general question about interfacing

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I'm working on a design with an ATmega644 driving numerous 4511 7-segment decoders in a multiplexed fashion.

In general, is it a good design practice to put resistors on interface lines? I'm wondering because in my design, the BCD3..0 lines and the latch enable lines are essentially short circuits. Will connecting the mcu and other support chips directly affect the longevity of this device, or is it better to put current-limiting resistors on all lines?

I know the limits of each port on the mega644, and I'm not exceeding them, but just wonder if it would be better to current-limit all lines.

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Quote:
In general, is it a good design practice to put resistors on interface lines?

in general, it is a good practice, especially during prototyping.

in general, it is not needed, especially if you have finalized your design onto a pcb.

Quote:
the BCD3..0 lines and the latch enable lines are essentially short circuits.

that should have never happened - it is a design flaw.

Quote:
Will connecting the mcu and other support chips directly affect the longevity of this device, or is it better to put current-limiting resistors on all lines?

to avoid accidentally shorting, or overloading the pins so that a logic high may be too low, or a logic low may be too high for the peripherals.

Last Edited: Sun. Oct 10, 2010 - 05:06 PM
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Is 'numerous' more than 8? There is a max chip that will drive em all... sorry I can't pull the part number out of the morass of tangled synapses that form my memory banks.

Imagecraft compiler user

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Thanks, gentlemen. I should elaborate:

1. I am driving four decoders, each of which drives two 7-segment displays in common-cathode orientation.

2. Each of the four latch lines is connected directly to !LE on its respective decoder. Each of the BCD3..0 lines is connected to the respective line on all decoders. That's what I meant when I said they're all shorted.

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without seeing the schematic, it is hard for me to envision what you are talking about.

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Joe, no resistors needed. Having your decoders in parallel, forming a bus going to all of them, is fine. The uC can drive four of them without difficulty.

If you connected the uC's pins to something that could, itself, drive the line, then the series resistor would be a good idea. Then if both the peripheral and the uC tried to drive the line at the same time the resistor would limit the current if one device drove the line high while the other drove the line low.

JC

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Doc (and others),

Thanks for the advice. Still relatively new to this, I'm (perhaps too) cautious when interfacing from the AVR out to other devices. Never had a problem when using current-limiting resistors in the 2Kohm range, but this design is pretty crowded already and it'd be great if I didn't have to complicate it further.

Thanks!

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You can never be too cautious..

One wrong assumption and your reaching for the extinguisher.

If in a case as JC stated..

A resistor would limit the current if one device drove the line high while the other drove the line low.

I would use 1N914 / 1N4148 Diodes to block that Scenario and still maintain +5 at the gate, rather than 2.5 (Created by that L Pad)

But.. any way.. That's just me..

Well may be not +5 volts exactly.. you will have a .7v drop across the diode.

..KDS

"We look for things.. Things that make us go."