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'Vih max only at VDD :('
1998\08\05@222619 by Frans Gunawan

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Why Vih max(input voltage high) on PIC only at VDD??? I think most
devices(uC,EPROM,etc) use VDD+0.5 or VDD+1.
This is a problem when we program PIC at VDDMin. It means all control logic
(data, pulse) must be <=4.5V. This is not convenience at all.
if it is at VDD+0.5 then we don't need to modify the control logic when
program at VDDMin. Is it OK to program it using 5V control logic when at
VDDMin?
Is there any reason why Microchip use VDD? any thought?
thanks

http://www.poboxes.com/f
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1998\08\06@113321 by John Payson

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[quoting]
Why Vih max(input voltage high) on PIC only at VDD??? I think most
devices(uC,EPROM,etc) use VDD+0.5 or VDD+1.
This is a problem when we program PIC at VDDMin. It means all control logic
(data, pulse) must be <=4.5V. This is not convenience at all.
if it is at VDD+0.5 then we don't need to modify the control logic when
program at VDDMin. Is it OK to program it using 5V control logic when at
VDDMin?
Is there any reason why Microchip use VDD? any thought?
thanks

[me]
The pins of the PIC (other than /MClr, RA4, VDD, and VSS) all have inherent diodes to VDD.  Microchip does not want to spec a minimum voltage drop for those diodes, even though in fact they're not likely to conduct with less than 0.6v or so.  For a programming fixture, there are two relatively easy ways of "conditioning" your signals to the PIC:

-1- Drive the signals with an open-collector output (or PIC outputs that are switched between "low" and "float") and use pull-up resistors wired to VDD of the PIC being programmed.

-2- Place resistors (100ohm to 1K) in series with your clock and data wires, and add some sort of load to the power supply of the PIC being programmed to ensure that the intrinsic diodes don't raise VDD.

Either approach should be fine; there's no need for anything unduly fancy.

1998\08\07@115914 by Claudio Rachiele IW0DZG

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                  Status Distribution August 06, 1998 15:40:45

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