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Altera_Forum
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14 years ago

Output Pin Driving Another Output Pin

Hello all, I've run into an unusual problem. I'll start off by describing my goal: I'm designing a circuit that can test for short circuits and open circuits in a wiring harness. The wiring har...
Altera_Forum's avatar
Altera_Forum
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14 years ago

The common solution is pulling up all wires with resistors and then drive the target line low with an open drain output.

In the CPLD you'll program bidirectional I/Os. The data out signal should be hardwired to a low level and you control the output pin through the OE signal.

If you don't want external resistors, you may use the internal pull up option, but I'd not recommend it if your wires are long: the built in pull ups are usually in the tens of kohm range and the signals can be affected by stray electrical noise.