Suitable replacement for legacy products with MC68701 part with same instruction set MC68008?

[tdiguy]

I am a firmware engineer at Power supply manufactirer and we have roughly 20+ legacy products with the motorola MC68701, MC6800 series processors used. We have exhausted sources of the MC68701 and need to replace it with micr controller that has the same or very similar instruction set. I have been reviewing the datasheets for current production parts for a viable replacement, but could use some help choosing the best match.

The MC68701U4S-1 is the most widely used part in our legacy products. The UV PROM is not used in any of the code any way, so that is not important for the replacement to have non-volatile. All of the code is very simple interfacing with parallel ADC's, DAC's, bus transceivers and then uses SCI (UART)  to communicate between master and slave micro controller in the same power supply.

In a perfect world:
The best case scenario is I could take the source code I have now (assembly) and change some register names, memory locations and compile it. Design a small adaptor pcb to adapt from DIP40 to the current layout. I would be curious to seek your opinion on this. Some of these design are used in military applications and the less code changes the better because of the re qualification testing needed.

From the reading I have done so far, the MC68008 seems like a good fit. The instruction set for the MC68008 seems to match up pretty close.  I know it is not reccomended for new designs, but we could make a lifetime buy now.

In the past I have used a PIC and completely reverse engineered the entire design, and basically started from scratch and wrote the code with C. This is a very time consuming process and if I can cut that down by using a Freescale product then I would like. After some research it looks like it would save a lot of time and be closer to original design (if not exactly the same).

Thanks,

Vince

[bigmac]

Hello Vince, and welcome to the forum.

The closest replacement for the 25 year old MC68701 device might be from the MC68HC11 family, which is probably about 10 years younger.  This has backward compatibility with the instruction set and addressing modes of the 68701, and is capable of interfacing with external memory. This family is also now virtually obsolete "not recommended for new design", but you might still be able to procure stock of a few of the device types.

To determine what firmware changes may be necessary, you will need to check for memory map compatibility, and obviously the peripheral modules, such as timer and SCI, may differ in operational detail.

Regards,

Mac

[celsoken · Accepted solution]

Dear  Vince,

Mac is right, as usual. But most HC11 parts are obsolete and the risk of getting trapped again is high.

Maybe it could be interesting to migrate to S12 wich is still upward code compatible with HC11 wich is upward code compatible with 6801. Maybe you could use the internal peripherals and avoid external memory access.

I've done some successful migration in the past so it can be done. I did not have mil reqs but automotive.

Since you'll probably build a translate table,do it for the S12 family.

You could possibly port to S08 family but since memory models upgraded from 6805 I would not recommend that.

6805 architecture was a disruption from 6800.

Cheers,

Celso

[tdiguy]

H iCelso,

I want to thank for the lead! After reviewing the S12D part will be a great fit. The "MEBI" module "Multiplexed External Bus Interface" will allow me to externally address my parallel ADCs, DAC, and bus transceivers and keep 90% of the original source code.

And to boot it is still an active part. Now if I could find a development board around I would be set. Then I could breadboard something to test this out.

Boy you have to love to the power of users forums, I have been in discussions with field apps engineers and support requests and they were not to helpful.

Thanks Guys.

And I also will check into the OTP 68HC711 at the very least for an interm solution. 

[celsoken]

Dear Vince,

In order to get things going, I suggest you to purchase at least the following items:

- one BDM interface e.g. Universal Multilink from P&E

- some of the S12 part of your selection

- some PCB adapters for the footprint of the S12 you chose

With the adapter you can wire directly to your target board.

Good luck,

Celso

[tdiguy]

Thank you for the ideas, I will look into them and see if I can make it work.

I have looked at the 68HC11 and most of the stock I can find are in ROMless parts and I need to use one chip mode. 

And the shame of it is I have converted a design from MC68701 to 68HC11 in the past and I actually have a DIP 40 to PLCC adptor pcb designed. If I could find significant stock of a ROM or flash part with a high enough pin count that would suit perfectly.

Vince

[celsoken]

Dear Vince,

As far as I can remember the most usual parts at affordable cost would be MC68HC711xx OTP ones. At this time there were E2P versions wich were bloody expensive and OTP parts. No flash parts available.

Good luck,

Celso