Luis Uribe

Isolation between digital output signals and the external pins

Discussion created by Luis Uribe on Feb 29, 2012
Latest reply on Mar 1, 2012 by Luis Uribe



Is there isolation between:
  - digital output signals, and the
  - external pins?

In the (new)  MC9S08QE128RM.pdf  Reference Manual, page 112:
  - the output signals PTxDn are represented as being DIRECTLY connected to the output pins.


In the (old) MC68HC908GP32.pdf (Data Sheet, 2008, page 119), or similar (original User Manual, for example...):
  - there is a TRI-STATE DRIVER that isolates the signal data from the output pin.

  And there is more: Data stored in the output registers, are read (when requested):

  - NOT from the outside pin, but
  - FROM the register's output (they are isolated !).


The difference is crucial.
  - If there is NO buffering driver, and the output pin is read directly from the outside pin

    (as in Read-Modify-Write instructions, like INC),

  EXTERNAL conditions WILL affect those operations.


Could anybody state, for sure, that the NEW MC9S08QE128 has isolation

- between Output Data (output registers)
- the Output PINs,


 as was the case on the OLD MC68HC908GP32?


I SUPPOUSE that, yes, THERE IS Isolation on the new devices too but, like the new documentation has dropped ALL references (pictorial and textual) to it, I  must  to  KNOW  for  SURE.


Thanks a lot.


Luis G. Uribe C.