PICAXE stability vs operating temperature...

[Grogster]

Hi all.


I came across this interesting information on hippy's page:
http://www.hippy.freeserve.co.uk/picaxeio.htm#Temperature_Effect

The Effects of Temperature
While the operating speed of the PICAXE-28, 28A, 28X and 40X is fairly stable across a wide temperature range through the use of a resonator or crystal controlled oscillator used for timing control, the PICAXE-08, 08M, 18, 18A and 18X are implicitly less stable because of their use of a simple internal "RC" oscillator and will be more severely affected by changes in temperature.

The operating speed of the internal" oscillator parts will drift as the temperature changes, whether they are changes in the ambient temperature or through self-heating while operating. The proximity of other components which may be warming up during operation, especially voltage regulators, may also heat up the PICAXE. It is fairly likely that the operating speed will change when a PICAXE is powered-up until it reaches a nominal operating temperature.

As the operating speed drifts then so will the accuracy of the timing of the serial output which the PICAXE is generating, and its ability to accurately read any serial input will be correspondingly affected. The problem can be compounded if the devices at each end of the serial link are drifting in opposite directions, possibly doubling the effect of the error.

Serial communications with a PICAXE are all done using one start bit, 8 data bits and one stop bit; 10 bits in total, with the bits being sent in that order. Should the PICAXE slow down by 6%, when the PC receiving this serial data comes to be reading the last data bit the PICAXE will actually be sending out the last but one data bit. This will result in PC accepting corrupted data or the data being ignored if the following stop bit is not of the correct value. The late arrival of the stop bit itself may also lead to another, non-existent, byte of data being received or further errors, and discarded data.

The resultant affect of temperature change may therefore be that the PICAXE no longer sends valid serial data or is unable to receive it. With large enough drift, it may become impossible to use serial data no matter what baud rate is selected.

It should be noted that this problem affects not just User Controlled Serial Interfaces but also the Program Download Interface.

I have never noticed any problems AT ALL with serial comms reliability on 18X's/14M's and 08M's at 2400 baud, no matter what temperature they run at - 0'C or 30'C - they always work, so is there any way to predict at what temperatures they might misbehave or is it essentially a case of trial-and-error with RC time-based products?

For the purposes of this thread, ignore all chips which can have external resonators or crystal arrangements - I'm talking about the internal resonator chips here...

 

[Andrew Cowan]

The internal RC clock accuracy depends on which PICmicro it is; the 16F88 and 12f683 state +/-1% at 25'C but +/-5% otherwise ( +/-10% at more extreme temperatures ).

That means that from a clock at 25'C, it could drift by just over +/-6% as temperature increases or decreases ( ie, starts at 4MHz*99% goes to 4MHz*105%, or from 4MHz*101% to 4MHz*95% ).

For the 16F627 Table 17-4 suggests +/-9%, but Figure 18-8 suggests +/-1% typically, but battery voltage will affect it also.

I'd suggest using a PICAXE which has a resonator/crystal as being far easier, cheaper and more accurate than trying to do compensation.

Back from 2006.

A

 

[westaust55]

PICAXE stability vs operating temperature

For the 28X1 and 40X1 the datasheet indicates:
The factory calibrates the Internal Oscillator block output (INTOSC). However, the INTOSC frequency may
drift as VDD or temperature changes, and this directly affects the asynchronous baud rate. Two methods may be used to adjust the baud rate clock, but both require a reference clock source of some kind.

The timing of the WDT is the best information I could see on the effect of temperature on speed.
Figure 18.2 indicates there is some tolerance in operation by as much as +/-30%.
The same figure also indicates that for the WDT, t Taking 25degC as the base line then the WTD time interval rises as the temp goes towards -40degC and that the WTD interval increases by around 22% when the chip temp is 125degC.

Figure 18.39 suggests the internal oscillator frequency can change in nominal frequency by around 0.25% as the voltage varies form 3V to 5V but there is a further tolerance of around +/-0.5% at any voltage.

Following curves indicate that as the chip temperature rises above 25degC, that the internal oscillator frequency reduces at all voltages by approx 0.5% but that the tolerance from the nominal/typical increases to around +2.5%/-1.5%.

 

[Dippy]

Grog,
When it comes to hardware electrical and physical specs ALWAYS read the actual Microchip DATA SHEET for the PIC in question.

They make/test the things so ALWAYS get that sort of Data straight from the lads.

www.microchip.com ... and take it from there.

In fact, go their website right now and find all the PICAXE PICs, download the Data Sheets and save them.
Then you will have ALL the infomation to hand.
It really is the easiest and most convenient way for you.

Note: Mirochip update things now and then ... on the sly

 

[westaust55]

Grog,
When it comes to hardware electrical and physical specs ALWAYS read the actual Microchip DATA SHEET for the PIC in question.

They make/test the things so ALWAYS get that sort of Data straight from the lads.

www.microchip.com ... and take it from there.

In fact, go their website right now and find all the PICAXE PICs, download the Data Sheets and save them.
Then you will have ALL the infomation to hand.
It really is the easiest and most convenient way for you.

Note: Mirochip update things now and then ... on the sly

Concur with Dippy.
I have a full set of PIC datasheets for PICAXE chips and Rev Ed components/boards plus many other items of interest.

Microchip is not the only one to update on the sly.
Rev Ed do that as well.

The Rev Ed "Whats New" page indicates Programming Editor V5.2.5 was released 19 June. Recall technical made a post to that effect.
But with no fanfare V5.2.6 is available (I downloaded days ago).

 

[BeanieBots]

so is there any way to predict at what temperatures they might misbehave

The blindingly obvious has already been stated, read the datasheet.
Don't forget, the datasheet gives values at which things MIGHT happen, NOT always what WILL happen with YOUR particular chip.

Also, if you have two chips talking to each other, BOTH are effected by temperature.

I have an 18X / AXE033 combo. Heat up the LCD PIC alone and the display goes erratic. Heat up both and all is well.

 

[Grogster]

Grog,
When it comes to hardware electrical and physical specs ALWAYS read the actual Microchip DATA SHEET for the PIC in question.

They make/test the things so ALWAYS get that sort of Data straight from the lads.

www.microchip.com ... and take it from there.

In fact, go their website right now and find all the PICAXE PICs, download the Data Sheets and save them.
Then you will have ALL the infomation to hand.
It really is the easiest and most convenient way for you.

...you have a point...

I will do that.

I MUST BE VERY CLEAR - I have no problems at the moment, I am only tossing around for comment, what hippy wrote on his site.

Personally, I have found the PICAXE to be exteremly reliable for serial comms over any temperature, but as a tech, I certainly understand the POTENTIAL that a change in temperature has to a basic RC resonator circuit...