02/03/2016 at 07:09 •
Since my EPROM burner is so old it connect via a parallel port I needed to find another solution for that. I found an old but never used Dallas DS1235 32Kx8 NVRAM module that I got in a goodie bag at some electronics seminar I attended way back in the past.
It's dated 1988 and looking at the data sheet the internal battery should last at least 10 years. Considering it's almost 20 years past the expiry date I wanted to make sure that the batteries wasn't dead.
The module consists of a SRAM with a DS1210 Nonvolatile Controller Chip and two lithium batteries potted in epoxy.
Luckily the top of the DS1210 was exposed at the bottom so it was rather easy to just use a hot soldering iron to scrape off the epoxy layer by layer to get down to the legs of the DS1210 so I could measure the voltages of VBAT1 and VBAT2.
It was dead... VBAT1 had just a few millivolts and VBAT2 had fared batter and was at 250mV.
I will dig out some more of the epoxy so I can cut one of the battery leads and attach an external battery to keep the module alive without VCC.
I'm not sure if I can use a single 3 volt coin cell battery and expect the SRAM to be within data retention limits since the DS1210 have a voltage drop of up to 0.3 volts. But the maximum voltage of VBAT1/2 on the DS1210 is 4.0 volts so I can't just series two coin cells.
So I have three choices. Either run it on a single standard 3v cell, get a 3.6 volt Li-ion coin cell or series two 3 volt cells and then hook it up with some diodes to drop off the extra voltage. But at nano amps the Vf of a diode is not much so I'd need a lot of them. Maybe I can use a LED instead....
02/03/2016 at 06:32 •
The Z8671 is the oldest of the two MCU typesand seems rather easy to use as well. The BASIC interpreter is fit into only 2K ROM instead of the 4K that the P8052 is using so it will be a lot more limited in functionality, but that will only add to the authenticity of the time period. ;-)
The Z8 (as the Z8671 is a part of) are using a muxed Data/Address bus for Address[0..7] so a latch is required to capture the lower address.
Then it also reads the 0xFFFD memory location and uses the three least significant bits of the data there to set the baudrate of the serial console port, so I need a three-state buffer to handle that.
And the Address[8..15] bus must be decoded into chip-select signals for the external RAM and the baudrate-buffer.
So I rummaged around in more junk boxes looking for old TTL chips that I could use and was able to find all I currently think I need and they are dated between 1982-1984. I even found an old used 7.3728 MHz crystal that is required for getting the correct baud rates.
No early 80'es computer can be without cheesy sound effects using something like a AY3-series sound effect generator. Unfortunately I couldn't find an old version of the chip, but I had some that seems to be made in 2006(?) so that have to be an half-anachronistic part in my design.