> The cheapest way to turn on the battery when the power is off is to
> use a diode. Also a small pnp + 3 resistors current charge is required
> when accu is used. I think you can't find a chip less expensive than
> all this devices. Vasile

Actually you normally need two diodes, one from supply and one from the
backup battery, to ram +vdd, and a pullup of 1M or so between ram +vdd and
/ce to keep it disabled. A tiny twin diode in SOT23 and 1M pullup will
work fine. Decouple the ram directly at its pins after the diodes with
0.1uF ceramic + 4.7uF tantalum, or as per manufacturer's data sheet.

It is also necessary that the unpowered circuitry connected to /ce does
not pull it low. One way is, to use a heftier pullup and a NPN transistor
as 'fake' TTL input stage (E to driver, B to weak divider holding 1.8V
when the base is not driven, and C to ram /ce). This circuit relies on the
base being drawn to gnd when the external circuit is not powered (and thus
the divider also not). Do not decouple the base divider with a capacitor.
Another is to use a single 4S66F CMOS gate as separator on the /ce line
with the enable wired to the reset or brownout sensor on the main powered
circuit. Yet another is to use the /ce line hard-wired to a reset or power
sense circuit that provides disable when the voltage goes away. Tieing it
to a known good CPU monitor's output /reset line is a good way to avoid
data corruption on turn on and turn off.

There are specific chips that provide standby power, plus input/stadby
switching, plus low battery sensing etc, like the Seiko S8230 and S8232
for exmaple. They also have a suitable /CE and reset driver built-in, plus
a regulator for Vsby when it is provided from external power. Maybe you
should look at these.

I once made a DS1230Y replacement by gluing these parts and a small
diameter (smaller than the 300mil wide 6116 body) lithium coin cell
directly on the back of a 6116 SRAM. The /ce pin was cut off short and its
remainder glued on below it with epoxy (with no electrical contact).
Worked perfectly. Data retention was well over 1 year with a homemade
design and can be >10 years with an industrial one.

hope this helps,

Peter

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