parallel If Shift = 1, register is shifted 5/11/2019

Registers (continued) Table: Symbol: 5/11/2019

Counters Definition: A register that goes through a prescribed sequence of states upon the application of input There are many types of counters, each providing a certain form of counting Two general types of counters: Ripple counter: the control input of some flip-flops are not derived by the circuit clock Synchronous counter: a synchronous sequential circuit, i.e. each flip-flop receives the clock in its control input 5/11/2019

Counters (continued) Ripple counter: Circuit Control inputs of three flip-flops are derived by the output of other flip-flops J and K inputs of all flip-flops are connected to 1, as a result for each rising edge on the control input, the outputs change Since two changes of output makes a pulse, the period of pulse is divided by two in the next flip-flop 5/11/2019

Counters (continued) Table: 5/11/2019

Counters (continued) Synchronous binary counter: A sequential circuit: design is similar to other sequential circuits Table 5/11/2019