AFC

[Jan Lichtenbelt]

AFC Automatic Frequency Control

Frequency tuning of 2 microchips with internal oscillators, in case of e.g. using RS232.

The internal oscillator of RS232-Rx microchip will be tuned to the same baudrate of the internal oscillator of the RS232-Tx microchip.

This is realised by a continously square wave of the RS232-Tx by means of sending continously characters 'U'. (Kind of function generator by means of RS232-Tx)
The frequency of this signal will be baudrate/2.

This frequency will be compared to the frequency measured in the RS232-Rx chip. By means of OSCTUNE both frequencies will be made equal.

The input RS232-Rx signal must be on pin T1CKI
(pin B6 for PIC16F11847)

Other input Rx paramters:
1) Baudrate
2) Accuracy, see details in the flowcode
Ouput on LCD screen:
1) Baudrate found (if correct this must be the given value)
2) OSCTUNE, the paramters for the tuning of the internal oscillator
(0..31 positive frequency shift, 63...32 negative frequency shift.
One bit change is about 0.09% frequency change)

Remark:
- Frequency counter macro used, can be replaced by any other on.
- I found an OSCTUNE value of 6, which means a deviation of 0.5% of both internal oscillators for 2 members of the PIC16F1847 serie.
- I prepared this flowcode in V5. Translated it into V6 did not give the correct results. Up to now I do not know why.

Jan Lichtenbelt, NL

[Benj]

Hello Jan,

Sounds like a good project, thanks for sharing.

I think v6 will need an additional component macro to initialise the RS232 component. Hopefully this is the only difference.