MC6800 OEM variant: SC44125P - two questions

[metertech58761]

One MPU I have at hand in a number of products is the Motorola SC44125P, and which I had asked about in multiple places over the past few years. 

I have since come to understand it was an OEM-style processor initially commissioned by General Motors for their earliest engine control modules.

Essentially, it has a 90% compatible pinout with the stock MC6800, but uses the MC6801 instruction set.

(edited to incorporate input from later in this thread)


There are only (three) pins that differ between the MC6800 and the SC44125P - and I (still have) questions relating to pins 5.

Pin 5:
6800 - Valid Memory Address (_VMA) output flag.
SC44125 - on the one paper schematic I have, it is shown as _VMAT, and is always grounded on the three different boards I'm aware of.

This means it serves as an input on the SC44125, but to what purpose?


And for completeness, the other two pins:

Pin 3:
6800 - Phi1 clock input
SC44125 - provides access to the normally internal IRQ2 line (except for one application, this line is simply pulled up to Vcc and therefore unused)

Pin 39:
6800 - Three-State Control (TSC) input
SC44125 - labeled _LIR (Load Instruction Register - more relevant to factory test than field applications / not used on any of the boards in question)

[woofy]

...
Pin 38:
6800 - unused
SC44125 - labeled _LIR, but on the three different boards, this pin is not used at all.

What would LIR represent?

100% guess as I have no idea.
Maybe Load Instruction Register. It could be used to indicate a good time to refresh any DRAM, similar to the Z80 M1/RFSH signals.

I would put a 'scope on it and see what signal can be seen,

[metertech58761]

Right now I don't have a setup where I can do power-up testing of one of the board sets, so I thought I'd give the brain trust here a chance.

(NXP was useless and I can't seem to get to the CPU World forum from here - I see it's up, just not reachable)

No biggie, it was mainly for my curiosity.

[SCSKITS]

Likely a custom part for GM. Motorola did a lot of custom variants back in the day. Not likely to find a data sheet except in an old GM archive.

[floobydust]

What year and make/model? The CPU is as I remember, revealed a bit in the GM patents especially the '81 Cadillac debut.
I know those who reverse-engineered these puppies and could ask for some details. A lot of it was the memory size/address decoding and custom timers/TPU. The TBI pickup truck one is still a popular junkyard find.
You can easily disassemble the F/W but it's still tables in EEPROM which makes for a hassle tuning.

[metertech58761]

Here's a pic of the SC44125 on one of the boards it was used on (part of a set of 4 panels; I have over 20 complete units I need to figure out what do with).

[fulovit]

This sounds like the GMCM (GM Custom Microprocessor) used in C3 (Computer Command Control Systems). From the information I have, the pinout is the same as a 6800 except (6800 usage in brackets)
pin3=IRQ2*   (CLOCK PHASE 1)
pin4=IRQ1*   (IRQ*)
pin5=NC        (VMA)
pin38=NC
pin39=LIR*    (TSC)

IRQ2* Same as IRQ1 but lower priority
LIR*  Load Instruction Register
The LIR* signal is active low on each CPU cycle when an opcode is being fetched. It is a buffered equivalent of the signal used to gate the data bus into the instruction register. This output is primarily used for test purposes and may be left open when in a system.

It looks like the bus  has some 6801 functionality implemented.

From the 1984 Motorola 6801 Reference (Internet Archive)
2.4.1 MC6800 Bus Comparison:
- The CLOCK PHASE 1 signal is derived internally
- VMA is not provided by  the MCU and is not required
- TSC, DBE, HALT* and BA have no counterpart on the 6801

I think  your hand-drawn pinout may have an error:
- VMA  if present it's an output (MCU to peripherals)
- PIN39 = LIR not PIN38

[metertech58761]

Here's the schematic where I got the pinout from.

[fulovit]

That source is a hand-drafted schematic so it could be in error. The differing pins are either not connected or not used so if there was a mislabelling error it would never have been picked up. Also the vma output is not tristate, so the fact that it is grounded could mean a labelling error.  My source is not official either,  so the only way to be sure is to see some printed documentation.

I also have this:
The GMCM cpu only has a single phase2 clock input.  It has an additional irq input  irq2. The tristate control input is removed however the address buffers will still go tristate during a halt. The vma is replaced by making the address lines go to all ones or high. All other inputs and outputs are similar to the m6800.

The only useful bit of information on the sc44125p I found was using Duckduck - 4 Star Electronics (obsolete electronics) has 4 for sale. My only suggestion is to use Kagi (they have a free trial), but I'd say no documentation exists.

[metertech58761]

Here are two more schematics using the same MPU. One is the Panel C shown earlier in the thread and the other of a logic board out of the 'base station' for this system.
Both were made by tracing and toning out all the parts on each.

Edit: After thinking on it a bit, I will accept that the paper schematic may be in error, have updated pins 38 / 39 on the symbol in my KiCAD library, and updated the schematics I have at hand.

That just leaves pin 5.

[fulovit]

The date on the schematic corresponds roughly to the end of production of the gmcm. This leads me to speculate that perhaps excess inventory was sold as a motorola part.
It may have had sparse data sheets or none at all, so perhaps the draftsman was being conservative with pins he wasn't sure of. I can't say what the t in vmat is though.

If you accept that the chip is identical to the gmcm  (the pinout is unique as far as I can tell) then vma is no longer used and pin5 is not connected. You could confirm this by checking out an old ecm.

[metertech58761]

I have seen date codes on the SC44125 stretching into 1990, so Motorola probably got the OK to offer it on a per-customer basis after GM moved on from that MPU.

However, all three products using this MPU were superseded by 1992, each ending up with a different processor.
(one changed to the 6809, the second changed to the Hitachi version of the 6801, and the last one switched to the 68HC11F1 circa 1992)

[fulovit]

Perhaps the sc44125 was only sold to large organizations and product data stayed in house, or was just never digitized.
In the attached motorola genealogy the reference to custom was probably this chip.

[metertech58761]

Very nice!

I definitely agree the "Custom CPU" between the 6800 and 6801 just about HAS to be the GMCM / SC44125!