Hello everybody,

I am currently developing a synthetizer board for a UHF transceiver. I need to reprogram the PLL when changing from receiving to transmitting. I have a PTT line which changes from high to low level when the transceiver goes to transmit(TX) status.
The PIC uP from Microchip has a inteseting feature, they can generate an interrupt when a pin goes from low to high and from high to low, that is it can detect a change level in a pin and then generate a interrupt, but 8051 series can´t do this. Then I emulate it using a XOR gate and an adittional pin in order to generate a falling or rising edge activated interrupt.
But I would like to do something similar without using external hardware. Any ideas?

I thought to use a level generated interrrupt, when PTT line goes from high to low an interrupt is generated, then the Interrupt service routine reprograms the PLL for TX frecuency, but consecutive interrupts will be generated while PTT will be low and therefore the PLL is continually being reprogramed for the same frecuency it has currently programed. Besides of this I must detect when PTT line changes from low to high and then reprogram the PLL for RX frecuency. I can use flags to detect if PLL is in TX status and then not to reprogram it over and over, but how can I program to RX status when I release the PTT line.

The code must be interrupt driven and not polling driven.

Any idea or suggestion would be appreciated.

Best regards.
Fran Dominguez