Activity 3.2.1 Asynchronous Counters: Small Scale Integration (SSI) Up/Down Counters
Asynchronous counters can be designed to count up or count down using Small-Scale Integration (SSI). The small-scale design can utilize almost any flip-flop type. To observe this process, we will simulate and analyze multiple 3-bit counters based on both D and J/K flip-flops. For both the D flip-flop and J/K flip-flop, we will modify circuits so that they will count up or count down. Remember, for up counters, connect the CLK to the Q output of the opposite polarity. For down counters, connect the CLK to the Q output of the same polarity.
Activity 3.2.2 Asynchronous Counters: Small Scale Integration (SSI) Modulus Counters
In the last activity we saw how easy it was to design asynchronous counters using either the D or J/K flip-flop. However, these designs had two big limitations.
First, the count limit had to be a power of two (e.g., 2, 4, 8, 16, 32, etc.).
All counts also started or ended at a count of zero. In the real world, we frequently need to set the count limit to some arbitrary value (10, 25, 85, etc.). More often than not, the starting or ending value will not be zero. For this reason we must design asynchronous modulus counters.
An asynchronous modulus counter, or mod-counter, uses the addition of simple combinational logic to a standard asynchronous counter to set the count limit and starting point. In this activity we will simulate and build a mod-5 counter that has a starting count of one.
This activity will also introduce using a clock signal with a PLD.
First, the count limit had to be a power of two (e.g., 2, 4, 8, 16, 32, etc.).
All counts also started or ended at a count of zero. In the real world, we frequently need to set the count limit to some arbitrary value (10, 25, 85, etc.). More often than not, the starting or ending value will not be zero. For this reason we must design asynchronous modulus counters.
An asynchronous modulus counter, or mod-counter, uses the addition of simple combinational logic to a standard asynchronous counter to set the count limit and starting point. In this activity we will simulate and build a mod-5 counter that has a starting count of one.
This activity will also introduce using a clock signal with a PLD.
Activity 3.2.3 Asynchronous Counters: Medium Scale Integration (MSI) Suspend/Reset Counts
Manufactures of integrated circuits frequently take digital circuit designs, which are commonly implemented with SSI gates, and create equivalent Medium Scale Integrated (MSI) circuits. The 74LS93 4-Bit Counter is an example of a Medium Scale Integrated (MSI) circuit.
In this activity, we will simulate and analyze a 4-Bit asynchronous counter using a 74LS93 4-Bit Counter. We will also explore how to suspend a count and reset a count.
In this activity, we will simulate and analyze a 4-Bit asynchronous counter using a 74LS93 4-Bit Counter. We will also explore how to suspend a count and reset a count.