My Korg M1 Synthesizer is due for battery replacement.
Since my M1 demands a battery change every 4-5 years I though it is a good idea to use FRAM instead of SRAM.
For those who do not know what the difference is: SRAM needs a battery to hold the data. FRAM is like a flash, it can be used like RAM, but also keeps the data for 155 years when the power is turned off. That should do me
So I took out the two 32Kx8 SRAMs and replaced them with FRAM.
However, FRAMs need the /CS signal to go low for each cycle. It's used to latch the address into an internal register.
FRAMs are not transparent like SRAMs are. So I needed a circuit which makes the FRAM compatible to SRAM.
In the Korg M1 for instance the /CS signal is actually the RESET signal. Once the system is up and running only /RD and /WE are used to access the SRAM. That means the /CS signal to the chip must be high whenever both /RD and /WE are high and whenever the incoming /CS (RESET) is high.
That's easy to achieve with 2 single gate logic gates.
Since SMD is involved and the ciruit needs to fitt into a 28 pin DIP socket I did layout the board and got a bunch of the made by Itead.
I basically desoldered the old SRAMS, soldered nice machined DIL sockets into the board and plugged the little modules into them.
They work great, but now the Synth came up with the "Low Battery" message everytime.
So I thought: "Ok, 10K to +5V should do". Well they do well but that japanese software designer from the end 80s was very aware that Lithium cells can't do 4.5V. So the display now showed "Battery error.
Ok, so after bit playing around with Ohm's law I soldered a 22K resistor in parallel to the battery.
The the voltage then is about 3.1V which is in the range of a new Lihium cell.
Yay, no battery error messages anymore. So it works
And yes, I have tested it by turning it on and off. And of course I reprogrammed it with the factory settings using a MIDI SysEx tool.
Here is the gallery:
