OK.....so my Q*bert works but there is this "super-cap" installed to replace the back-up batteries on my PCB.
However, a "super-cap" doesn't keep the power for as long as I want (without powering up the machine every day).
The most common used solution is to use some Dallas battery backed SRAM modules like this:
http://www.jrok.com/sohs/qb-upg.htmlHowever, these modules have some disadvantages:
- They're insanely expensive at about 16 dollars....that makes 32 just for the units, without shipping...
- relatively hard to find
- when you're going to have to replace them.......your hi-scores are gone....OK it won't be until (over) 10 years....but it's the idea
These modules are not much more than an SRAM (those are already on the board) and a lithium cell in one case.
My preferred method is to install a CR2032 holder and battery. Done that successfully on my Berzerk f.i.
The one thing you need to take care of is that there is no charging going on anymore.
Here's the battery circuit of Q*bert:
It works like this: When the power is off the battery supplies 3.6V through diode D3, bypassing R40.
When power is on, the +5V is parallel to the battery voltage and charges it through R40.
D2 stops the battery voltage from being fed-back towards the other +5V powered chips (about the entire board
)
C27 is there to straighten any fluctuations I suppose.
Now...if I replace B1 with a non-chargeable battery I have to make sure it won't be charged anymore. In this cas, I can simple remove R40.
This way the +5V will be still going through D2 but will be stopped by D3 when power is on.
When power is off, the battery will still provide it's voltage through D3 and D2 stops it being fed-back to the rest of the +5V power chips.
Normally I'd have to use Schottky diodes to minimize the energy loss across the diode when the battery is supplying power. However, I checked the datasheet of the 1N4454 diode (
https://www.fairchildsemi.com/datasheets/1N/1N4454.pdf ) and it specs it as a "high conductance" diode but doesn't say "Schottky" anywhere.
I guess I'll just replace D3 with a 1N5817 anyway...that way I'm sure of things (works on Berzerk board nicely too
). It's not like that diode will break the bank (not even mine
)
Costs:
- CR2032 battery: €0,45
- holder: €0,56
- 1N5817 €0,061
Total: about 1 Euro
So this solution has the following advantages:
- a LOT cheaper
- possible to replace battery without loosing hi-scores (cab must be powered on during replacement!)
Disadvantage: a little bit harder to install the socket, may require drilling a hole, haven't looked into it yet.
