Sutton:

Welcome to 8052.com.

I cannot give you an exhaustive discussion about all the differences of the FA series parts but I can make a strong guess from how you described the problem as to why it is not working. Take a close look at the description of the POWER DOWN MODE on page 6 of the attached data sheet for the Intel 'FA/FB/FC series parts.

Intel_87C51FA Datasheet

You will note that power down mode for the Intel parts can be exited by either a pulse on the reset pin or the assertion of an appropriately enabled level mode external interrupt.

I helped some folks about 8 or 9 years ago with the conversion of their product from the EPROM based P87C51FA/FB/FC series part over to a flash based "equivalent" part. At that time we found that their product design depended upon the design of the external interrupt scheme to exit the power down (STOP) mode. We had tried to use the similar Atmel flash based 89C51 type part only to discover that the part only is able to exit STOP mode via the pulse on the reset pin. Other 8051 variants may have had the same difference in STOP mode.

If your product design and the way the firmware is written depend upon the STOP power down mode to conserve power and expected the external interrupt mode to exit to full active mode then that may explain why your parts do not work. Study carefully to make sure the parts you try to use offer the exit from STOP mode that matches the product design.

Michael Karas

PS - P87C51FA/FB/FC parts differences are 8K/16K/32K onboard EEPROM.