MSI Counters Counter ICs Technician Series ©Paul Godin Updated Feb 2015 gmail.com

Synchronous Counters In a synchronous counter each of the Flip-Flops receive the same edge at the same time. Eliminates “glitch” states caused by delays in asynchronous design Additional gates are required Counters.2

State Machines A State Machine is a counter that is designed to produce a specific output sequence that may or may not follow a natural count sequence. Counters.3

IC Complexity Scale SSI: Small Scale Integration. Usually refers to IC’s that contain individual gates or flip-flops. MSI: Medium Scale Integration. Usually refers to IC’s that contain counters, encoders, etc… LSI and VLSI: Large and Very Large Scale Integration. Refers to IC’s that can perform large logic functions, such as CPLDs, microprocessors, etc. ASIC: Application Specific IC. Refers to IC’s that are custom built for specific functions, and are vendor- specific. An example is the chip in a TI Calculator. Counters.4

Counter ICs There are many counter ICs available, each with a specific set of functions. Examples: Count Sequence: Count to 10 (Decade Counters) Count in a full binary sequence (Binary Counters) Count Direction: Up or Down Counters Reset (0000) or Preset (1001) capabilities Many MSI counters are combinations of synchronous and asynchronous design Counters.5

Counter ICs: Additional Features Examples of additional functions or features include: Frequency division: Divide specific input frequencies (Crystal or AC) Output patterns Multiple counters within a single package Borrow and Carry for additional counters Blanking leading or trailing zeros Counters.6

Counters.7 12-bit binary counter. Often used as a frequency divider: input of 4096Hz produces a 1Hz output CD4040 Binary Counter

Counters.8 Features: -up or down counting -borrow or carry outputs -ability to clear (0000) -ability to load any BCD value The 74LS192 BCD Counter

Counters.9 A more unusual counter, it has these features: -Up count (BCD) mode or frequency divide mode -Symmetrical Divide-by-10 -Divide by 5 with either 20% or 40% DC -Requires external wiring to set the mode of operation -Contains 2 active high ANDed resets and 2 ANDed presets The 74LS90 Counter

Counters.10 This dual-counter chip has these features: -Up count only -Selectable edge input -Independent Master Resets The CD4518 BCD counter

Cascading Counters Cascading means connecting one device to another device for it to continue the logic operation. When designing a digital clock, counters need to be cascaded. Consideration must be given to how the next counter in the cascade will be incremented. The MSB that changes will produce a negative edge when the count returns to zero. Counters.11

MSB Edge Decade DCBA A B C D LSB MSB Negative edge produced by 9-to-0 transition on D Counters.12

Frequency Division Counters are often used as frequency dividers. Example: A common frequency for crystal oscillators is KHz. We can divide this frequency into a 1 Hz pulse by using full-sequence counters: ÷16 ÷ Hz 2048Hz 128Hz 8Hz 1Hz How many flip-flops if we built this as a single counter? Counters.13

MSB Edge Cascading Decade DCBA Mod-6 DCBA Negative edge produced by 5-to-0 transition on C D does not change so it isn’t used for cascade Decade LSB MSB Counters.14

In-Class Exercise Use EWB to analyse a 12 hour clock: AM and PM settings seconds, minutes and hours Special consideration to the hours: The clock counts from 12 to 1 Consider the state “1 and 3” must be detected and preset the counters to 0 and 1. Counters.15

End ©Paul R. Godin gmail.com Counters.16 Why was the counter defective? One of the flip flops couldn’t give a bit