7-Segment Display -- Serial

The previous 7-segment designs are good, but do suffer from port pin gluttony. In this exercise we ameliorate things somewhat by going sequential. Neither the hardware nor the software requirements are overly burdensome.

We'll use a pair of CD4026 counter/decoder chips. The slick aspect is that this CMOS integrated circuit (like the CD4511 employed in the previous project) contains the necessary character decoding logic for the seven segments. Even better, it comes outfitted with the current limiting diodes built-in, keeping the circuit board design quite minimal. So, the wiring is a snap. Here are the schematics:

Observe that only three port pins are required.

The technique is similar to what we saw in the LED Dot Graph with Counter project earlier. First, the segment driver outputs are shut off (via the Enable pins), then the counter is forced back to zero (by the Reset pins), next the counters are clocked along up to the desired number (by the Clock pins). Finally, the outputs are re-enabled and and the displays show the desired number. This all happens so quickly, that to the naked eye it simply appears that the two digits went wham-bang all at once. And since the units chip chains forward to the tens chip, numbers from 00 to 99 are obtainable.

As usual, the PIC Micro Pascal code is oh-so-easy to whip up. There really is nothing this cross-compiler can't handle, and handle elegantly.

A rather neat approach for such old fashioned CMOS devices, don't you think?

Click to get the source code.
Click to get the schematic PDF.

Next Project: 7-Segment Display -- Four Digits

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