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SRAM ce line

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I have some old boards using intel 80188uC with 2 x sram chips . I want to retain data in the sram when VCC fails. I connected a wired or diode pair - one to vcc and 1 to a nimh battery pack . When battery pack is powering the system the other side of the diode goes only to the power pin of the SRAMs and a pull-up resistor to both CE lines.

The battery runs down rapidly - it appears as if when the CPU has no power the output CE lines ( lcs & ucs) are draining the battery , I suspect through protection diodes internal to the chip . I cut the traces on the PCB to verify this , which does seem to be the case .

Request for help - please can someone send a diagram of how I could perhaps insert a mosfet “ in line “ on the two CE lines so that they effectively go “ open circuit “ when battery is powering the memory and then close when VCC is restored.

Any help much appreciated .


can you show us a small schematic of your mod?

A possibility could be a 74HCT1G66 in series with the CE line powered from the OR'ed battery/VCC line. Connect the control input to VCC.

A better idea would be using FRAM or NVRAM instead of SRAM. Would also eliminate the battery.

David Hess:
The way battery backup is normally done is to include some extra discrete logic between the processor bus and memory to control -CE which itself is powered by the same backup supply so when power to the processor is removed, no extra load is presented to the battery.

Firstly excuse poor drawing - its an age thing  :(. I was needing advice on how to insert the 2N7000 in the circled areas. The thinking is that when there is VCC the FET will be on thereby providing a low impedance patch for the 2 * CE lines.
Then when VCC is removed the fet will turn off thereby preventing the battery VBAT draining through the CE line on the CPU , as it seems to be sinking current through the esd protection diodes internal to the chip as the pin is pulled high by the 10K resistors
therefore forward biasing the high side esd diode . FRAM is not an option as we have over 1000 of these boards in the field for the last 25 years and they use a K6XX4008CIF-GB70 OR BS 62LV4006SIP55 OR BS 62LV4007SC-70 all of which are 32 pin form factor so I have been unable to find the same foorprint FRAM or NVram + they are very expensive so I was hoping to Frankenstein a solution


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