Minimum Voltage of Picaxe 28x1

M

Mikler

Member
Hi!

I would be grateful for newbie comments/help regarding the minimum, reliable, operating voltage of the 28X1.

ONE:
I have looked up the datasheet of the PIC16F886 which states 2v to 5.5v.
I have seen elsewhere that one needs a minimum of 3 volts?
Which is it really?

TWO:
Also, does programming need a higher voltage?

THREE:
Finally, if it does operate down to 2v, can anyone comment as the the accuracy of the delay command as the voltage drops from 3v to 2v (I am NOT using a resonator), or does it stay fairly constant? I suppose 2%-3% for my application would be fine.

Thanks

Mik

P.S. By way of explanation, for various reasons, I HAVE to use two AA (alkaline) batteries.
 
M

MPep

Senior Member
Hi Mike,

1- Best way is to try it. 3V would be better, although some people have used lower voltage rails. Could you use a small DC-DC converter? Something like this http://www.talkingelectronics.com/te_interactive_index.html look for the "5V solar charger".

2- Best if programmed using 5V. Previous forum posts have mentioned this already.

3- Sorry, I am not the person to answer this question.

Regards,
Mark.
 
M

Mikler

Member
Regretfully, a dc-to-dc convertor would not be possible - for various reason, I am tied to two AA alkaline batteries....

Thanks anyway
 
R

Ralpht

New Member
Several months ago I experimented around with the working voltages of the Picaxes I had.

Of the 8 X '08M's I have, five of them worked at 2.2 volts which was the lowest I could go with my Power supply. By worked, I mean they started ok on switch on and ran a test program reliabily every time. But I could not program them until I raised the Vcc to 4.5V.

The other three didn't start till the Vcc was 3.3 volts and would only program when Vcc was 4.5V+ .

The 18X's gave similar results (all started at 2.2V) but I only have 3 of them so not a valid example.

I have a dozen 28X's and they all worked at 2.2 volts but would not program till Vcc reached 4.02V.

I have access to a Lab standard power supply that is known to be accurate to 0.01% - it gets calibrated once a year so the voltages and metering on it are accurate.

Don't have 28X1's so can't help there but I suspect it won't be too much different to my results with the 28X's.

The above results are statistically meaningless but will give you an idea of what to expect.

Hey - the best way to see if yours will work it to try it! I doubt it will program at anything less than 4V though.

Two AA's will give you about 3V depending on load, so your 28X1's will work but "in field" programming may be an issue.
 
W

womai

Senior Member
Data sheets are your friend. In this case the one for the PIC16F886, which is the chip the 28X1 is based on.

Look under chapter 17 (Electrical characteristics) for details, but in a nutshell:

- you can run down to 2V at up to 8 MHz on the internal oscillator.
- you can run down to 2V at up to 4 MHz or down to 3V up to 10 MHz on an external resonator.

I use several PIC16F886's at 3.3 V running at 8 MHz with an external resonator and so far haven't seen any problems.

Wolfgang
 
M

MFB

Senior Member
I routinely run 28X1 chips at 3 V when, operating from an external 16MHz crystal, and can reliably download data at 19.2 kbps using the ser rx port. However, these results are also statisticaly meaningless.
 
B

BCJKiwi

Senior Member
When programming, there is an internal circuit which raises the voltage to a critical value to enable the write to the eeprom. If supply is low then the required programming voltage will not be acheived in internally.

So would always recommend programming at or near 5V.
 
hippy

hippy

Ex-Staff (retired)
Also note that the internal oscillator accuracy can worsen when at lower voltages. For a 28X1, it's +/-1% around 3V5, +/-2% for 2V5 to 5V5, but below 2V5 it is +/-5%.

Using a low voltage and internal oscillator may cause errors with inter-PICAXE serial communications on some chips and all timing commands could be affected. SERTXD, SERRXD, SERIN and SEROUT with a PC should be okay as it takes +/-6% error to be problematic and the PC should be close to 0% error.
 
hippy

hippy

Ex-Staff (retired)
@ BCJKiwi : 4V5 to 5V5 is required for writing the internal Flash which is where program code is stored so, yes, 5V is recommended for when downloading.

On-chip Data Eeprom can be written at any voltage above chip minimum so WRITE commands will not have a problem when the code is running.

For PICAXE's which share code and data space, downloading at lower voltages should therefore also work ( 08, 08M, 14M, 18, 18A, 18M, 20M ).
 
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