EE345: Introduction to Microcontrollers Register and Counters Prof. Ahmad Abu-El-Haija

August 13, 2015Digital System Design2 Acknowledgement This presentation is a modified version of lecture notes prepared by Dr. M. Sachdev, University of Waterloo, other slides from unidentified authors, and original slides from the publisher.

August 13, 2015EE345 - Introduction to Microcontrollers3 Contents Registers Shift Registers Ripple Counters Synchronous Counters Other Counters

4 Registers  Register is a group of flip-flops. Each flip-flop is capable of storing one bit of information.  n-bit register has n flip-flops.  Can hold n bits of binary data.  Register may also contain combinational logic that determines how information is transferred into register. A counter is essentially a register that goes through a predetermined sequence of states.

5 4-Bit Register Common clock input triggers all ff’s on positive edge of each pulse, and binary data available at inputs are transferred into register. Clear input is asynchronous

6 Register with Parallel Load  Specific control signal to load n-bit data Load =0, register retains the data Load = 1, register accepts new data.

7 Shift Register  Capable of shifting data in one or both directions Clock controls the shift operation  Figure shows a simple shift register with left to right data shifting capability

8 Serial Data Transfer Serial mode Data is transferred one bit at a time

Serial Transfer Example August 13, 2015EE345 - Introduction to Microcontrollers9

10 Serial Addition  Parallel adders Faster, cost more logic  Serial adders Slower n-bit addition → n clock cycles Less hardware

State Table for Serial Adder August 13, 2015EE345 - Introduction to Microcontrollers11 J Q = x y K Q = x’ y’ = (x + y)’ S = x y Q

12 Second Form of Serial Adder

13 Universal Shift Register

14 Ripple Counters Counters are available in two categories: ripple counters and synchronous counters. In a ripple counter, the flip-flop output transition serves as a source for triggering other flip-flops. In a synchronous counter, the C inputs of all flip-flops receives the common clock. Binary and BCD ripple counters

15 Ripple (Asynchronous) Counter  Counts the binary sequence Negative edge triggered Output of one flipflop → clock to the next Clock skew adds up

Binary Count Sequence August 13, 2015EE345 - Introduction to Microcontrollers16

17 BCD Ripple Counter A decimal counter follows a sequence of ten states and returns to 0 after the count of 9. Counter must reset itself after counting the terminal count. A decimal counter follows a sequence of ten states and returns to 0 after the count of 9. Counter must reset itself after counting the terminal count.

18 Q1 changes state after each clock pulse. Q2 complements every time Q1 goes from 1 to 0 as long as Q8 = 0. When Q8 becomes 1, Q2 remains at 0. Q4 complements every time Q2 goes from 1 to 0. Q8 remains at 0 as long as Q2 or Q4 is 0. Q8 is cleared on the next transition of Q1. BCD Ripple Counter

19 Three-Decade Decimal BCD Counter

20 Synchronous Counter Common clock is applied to all ff’s. Clock skew does not add up. Faster than ripple counters. Design of synchronous binary counter is so simple that there is no need to go through sequential logic design process, but can be used. FF in least significant position is complemented with every pulse. A ff in any other position is complemented when all the bits in lower significant positions = 1.

21 Up-Down Counter  Can count up (0000 →1111) or down (1111 → 0000) binary sequence

22 Synchronous BCD Counter Design a synchronous BCD counter with T flip-flops

23 4-Bit Binary Counter with Parallel Load  Count is inhibited when is Load enabled

August 13, 2015EE345 - Introduction to Microcontrollers24 4-Bit Binary Counter with Parallel Load

25 BCD Counter with Parallel Load

August 13, 2015EE345 - Introduction to Microcontrollers26  A circuit with n flip-flops has 2 n states We may have to design a counter with a given sequence (unused states) Unused states may be treated as don’t care or assigned specific next state Outside noise may cause the counter to enter unused state Must ensure counter eventually goes to the valid state Counter with Unused States

27 Counter with Unused States

28 Ring Counter A ring counter is a circular shift register with only one flip-flop being set at any particular time, all others are cleared. The single bit is shifted from one flip-flop to the next to produce the sequence of timing signals. The timing signals can be generated also by a 2- bit counter that goes through four distinct states. To generate 2 n timing signals, we need either a shift register with 2 n flip-flops or an n-bit binary counter together with an n-to-2 n -line decoder.

29 Generation of Timing Signals

30 Johnson Counter A k-bit ring counter circulates a single bit among the flip-flops to provide k distinguishable states. The number of states can be doubled if the shift register is connected as a switch-tail ring counter. A switch-tail ring counter is a circular shift register with the complement output of the last flip-flop connected to the input of the first flip-flop. In general, a k-bit switch-tail ring counter will go through a sequence of 2k states.

31 Construction of a Johnson Counter Number of states of a ring counter can be doubled

Other Counters August 13, 2015EE345 - Introduction to Microcontrollers32

33 A 2-Bit Asynchronous Counter

34 3-Bit Asynchronous Counter

35 Clocked Asynchronous Decade Counter

36 A Synchronous 3-Bit Binary Counter

37 4-Bit Synchronous Binary Counter

38 3-Bit Counter