pic12f675 with battery and RF problems!!!!!!!

[nicenoise]

hello
i'm developping an aplications using the pic12f675 with the HP3 linx transmitter with a 3.6V nimh battery.
I'm using a clock of 20Mhz.
when the battery is near to 3.6V the circuit not works very well... in this moment the range of the signal transmitted is very short (50ft)... when the battery es full (near to 4.20V), the system works perfectly and the range is 1000ft....

its extrange that affect only the range of the transmitter...
the voltage especifications of the transmitter are:
2.1V-12V
and the pic12f675
2V-5.5V

I think that the problem is the clock... its possible? anyone say me that microchip recomended a clock of 10Mhz maximun for 3.6V.... is this correct???
my problem is that i change the clock to 10Mhz... my rs-232 frecuency (31.25Khz) change a little bit....

pleas help...

thank you

Alex

[Ttelmah]

Get the chip data sheet. Look at the electrical specification. The chip is only guaranteed to work at 20MHz, with a supply voltage of 4.5v or more. That it runs at 4.2v, is 'luck'...
For 3.6v, the highest guaranteed clock rate is 10Mhz. You should be able to get your RS232 rate right at this rate. If it is fractionally 'off', tweak the declared 'clock' value to get it right. The software RS232, is not that accurate, but should not be far off at this rate. This does change slightly with the compiler version.

Best Wishes