MM58167 RTC

[Michelle Lawson]

Since I had a few of these laying around, I figured I'd whip something up. Granted it's probably not 100% RC2014 compatible, it does work and I have my code running and working as expected. Based on what the planned use if for my RC2104 system, my code may not be of much interest; but I'm happy to share it if anyone is interested. Anyway, here's the schematic....

[Ed Silky]

Hi Michelle,

Very cool!

Quickly looking at the schematic, I have one suggestion to make it applicable to more RC2014 systems. On the '138, instead of tying the HIGH ENABLE (pin-6) to 5V, drive it with M1- from the Z80 (RC2014 bus). The reason is that the Z80 takes IORQ- low for I/O (port) RD/WR operations, but it also takes it low for interrupt acknowledge. The difference is that during INTACK the M1- signal is low (along with IORQ-) while during the port operations the M1- signal is high. 

If this board is used on a system that uses Z80 peripherals (CTC, SIO, PIO, DMA, etc) it is important that it doesn't get enabled onto the bus during an interrupt acknowledge cycle when one of these devices generates an interrupt. Using M1- on the HIGH enable for the '138 will ensure that.

-Ed

On Wed, Mar 25, 2026 at 7:12 AM Michelle Lawson <michelle...@gmail.com> wrote:

Since I had a few of these laying around, I figured I'd whip something up. Granted it's probably not 100% RC2014 compatible, it does work and I have my code running and working as expected. Based on what the planned use if for my RC2104 system, my code may not be of much interest; but I'm happy to share it if anyone is interested. Anyway, here's the schematic....

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[Mark T]

A better use of E2 and E3 would be to reduce the address range.

Assuming the MM58167 requires either RD or WR in addition to CS, then M1 does not need to be used. RD and WR will both be inactive during interupt acknowledge.

[Michelle Lawson]

Well, my wire wrapped prototype is wired based on the schematic, and it's working; and well, JLCPCB is spinning boards as we speak. That and I'm not big on interrupts. All that said, I'm happy to pass along any of the KiCAD files if anyone wants to tweak it for another design.

[Ed Silky]

Excellent. Just something to keep in mind. MarkT's suggestion is also something to keep in mind (as long as RD- keeps it from putting anything on the data bus).

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[Ed Silky]

Hi Michelle,

I'm curious, you say that you're "not big on interrupts", but does your system not use any? I thought you were running CP/M - are you doing that with all polled I/O? Or are you completely running with your own firmware?

-Ed

[Michelle Lawson]

Ed, I did originally plan to base things on a CP/M OS platform, I decided that it would be better to just go with an embedded firmware and cut down on a lot of overhead. Thus, the WOPRjr is taking shape. I have gotten development from testing the various segments and onto an integrated embedded OS. Reading and writing to & from the various comm ports work, I can format and do a directory of the CF card, and now the RTC is working. The printer will be just a port from the same design I have running on another project. So far, so good. Hey, at the end of the day, it's all just ones and zeros; you just have to get them in the right order for them to play nice..... Keeps my brain functional and a great hobby. Thanks

[Ed Silky]

Hi Michelle,

Very good. Yep, it's all about what works and what you want to spend time on. Keep up the good work. Please continue to provide updates.

-Ed

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