PIC/AVR internal RC oscillator stability

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The datasheets I read on the PIC MCU says the precision is 2% for most
common voltage and temperature ranges (and 1% for the sweet spot).
I assume the AVR MCUs have similar characteristics.

This should mean that for say 10 clock periods at 4 MHz the period would
vary between 2450 - 2550 ns?

Is the stability higher for a shorter timescope wherein the temperature
and voltage should not have the ability to change as much. Such that between
two timepoints spaced 25 us apart. The period for 10 clockcycles would vary
less than the specified 2% ..?

,,,,

For a rs232 byte transmission the dataspec precision should mean 2% per bit,
and 21% for the full transmission? (maybe this is still acceptable for a pc
serial port?).

Maybe it's possible to work around this by creating a bitbang protocol that
resync with software at every bittransition such that any bitperiod will only
deviate with 1-2%?

1 us = 10^-9 s

 

[Phil Allison]

The datasheets I read on the PIC MCU says the precision is 2% for most
common voltage and temperature ranges (and 1% for the sweet spot).
I assume the AVR MCUs have similar characteristics.

This should mean that for say 10 clock periods at 4 MHz the period would
vary between 2450 - 2550 ns?

Is the stability higher for a shorter timescope wherein the temperature
and voltage should not have the ability to change as much. Such that
between
two timepoints spaced 25 us apart. The period for 10 clockcycles would
vary
less than the specified 2% ..?

** You think the temp and supply voltage can change *at all* in 25uS ????

For a rs232 byte transmission the dataspec precision should mean 2% per
bit,
and 21% for the full transmission? (maybe this is still acceptable for a
pc
serial port?).

Maybe it's possible to work around this by creating a bitbang protocol
that
resync with software at every bittransition such that any bitperiod will
only
deviate with 1-2%?

1 us = 10^-9 s

** Not last time I looked - pal.



........ Phil

 

[David L. Jones]

The datasheets I read on the PIC MCU says the precision is 2% for most
common voltage and temperature ranges (and 1% for the sweet spot).
I assume the AVR MCUs have similar characteristics.

This should mean that for say 10 clock periods at 4 MHz the period would
vary between 2450 - 2550 ns?

Is the stability higher for a shorter timescope wherein the temperature
and voltage should not have the ability to change as much. Such that between
two timepoints spaced 25 us apart. The period for 10 clockcycles would vary
less than the specified 2% ..?

The temperature isn't going to change much in 25us!
The oscillator is temperature and supply voltage dependant. Keep those
two stable and the oscillator will have the stability to match.

Dave.

 

[Jasen Betts]

The datasheets I read on the PIC MCU says the precision is 2% for most
common voltage and temperature ranges (and 1% for the sweet spot).
I assume the AVR MCUs have similar characteristics.

IIRC the AVR oscillator is particularly temperature stable at 2.7V

This should mean that for say 10 clock periods at 4 MHz the period would
vary between 2450 - 2550 ns?

Is the stability higher for a shorter timescope wherein the temperature
and voltage should not have the ability to change as much. Such that between
two timepoints spaced 25 us apart. The period for 10 clockcycles would vary
less than the specified 2% ..?

,,,,

For a rs232 byte transmission the dataspec precision should mean 2% per bit,
and 21% for the full transmission? (maybe this is still acceptable for a pc
serial port?).

you are reccomended to be within 5% of correct clock-rate for sucessful rs-232
(async 8 data bits)

Maybe it's possible to work around this by creating a bitbang protocol that
resync with software at every bittransition such that any bitperiod will only
deviate with 1-2%?

manchester coding

 

[TTman]

For a rs232 byte transmission the dataspec precision should mean 2% per
bit,
and 21% for the full transmission? (maybe this is still acceptable for a
pc
serial port?).

Maybe it's possible to work around this by creating a bitbang protocol
that
resync with software at every bittransition such that any bitperiod will
only
deviate with 1-2%?

1 us = 10^-9 s

AVR's have internal calibrated osc. It works fine on baudrates to 56K above
that it gets tricky at 115K ,beyond that impossible with int. RC.
AVR have an app. to calibrate the RC more accurately but with xtalsa costing
a few cents now, it's hardly worth the effort....

 

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1 us = 10^-9 s

Mmm.... I think not...

TYPO!, late night