Moving Serial application from 16F628 to 16F648a

[RogerM]

I have an application that runs on a 16F628 and uses the software rs232 function at 9600 baud.

I need additional program space so I'm using a 16F648a to take advantage of the increased program space. Ii do not want to change the RS232 part of the application so I want to keep on using the software function.

It does not run reliablely on the 648a [ it runs fine on the 628 and 84]. I use ports A2 and A3. A2 is the RX and I have a 750k pull down resistor and a 22k serial resistor. I have tried different values for these and it makes no difference to the 628 and 84 but these valuse seem to make
the 648a work better but I still get significant transmission errors. As part of the soft RS232 I use the invert option. The PIC based device is hard wired to a Palm Pilot.. that part seems OK.

My question is - is there some thing special about the F648a that requires different configuration or setup - either hard ware or software compared with the older 628.

I have the setup_comparator(NC_NC_NC_NC);

I have used a number of different version of the CCS C compiler and it makes do difference. Currently i have 2.202

Thanks ..

Roger

[PCM programmer]

This Piclist thread complains about the 16F648A. It mysteriously
doesn't work. No solution found.
http://www.google.com/groups?&threadm=eprohv0u9kdrfefpvdgqnlhhk13nppc3lv%404ax.com

Same thing here:
http://forum.microchip.com/tm.asp?m=28234&mpage=1&key=16f648a&anchor#28234

And here:
http://forum.microchip.com/tm.asp?m=24815&mpage=1&key=16f648a&anchor#24815

Mysterious problems with data eeprom that don't occur
on other chips:
http://forum.microchip.com/tm.asp?m=20295&mpage=1&key=16f648a&anchor#20297

Microchip's migration document:
http://ww1.microchip.com/downloads/en/DeviceDoc/40048a.pdf
From that document:
"Because the PIC16F627A/628A/648A family uses a newer process
technology, there will be subtle behavior differences between the
PIC16F62X and the PIC16F627A/628A/648A parts. Before starting on a
design migration, check the data sheets and verify that the electrical
specifications for the new part are appropriate for your application".

People on this board have noted that the 'A' version chips require
more careful attention to power supply bypassing.

From reading this board, I have drawn the overall conclusion that
the 'A' chips have much less margin for error, in all things.
So, I don't use them.