28X1/X2 operating voltage

L

lanternfish

Senior Member
A question for the more knowledgeable.

The Microchip datasheets for the 16F886 (28X1) and the 18F2520 (28X2) seem to indicate that they can operate at 3 (3.3) volts. Is this correct or have I read the datasheets wrong?

Cheers
 
W

westaust55

Moderator
The 4.5V and 5V are nominal values

the 28X1 and 40X1 datasheet indicates operation down to 2V. so, yes, you can operate them at 3V.

Suggest no lower than 3V while programming as others have had problems trying to program at lower voltages.
Maybe even 3V could be too low but I have successfully programmed 08M and 18X chips many times at 3V.

Note also that as the voltage reduces, the max operating frequency can also drop off.

For the 28X2 and 40X2, the PICAXE manual 1 page 12 indicates the 18F4520 and 18F2520 PIC versions are used.

These hava a minimum working voltage of 4.2V by the datasheet.

However rev Ed also have the 3V version of the 28X2 and 40X2 using a slightly different chip (see PICAXE manual 1 page 12)
 
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L

lanternfish

Senior Member
Cheers for that WestAust55. I am aware of the 3V versions (have a couple now) and was interested to see if others had tried to use the 5V versions at 3.3V.
 
J

jmumby

Senior Member
I have tried a 28x1 at 3.3v because I didn't want to use a logic level converter for a GPS. Flakey, random, unreliable. Nice to know you can get a 3.3v version.
 
D

Dippy

Moderator
I've been using a 28X1 with 2 fresh AAs becuase I too didn't want to use level shifting with an SD controller.
Serial comms to my PC and downloads were fine 90% of time.

I haven't tried X2 but I use 18F2520s a lot. I've been happily programming and serial comming with them at 3.3V. However, I should add, that in the real world at these voltages I use the K versions or LF versions.

ALSO, please be careful when reading Microchip Data Sheets.
They often make generalised comments which can easily be taken for granted - only when you read further you find the devil in the detail.

Example:
PIC 18F2520.
Be CAREFUL. Look at frequency of operation versus supply voltage.
Look at figures 261.1 26.2 (in my old Data Sheet) showing PIC Frequency vs Voltage.

There are various other parameters to look at too.

And then read this:-Like all Microchip PIC18 devices, members of the
PIC18F2420/2520/4420/4520 family are available as
both standard and low-voltage devices. Standard
devices with Enhanced Flash memory, designated with
an “F” in the part number (such as PIC18F2420),
accommodate an operating VDD range of 4.2V to 5.5V.
Low-voltage parts, designated by “LF” (such as
PIC18LF2420), function over an extended VDD range
of 2.0V to 5.5V.


Microchip err on the side of caution so sometimes you can get away with things.
But ALWAYS be aware and ALWAYS read further than the HEADLINES ;)
(I know it's a pain, but it's worth it)
 
hippy

hippy

Ex-Staff (retired)
I tend to run all my PICAXE off 3.3V in case I inadvertently insert a 3v3 PICAXE and haven't run into any problems as such. This is out of specification for some PICAXE so not guaranteed to work and the recommendation is to run PICAXE within specification or to choose a PICAXE which has a specification which fits the power supply.
 
D

Dippy

Moderator
Which bit was flaky? PICAXE to SD? PICAXE to PC? Maybe your PC isn't happy with 3V logic?
Or it could have been crappy layout.... if 10,000 people can make it work and one can't then... well, no need to call in Sherlock Holmes eh :)
A slight generalisation I admit. You'll get is sussed I'm sure.
 
W

westaust55

Moderator
lanternfish said:
Cheers for that WestAust55. I am aware of the 3V versions (have a couple now) and was interested to see if others had tried to use the 5V versions at 3.3V.
Click to expand...
By the datasheet the "nominal" 5V 28X2 has a minimum voltage level of 4.2 V as I mentioned in post 2.
 
J

jmumby

Senior Member
Dippy said:
Which bit was flaky? PICAXE to SD? PICAXE to PC? Maybe your PC isn't happy with 3V logic?
Or it could have been crappy layout.... if 10,000 people can make it work and one can't then... well, no need to call in Sherlock Holmes eh :)
A slight generalisation I admit. You'll get is sussed I'm sure.
Click to expand...
Yep I got it sussed alright, I found out that my GPS was 5v tolerant. That project is long gone now however, canabalised for other projects. Althought this was a 28x1 with the infamous scratchpad bug.
 
Technical

Technical

Technical Support
Staff member
The values depend on the PIC type and whether the BOD can be turned on or off under software control - this was only introduced in the 'newer' PICs.

08 - 2.1V
08M, 14M, 20M, 28X1, 40X1 - 2.1V
18, 18A, 18M, 18X, 28A, 28X, 40X - OFF (None)

20X2 - 1.9V
28X2, 40X2 - 3.2V
28X2-3V, 40X2-3V - 1.8V


So if trying to use a (normal 5V) 28X2 at 3V (which is not recommended - use the 3V part!) turning off the BOD would be useful. However on a 28X1 not a problem (as it's BOD is 2.1V).
 
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