pic microcontroller discussion list <> wrote:
> In members' experience, can the 16f628A (which claims to have
> a precision
> internal oscillator) adequately handle 9600bps reception when
> using the
> 4MHZ internal oscillator?  I have a design where freeing up
> two pins would
> be extremely helpful, so would like to avoid a crystal or resonator.

The official answer should probably be a FAQ (starting with "No").  The
real life answer is:
IME, it usually works, but I wouldn't use it for volume production or
anything critical.

A serial byte is about 10 bits long (including start bits etc), and is
usually sampled at the receiving end in the middle of each bit.  Therefore,
if the timing slips by more than 1/2 bit width during 10 bits, there will
be errors.  That's an error of 0.5/10 or 5%.  If you assume that is shared
across both transmitter and receiver, they are allowed 2.5% each, but it
would be better to keep it under 1%.  The 628A internal clock (IIRC) is
about 3-4%

The real life answer comes about because:
1) when I have used them for this purpose, I haven't needed them to be
functional over a wide temperature or supply voltage range,
2) the typical accuracy is better than the worst case quoted in the
datasheet
3) I don't usually have a PIC at each end, the PC end is probably a
crystal, with accuracy better than 0.1% so the PIC can get away with a
larger clock error

If you can test each part across the full expected range, and afford to
throw away some PICs, it might be worth a risk for a small controlled
production run, but that's what you have to figure out- I wouldn't try it
personally!

As someone else has mentioned, the baud rate is irrelevant, the above
problem is just as bad at 300 baud and 19200 baud

Nigel
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