ECE 171: Introduction to Digital Circuits

Fall 1999

Rev: 11.30.99

Lecture Notes 16

Last Time

• Flip flops
  • JK Flip Flops
  • T Flip Flops
• Ripple Counters/Frequency Dividers
  • Example Application
  • Glitches & Delays
• Synchronous Counters

This Time

• MSI Devices
  • Counters
  • Registers
• Vending Machine
• Exam 2 Returned

MSI Synchronous Counters

• Most counters come with many bells and whistles.
• Example: 74ALS193
  • Can count up or down
  • Has an asynchronous load
  • Has an asynchronous clear
  • Active low carry-out and borrow-out to enable this device to be interconnected with like devices to form larger counters.
  • Internally, this device is made of the basic gates and flip-flops that we have discussed in class

Registers

• Serve as temporary storage for binary words.
• Most microprocessors contain registers
• Draw diagram of a PIPO 3-bit register.
• These are conceptually simpler than counters.

4 Types of Registers

• PIPO: Parallel in, Parallel out
• SISO: Serial in, Serial out
  • Also known as shift registers
  • Used to get data into an IC using only a few pins
  • Also used in communications applications.
• PISO: Parallel in, Serial out
• SIPO: Serial in, Parallel out

Ring Counters

• Can be used to generate repeating waveforms
• Review data sheet for 74AS194
  • 4 Modes: left shift, right shift, inhibit (latch), parallel load
  • Go over truth table
  • Logic diagram is made up of gates and flip flops
  • Go over timing diagram

Vending Machine

• Suppose we are to design the logic for a vending machine
• A mechanical engineer has designed a coin detector and the release mechanisms
• The machine only accepts quarters, nickels, and dimes
• The maximum anticipated price is 55 cents
• The ME has provided switches that generate a pulse (possibly with bounce) that opens when a coin drops through. There is a separate switch for nickels, quarters, and dimes.
• The ME has also designed a pair of dials to let the client set the price of the product. The dials are designed such that the price is given in binary encoded nickels.
• The outputs are:
  • Release Product
  • Release 1st Dime
  • Release 2nd Dime
  • Release Nickel
• We will only discuss architectures. We won't select devices to actually build this system.

3 Possible Architectures

Get ideas from the students. It is likely that, with some direction, they will come up with the following possible architectures.

• Separate counter for each coin, use combinational logic and adders to convert to nickels.
  • Draw diagram.
• Separate pulse generates for quarters and dimes. One counter that counts the price paid in nickels.
  • Draw diagram.
• An adder and register that adds the amount of the coin in nickels to the current amount paid every time a coin is dropped. Requires an adder and a register.
  • Draw diagram.

Input Signal Conditioning

• Switches for coins are either open or closed.
• We need to convert to valid signal levels.
• The ME has tied one end of each switch to a common terminal. This is very similar to a common-anode/common-cathode LED.
• How do we configure to get an active high pulse every time a coin is dropped?
  • Draw solution.
• How do we configure to get an active low pulse every time a coin is dropped?
  • Draw solution.

Exam 2 Feedback

• The front of the exam says that there is 85 possible. This is an error; there are 90 possible points.
• Overall, the performance was much better than Exam 1.
• Very few people got the Karnaugh maps correct, but most had the right idea.
• I gave a lot of partial credit.
• Keep up the good work for the final.
• Personal recommendation: Many people left early, before the time was up. It is usually worthwhile to stay and double-check or triple-check your answers so that you catch the many small errors that were made.
• Solutions and distribution are online.