Substituting an obsolete hc908 MCU

Thread Starter

narkeleptk

Joined Mar 11, 2019
532
Have a thought hanging in my mind for awhile now but have not started pursuing much yet. Figured I would toss it out to see if anyone may have some deal breaking information before I dig into it to much.

So... I have a quite a few control modules that come in with failing mc68hc908as60 PLCC52 mcu's. For these repairs I generally just replace the module or scavenge a good mcu from another more common module. I have tried sourcing these chips from many different places with no luck. The Sellers never can deliver. However, I can easily get the 908as60 qfp64 package's still and I have enough room in most of the units to place a qfp64 to plc52 adapter if I was to make one. I can unsecure the original mcu's for cloning data or just use the memory from others I have but I have no access to any source code.

My main concern is that the different packages my behave differently with the programming. I have looked a little in the datasheet and will dig into it a lot more but I have not really seen anything in there yet where there is much difference in the two packages other then the 64 having some extra ports. I'm hoping I can just float them and everything else will be ok. I know very little of source coding in general so this may be crazy talk and not possible at all. Any input on the subject is appreciated. Thanks!

datasheet if interested:
https://www.nxp.com/docs/en/data-sheet/MC68HC908AS60.pdf
 

jpanhalt

Joined Jan 18, 2008
11,088
I am not familiar with that device and it's a lot of datasheet. Apparently, the ports come up as inputs. If an additional port is the only difference, floating that port might lead to problems. You might get away with just tying it high or low, though. If you can disassemble the code, making it an output (page 324) should be easy without having actually to understand all of it. I didn't check peripherals. Presumably, they would not be changed for the corresponding ports.
 

Thread Starter

narkeleptk

Joined Mar 11, 2019
532
Thank you for your reply. When I get a little free time I will check to see how it handles unused ports on a working module. I think I remember one or two unused and grounded but will need to verify. If this is the case then its safe to assume I could just ground the added ports from the qfp as well yes? I might even just program a qfp on a breakout and solder some headers on to the plc footprint see how it goes.
 

jpanhalt

Joined Jan 18, 2008
11,088
Based on my experience with PIC's, inputs are high impedance and it probably doesn't matter functionally whether they are grounded or high. I would ground as well to limit current drain.

Here's what NXP says (p. 302):
20.2 Introduction Forty bidirectional input/output (I/O) pins form six parallel ports. All I/O pins are programmable as inputs or outputs. NOTE: Connect any unused I/O pins to an appropriate logic level, either VDD or VSS. Although the I/O ports do not require termination for proper operation, termination reduces excess current consumption and the possibility of electrostatic damage.
 

be80be

Joined Jul 5, 2008
2,051
It maybe just like the pic chips you can run code for the smaller chip like the 18f2550 on a 18f4550
Just as long as the code dont use pins that the bigger chip has it works fine .

Pic has some 28 pin chips that have 40 and 44 pins to but the core has the same memory map.
Hope you are in luck with your chip
 
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