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