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Digital Display Circuits
15 questions By Tony R. Kuphaldt
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Question 10 of 15
The MM58342 high-voltage display driver IC from National Semiconductor serves as an interface between either a microprocessor or microcontroller and a high-voltage vacuum fluorescent (VF) display panel. The IC reads and conditions 20 bits of data to drive 20 “grids” in such a display. When combined with a similar driver driving the anodes of the same VF display, individual pixels (or combinations of pixels) may be controlled (lit).
An interesting feature of this IC is that it receives the 20 bits of data serially (one at a time), through a single input pin:
Read the datasheet for this device, then comment on why you think a serial (rather than parallel) data input format was chosen. Also describe the sequence of operation for loading data into this IC and outputting that data to the 20 output lines.
Reveal answerIf it were not for the serial input, this IC would have quite a few more pins! The timing diagram and description in the datasheet should provide plenty of information for determining how to send data to the display using this IC.
Notes:While this question introduces the concept of a vacuum-fluorescent (VF) display, it also serves as a review of shift register and latch technology. The block diagram should be informative enough for most students to be able to figure out at least an approximate procedure for loading and outputting data.
It is interesting to note (and discuss with your students) that this IC does not decode characters. It merely conditions and outputs bits of information to the grids of a VF display. Ask your students, then, where they think the patterns of “on” and “off” pixels must be generated to form specific characters on the display.
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Question 11 of 15
One new technology entering the market is organic light-emitting diodes, or OLEDs. Describe what these are, and why they hold so much promise for electronic display device elements.
Reveal answerAn “organic” LED is one made from organic (carbon-containing) molecules instead of crystalline silicon, gallium arsenide, or other more traditional semiconductor substances. One distinct advantage of these devices is their ease of manufacture, but I’ll let you research exactly why (as well as other advantages of these devices).
Notes:This question is destined for obsolescence, as organic LEDs will either become so popular as to lose their novelty (“new technology”) or become supplanted by something even better. But for now (May 2005), they are worthy of their own question in the Socratic Electronics project!
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Question 12 of 15
Two electronics students attempt to build 7-segment display circuits, one using a 7447 decoder/driver IC and the other using a 7448. Both students connect their ICs to common-cathode 7-segment displays as such:
The student using the 7448 notices the LED segments glowing faintly, but the patterns are not correct for the digits that are supposed to be displayed. The student using the 7447 has an even worse problem: no light at all! Both have checked and re-checked their wiring, to no avail. It seems as though all the connections are in the right place.
What do you think the problem is? Hint: consult datasheets for both chips to find clues!
Reveal answerNeither the 7447 nor the 7448 are designed to source current to the LED segments, only sink current. I’ll let you figure out why the 7448 chip has the ability to make any of the LED segments light up at all.
Follow-up question: trace the direction of electron flow through the wires between the decoder chip and the display.
Notes:This question provides an excellent opportunity to discuss the difference between sourcing and sinking current, as well as the importance of knowing what the output stage of an IC looks like internally.
