Max.OSC.Freq. to communicate with PIC18 UART at 115200 baud?

[BasePointer]

pic18 uart

Hi,

I'm using PIC18F452 at 40MHZ.
Its UART doesn't support 115200 baud rate with 40MHZ XTAL.

BaudRate = 40MHZ / (16*(21+1)) = 113.636,3636
Error = %1.37 is not acceptable for me.

Which frequency should I use to make Error=0.0 ? It also must be nearest value to 40MHZ.

10x
BP

 

[FoxyRick]

uart pic18

This should be close enough for normal communication I would think. But if you really need it closer, here are the best figures for 115200 baud for standard value crystals:

18.432 MHz gives an error of 0.000000% with reload value of 9
24.000 MHz gives an error of 0.16% with reload value of 12
33.330 MHz gives an error of 0.46% with reload value of 17
38.880 MHz gives an error of 0.45% with reload value of 20

If you really want 0% error close to 40MHz, the closest I can get is 38.707200 MHz which gives an error of 0.0000% with reload value of 20. This is not a standard value of course, just to keep life interesting. Maybe you could persuade the 38.880 crystal to pull that far?

Or, you could ask these nice pwople to custom make one for you:

https://www.icmfg.com/buildcrystal.html

Of course, if you really need this level of accuracy the instrument must be kept in a controlled temperature oven as the crystal's frequency will alter too much with temperature variations (and ageing, and external vibrations, and capacitor ageing, and out of pure contrariness)

Good luck,
FoxyRick.

 

[BasePointer]

44.2368mhz osc

Is overclock possible for PIC uP?

I found 11.0592MHZ XTAL. It becomes 11.0592*4 = 44.2368MHZ with PIC's internal PLL and thus
BaudRate = 44.2368MHZ/(16*(23+1)) = 115200 baud that I exactly want.

Does PIC's internal PLL allow this ?

10x
BP

 

[FoxyRick]

uart max baud pic18f2550

As far as I am aware, it is possible to overclock a PIC. There are no checks on what frequency it is running at. However, operation is only guaranteed up to its stated speed.

You are looking at over 10% higher - quite a lot. The only way is to try it and see if it is stable (or runs at all - the PLL might not lock, if it locks, the chip might need a little extra voltage to ensure clean logic transistions - this produces more heat etc etc.).

Remember though that results may be completely dependant on individual samples - don't expect this to be a commercial solution unless you do a *lot* of testing.

I don't mean to sound negative - I hope you succeed. I am interested in your results.

Cheers,
FoxyRick.

 

[BasePointer]

pic18 baud 40mhz

I'll tell you the result as soon as I try.

10x for your interest.
BP

 

[muko]

pic18 how to overclock

I have worked up to 1.2 Mbaud with 18F452 device and works fine.