HP5315 Counter MK3870 Micro & MRC Chip

[Kjo]

I am heading down a rabbit hole with this project an hope there are a few old farts out there that may have access to some proprietary data. I have build a USB based daughter card for this series of frequency counters that interrogates the MK3870 for all of the info that is sent to the display. From this, the firmware figures out what mode the switches are in and formats the data for transmission over the USB to a logging application.
This all works quite well. While I have several of these meters, I managed to brick a MK3870 after many removals. I think the plastic package actually bent in the middle.
I have the binary ROM image and 5 MK38P70-02H micros. But I can only get one of them to run. I can’t figure out what is the problem with the others.

Now for the rabbit hole. A modern PIC should be eminently better in this application. I’m using a 16F887 cause I have a bunch. Even with a disassembly of the 2K ROM, it is a near impossibility to figure program flow. There are just no good tools around any more. Even ADA PRO doesn’t support the F8/MK3870 hardware.

But HP has described the 5315 both in manuals and the HP Journal. So I understand it quite well.
Using a daughter translation card, I have all of the front end firmware working, including reading the switches and programming the display.

The tricky part is programming the MRC and reading it. I have decoded the basic MRC programming for Freq, period, gate time & osc check modes by logic analysis. I’m confident I can decode the other modes also. But I fear that that there are settings that I will never figure out this way.

There are 4 registers in the MRC. Three are read only. They are event, time, status & control. Event and time are easy. But while I can read and write the others, I can’t really determine what the bits mean. The status is 12bits & control 20bits.

I am hoping there is someone who might have undocumented specs on the HP MRC as it was used in quite a few other instruments over the years. Any help or pointers is appreciated.