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Pi85: A small 8085 system with Raspbery Pi IO and ethernet [message #3898]

Mon, 04 December 2017 08:01

ab0tj
Messages: 70
Registered: October 2015
Location: Colorado

Member

Hi all, I just wanted to share what I've been working on lately... I didn't want to leave an S100 system running while writing and testing code for the S100 ethernet board I built, so I whipped up a small PCB for a system do do development on. The result was the Pi85 - an 8085-based system with 64K of RAM (actually 128K but it has no banking functionality) and a Raspberry Pi Zero W that handles IO and storage duties. The Pi currently provides a console device and up to 4x 8MB virtual hard drives, but it could be expanded to do more. The Raspberry Pi is 3.3V IO and not 5V tolerant, but testing with an oscilloscope shows the outputs of the 8085 chip can barely get above 3.3V even when unloaded, so level conversion was omitted and the rest of the circuit uses 3.3V devices. A WIZ830MJ module is included for ethernet connectivity on the 8085 side. At this point the hardware works and CP/M boots, but software development and testing still continues.

Due to economy of scale when having PCBs manufactured, I have a few boards available for the price of shipping. But beware: SMD soldering is involved (1.27MM pitch minimum though) and I'm no professional so I claim no responsibility if it burns your house down or terrorizes your cat.

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[Updated on: Mon, 04 December 2017 08:06]

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Re: Pi85: A small 8085 system with Raspbery Pi IO and ethernet [message #3900 is a reply to message #3898]

Mon, 04 December 2017 08:35

Andrew B
Messages: 467
Registered: October 2015
Location: Near Redmond, WA

Senior Member
Administrator

Interesting.

Is the Pi software 'bare metal' or running on top of Linux? If it's running on top of Linux, how do you solve the problem of the non-realtime nature of the Linux kernel/long pipeline of the ARM causing GPIO pin interrupts to take many clock cycles? Is it something like the PropIO board where the CPU is automatically put into a WAIT state until the Pi replies with data?

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Re: Pi85: A small 8085 system with Raspbery Pi IO and ethernet [message #3901 is a reply to message #3900]

Mon, 04 December 2017 08:44

ab0tj
Messages: 70
Registered: October 2015
Location: Colorado

Member

Andrew B wrote on Mon, 04 December 2017 09:35

Is the Pi software 'bare metal' or running on top of Linux? If it's running on top of Linux, how do you solve the problem of the non-realtime nature of the Linux kernel/long pipeline of the ARM causing GPIO pin interrupts to take many clock cycles? Is it something like the PropIO board where the CPU is automatically put into a WAIT state until the Pi replies with data?

The Pi is running Linux. In fact I'm using the WiringPi library so it's not exactly fast. What happens, in a nutshell, is this:

- The 8085 writes a command byte and maybe some data to a predetermined area in memory
- The 8085 outputs to port 0 which sets a flag to tell the Pi that it wants something
- The Pi takes over the bus via the HOLD line
- The Pi reads from memory to find out what the 8085 wants and performs that operation
- The Pi returns control back to the 8085

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Re: Pi85: A small 8085 system with Raspbery Pi IO and ethernet [message #3908 is a reply to message #3898]

Tue, 05 December 2017 20:35

mikemac
Messages: 249
Registered: March 2017

Senior Member

I'm slow tonight. I don't see anything S-100 related. Is this board so you can write 8085 code?

Checking out the actual power level of 8085 looks like it paid off. I never would have thought that that might work. Good job!

Mike

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Re: Pi85: A small 8085 system with Raspbery Pi IO and ethernet [message #3909 is a reply to message #3908]

Tue, 05 December 2017 21:07

ab0tj
Messages: 70
Registered: October 2015
Location: Colorado

Member

mikemac wrote on Tue, 05 December 2017 21:35

I'm slow tonight. I don't see anything S-100 related. Is this board so you can write 8085 code?

Checking out the actual power level of 8085 looks like it paid off. I never would have thought that that might work. Good job!

Thanks for the encouragement. It has been a fun project so far.

I guess I could have been more clear on the S100 part... I already built an S100 board that hosts an WIZ830MJ ethernet module so that I can attach my IMSAI to an ethernet network. As a side project, I made Pi85 so I could develop software for the S100 board without having to run the old IMSAI the whole time I'm working on the ethernet software. This little board has the same ethernet module, takes a lot less power, and is a lot easier to develop code on. I get the best of both worlds because I can use a powerful text editor like "vim" in Linux to edit an assembly file and then easily transfer that into CP/M to assemble and test.

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Re: Pi85: A small 8085 system with Raspbery Pi IO and ethernet [message #3910 is a reply to message #3909]

Wed, 06 December 2017 09:01

ab0tj
Messages: 70
Registered: October 2015
Location: Colorado

Member

Posted the related code on GitHub in case anyone is interested: https://github.com/ab0tj/Pi85

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