Counters 2.1
MSI Counters
Counter ICs
©Paul GodinUpdated Aug 2013prgodin @ gmail.com
Counters 2.2
Introduction• 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 2.3
Counter ICs• There are many counter ICs available, each with a
specific set of functions. Examples of functions include:• Loading a value (instead of resetting to 0)• Counting up or down by either:
• Up/Down selection input• Dual clock inputs (one for up, one for down)
• Frequency division:• Specific input frequencies (i.e. ÷ 60 or by a crystal
frequency)• Output patterns
• Multiple counters within a single package• Decade or binary counting• Borrow and Carry,...
Counters 2.4
The 7490• The 7490 is a decade counter (Modulus = 10)
• List the features for the 7490:
Counters 2.5
7490 Frequency Division• The 7490 has two modes of operation:
• Mod-10 up count• Symmetrical divide-by-ten• Bi-Quinary frequency division
Counters 2.6
The 74192• The 74192 is a decade counter (Modulus = 10)
• List the features for the 74192:
Counters 2.7
The 74192• The 74192 has several features.
• Note the specification sheet does not include a function table, but uses a sample timing diagram to explain the functions of the device.
Counters 2.8
The 14518• The 14518 (or 4518) is a decade counter (Mod = 10)
• List the features for the 14518:
Counters 2.9
The 14518• The 14518 is a popular counter IC because it
includes two decade counters within a monolithic package.
• Note that this device is electrically different from the 74xxx series of devices.
• This device can be configured to accept either a positive edge or a negative edge.
Counters 2.10
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 2.11
MSB Edge
Decade
D C B A0 0 0 00 0 0 10 0 1 00 0 1 10 1 0 00 1 0 10 1 1 00 1 1 11 0 0 01 0 0 1
A
B
C
D
LSB
MSB
Negative edge produced by 9-to-0 transition on D
Frequency Division• Counters are often used as frequency dividers.
• Example: A common frequency for crystal oscillators is 32.768 KHz. We can divide this frequency into a 1 Hz pulse by using full-sequence counters:
Counters 2.12
÷16 ÷16 ÷16 ÷832768Hz
2048Hz128Hz
8Hz1Hz
How many flip-flops if we built this as a single counter?
Counters 2.13
MSB Edge Cascading
Decade
D C B A0 0 0 00 0 0 10 0 1 00 0 1 10 1 0 00 1 0 10 1 1 00 1 1 11 0 0 01 0 0 1
Mod-6
D C B A0 0 0 00 0 0 10 0 1 00 0 1 10 1 0 00 1 0 1
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 2.14
In-Class Exercise• Use EWB to design a 12 hour clock with AM and PM
settings. Include seconds, minutes and hours.
• Use the appropriate counter IC for this exercise.
• Use EWB Sub-Circuits.
• Special consideration to the hours:• The clock counts from 12 to 1• Consider what state must be detected and what must
happen.
Counters 2.15
End
©Paul R. Godinprgodin @ gmail.com