Can this work? Using super cap for SRAM retention

[wilykat]

What I wanted to try is to replace a mess of components as seen in this page: http://wiki.pcengine.info/pmwiki.php?n=Hardware.Tennokoe2 second image. The SRAM chip is on the right side of the board and the mess of parts are on the left side. It uses 2 voltage sensing chip (looks like transistors), 1 darlington transistor, and 1 odd diode plus few resistors and capacitors to handle voltage switching between the 3v battery and 5v mains. When the game system is shut off, the circuit is supposed to switch over to batteries.

I am trying to shrink that board and if the super cap can work, it would cut about 1/3 of the board out. The cap charges normally from 5v source when the system is on, and when it is off, the cap keeps the SRAM powered and a diode prevents the cap from powering the whole system. I forgot in the above picture a resistor (10k probably) to pull up /CE line to capacitor's + end to put the chip in standby when there's no external activity.

Would that work or would a 2k SRAM still need convoluted power switching logic to preserve the data?

PS has the SRAM changed in power requirement in the last 20 years? If modern 2k SRAM that is pin compatible but has lower power requirement in standby, I may need to swap the chip too! The also shows a large 64 pin chip, that is a custom chip that handles the communication from the host system and controls when SRAM can be read or written to. Since it's custom and propriety, this would have to stay intact for now.

 

[Ian Rogers]

The MK48z02 can still be purchased... 2k sram with battery...
They have this arrangement..
**broken link removed**

 

[wilykat]

Mouser has that listed as obsolete and not stocked. Allied and Digi didn't have them. I checked eBay, they have NOS up but NOS means the internal battery could have run out if not soon. So unless the battery is easily replaced, I doubt eBay would be a safe source of working mk48z02.

There's another problem I saw when I was looking for it, the picture shows it being rather tall. It may end up being too thick. With the 1F capacitor I've looked at, they can be only 4mm thick with leads sideway. The current SRAM and PCB together is about 8mm tall total. The available space inside the game console is maybe 12mm at most. Factor in some wiring and there won't be a lot of room left for large chip.

 

[Nigel Goodwin]

Super caps have commonly been used for such purposes, however they proved extremely unreliable (more so than batteries were) - and a nice little earner replacing them

 

[KeepItSimpleStupid]

Can FRAM work for you?

 

[wilykat]

Can FRAM work for you?

Doubtful. It seems they are designed for lower voltage and none that would operate at 5v. I would need level shifter for all inputs and outputs, basically the whole chip to make it work with existing 5v system.

I have that mk48z chip ordered, I'll see if it's too big or not.

 

[Ian Rogers]

I use fram at 5V... Its SPI but I beleive they do parallel .. .............. Just checked.. Yes they do for £11.44 but you'll need an SOIC to dip adapter..

I'm sure the MK48Zxx will do though.... I did have some small battery ones somewhere... Very low profile

 

[wilykat]

Is there a 2kx8 FRAM? The smallest I could find is 32k.

 

[Ian Rogers]

No! That really wont matter.. The SOIC adapter can be fooled... the jedec format allows you to dismiss several pins..

See how to fool the system..

**broken link removed**

Two or three wires and the system will see only 2K

 

[Tony Stewart]

https://uk.rs-online.com/web/p/fram-memory-chips/8282783/

 

[Ian Rogers]

https://uk.rs-online.com/web/p/fram-memory-chips/8282783/

You're kinda missing the point.. He needs non volatile parallel ram to replace the SRAM... SPI or I2C isn't what's required..

 

[Tony Stewart]

You're kinda missing the point.. He needs non volatile parallel ram to replace the SRAM... SPI or I2C isn't what's required..

gotcha, ( obviously I missed your threads)

Vbat backup is pretty simple with diode logic and higher voltage supplies the current.
THe reliability is ensured with a low ESR cap on the chip to prevent transients and careful examination of CE or chip enable during power down to prevent illegal writes. So at minimum all you need is a pair of Schottky diodes and a good cap. THe battery can be carefully selected knowing the static current.

Coin Cells used tend to have 3k series resistance, but CR123 Lithium Cells are << 1Ohm. which could last years. You can measure standby current using Ohm's Law, V/R=I current with 100 Ohm series resistor or similar to the chip and low leakage cap to drop at least 50mV.

A simple transistor to disable CE ( wired OR PNP if active low) from sensing the bus voltage not present by sensing.

It's not that complicated. ASk for more details after you make a measurement on current.

BTW a CR123 battery has hundreds of Farads and is cheap compared to a super cap and exactly 3.0V

I suspect your SRAM is rated for a voltage range of 2.7V to 5.5V and uses < 1uA in standby mode. You can also disable WE instead of CE with a pull up R.

Be careful about enabling the chip while the game PCU has no power as CMOS latchup can occur when you apply power and the signal voltages from SRAM are higher than the BUS chips powered down. THis is a well know phenomenon that burns up the chip in an SCR latchup mode for CMOS. It may only just get warm or hot depending on vintage of chip, so CE disables the signals to float on the bus when power OK goes low. N.B. look for Power OK signal or make one so you avoid applying bus voltages (tri-state mode) from SRAM when other devices are powered down.

Vcc must go high before any signals on the bus from SRAM.