Re: Hex Display Subcircuit

From: Spehro Pefhany (speffSNIP_at_interlogDOTyou.knowwhat)
Date: 06/11/04 

Date: Fri, 11 Jun 2004 15:31:05 GMT

On Fri, 11 Jun 2004 07:57:30 -0700, the renowned John Larkin
<jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

>On Fri, 11 Jun 2004 08:21:10 -0400, "Eric Adamson"
><jNOunk@eSrPiAcMadamson.com> wrote:
>
>>I'm developing a multiplexed two-digit hex display using 7-segment displays.
>>Multiplexing and BCD-hex encoding are being handled by a PIC. Diagram can
>>(hopefully) be seen at:
>>
>>http://forum.microchip.com/upfiles/23628/Rp42593.png
>>
>>My question relates to the inverter which is used to select the digit to be
>>driven, and how it should be implemented. I'm looking at a 74HC04, but find
>>that it's current rating is only around 4mA. Apologies for not having the
>>LED specs handy, but I'm headed off to work -- general advice would be fine.
>>It appears to be relatively efficient, and given that only one segment will
>>be driven at any given instant, my current requirements should remain fairly
>>low, save perhaps any need to overdrive for brightness compensation.
>>
>>I'm not using the 74HC04 for anything else, so wiring several gates in
>>parallel seems a reasonable way to increase current capacity. The PIC can
>>source/sink at least 20mA, so it's probably fine to drive the segment which
>>appears at the inverter input, but I'm wondering whether it still might be
>>preferable to lose the inverter IC and buffer the RA? line with discretes.
>>
>>BTW, the entire post can be accessed at this URL:
>>
>>http://forum.microchip.com/m.asp?m=42520&mpage=1
>>
>>Thanks in advance, for any advice you can offer.
>>
>> Eric Adamson
>> Lansing, Michigan
>>
>
>
>An HC04 will actually source and sink a lot more than 4 mA, certainly
>enough to drive a 20 mA led. It sinks a bit better than it sources.
>
>John

Umm... you are using a single series resistor in the common (anode or
cathode). That means that an "8" will be much dimmer than a "1" unless
you play some software tricks.

Unless your display is internally multiplexed (rare on 2-digit
displays) you can do this with the same number of port pins and no
inverter, you know....

Best regards,
Spehro Pefhany 

-- 
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