TK68HC24 Port Replacement Unit

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Tekmos, working in cooperation with NXP, has designed a replacement for the NXP MC68HC24. This part is designed to be identical to the NXP part, and is a drop-in replacement for the NXP part. We provide the part in both the 40 PDIP version and the 44 PLCC version. All parts are at the fastest speed grade.


  • Utilizes the Tekmos 68HC24 Core
  • Replaces ports B and C of 6811 type microprocessors.
  • The chip-select function allows multiple TK68HC24s to be used in systems requiring multiple parallel ports.
  • 3 – 5.5 Volt Operation.
  • 0 – 5 MHz Operation.
  • Supports all handshake and I/O modes.
  • Available in 40 PDIP and 44 PLCC versions.


Design Details

This is a new part that has been designed to be identical in performance to the original part.  It is not the original part.  The only measurable difference we are aware of is that we deliberately added Schmitt triggers to all inputs.  The parts still meet the original DC and AC specifications.  But since it is being built in a newer, 0.8u technology, we were concerned that slower rise-times which worked on the original parts might cause problems in the newer technology.  The addition of a Schmitt trigger prevents that from being a problem.

There is one issue that some customers have faced.  The specification for input rise time is 20 ns.  However, some customers are using a RC circuit on the reset input that produces substantially longer rise times.  This problem may be aggravated when the reset signal goes off board, adding to the capacitance.

Parameters vary from lot to lot.  But generally, the input threshold for reset on the TK68HC24 is higher than both the original NXP 68HC24 and that on the 68HC11 microprocessor.

If this is the case, and If reset has a slow rise time, then the 68HC11 processor can become active while the TK68HC24 is still in reset.  We have seen a case where the processor wrote initialization data to the TK68HC24 while it was still in reset, which caused a system failure.

The best fix is to not violate the input rise time specification.  A second fix is to install a software delay during initialization to allow reset to become a valid one level before writing to (or reading from) the TK68HC24.


The data sheet is available here: pdf TK68HC24 (149 KB)


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