The collected works of rscottdrysdale

i've never played with that particular dallas chip, but have done the external battery hack to to the type doesn't have the socket on top, but a square of black epoxy which conceals the battery. on that model, there is no circuit board - the RAM and clock are in a 24/28 pin chip with the battery on top covered in goop.

to hack that guy, you dremel away the epoxy above two of the pins, which exposes the battery leads. cut them and solder a new external battery in place, and viola.

i'm guessing that on your flavor of dallas chip, the battery is under the circuit board hosting the RAM socket. it's probably a safe bet that the four pads visible on the circuit board are indeed the battery connections - but since the battery is probably dead as a doornail at this point, figuring out which is which is gonna be difficult.

solution 1:

it looks from your photo that the underside is shiny - in other words, they've got the battery tucked under there and covered with epoxy.

if you're friendly with a doctor or dentist, get it x-rayed then you know where to drill into the epoxy to find the battery terminals. once exposed, you can ohm to find out where they go on the chip, then disconnect the old battery and solder a new one in a more exposed place. the battery is likely 3.3V, but i'd do some research first.

solution 2:

get the datasheet ( http://datasheets.maxim-ic.com/en/ds/DS1216-DS1216H.pdf ) and compare to current dallas "timekeeper RAM" products. you might find one that's nearly pin compatible and nearly register compatible, build an adapter socket, and see if the firmware is willing to talk to it.

solution 3:

i didn't find anything in quick search (and didn't read through the datasheet), but if you can find the pinout of the 1216 chip on the circuit board, you could just cut a trace or nip a pin to disconnect the old battery, and wire a new outboard one.

essentially, yes. pull the RAM chip and bend it's VCC pin skyward. solder a small (wire-wrap) wire from VCC on the dallas carrier through a diode to the RAM's VCC pin. hook up the "+" of your new battery through another diode to the same RAM VCC pin. the diodes will drop .7V (so the RAM VCC pin will see .7V less than board VCC or battery backup voltage), but it should work. of course, don't forget to hook the battery "-" to GND.

however, you will have achieved NVRAM retention only. the clock (in the dallas chip) will not keep time when power is off, as it's still (not) powered by the old (dead) battery. once your system comes up, you could have it get time via an NTP server, but in the mean time logs and filesytem timestamps will be wrong.

okay, here's the deal:



your mission is to figure out which pins on the dallas chip are battery in, VCC in, and RAM VCC out.

BATTERY -, CARRIER GND, and RAM GND should all be the same.

as suggested earlier, the higher voltage you're seeing is probably the raw battery output. find the same voltage on the dallas chip pin(s) - hopefully it only goes to one place. the lower voltage is probably the output of the switch/rectifier in the dallas chip, and it should also be present on pin 28 of the socket to feed the RAM.

if the higher voltage does indeed go to one place on the dallas chip, find a trace to cut to that pin, or nip the pin as close the circuit board as possible. after doing that, make sure the lower voltage you found is no longer present anywhere.

if that happens, you've removed the old battery from the circuit and can solder the + terminal of your new battery (it is 3.3V) directly to the dallas pin you cut.

now prepare for the joys of non-Y2K compliant clock chips - you're on your own there!!!

yes, the the question is whether the clock chip itself rolls over correctly at the end of the year.

you'll have trouble if:

1) the year rolls over while the machine is off, or
2) the year rolls over while the machine is on, but the software doesn't write the new date to the chip on the assumption that it works correctly.

it shouldn't be a big deal to fix the date once a year. and the next question is whether the counters in the dallas chip actually support (have enough bits to store) years > 2000.

ok, looks like if the A,B,C,D labels on the chip pins correspond (check with an ohmmeter) to the A,B,C,D labels on the circuit board pads in your picture, then you can hopefully just cut the trace stub leading "up" from the A pad. the assumption is that battery + is wired to that pad underneath the carrier board, and cutting that trace will disconnect the battery from the whole circuit. use a SHARP exacto-type knife, being careful not to cut any other traces (or yourself).

try it. you can always bridge the trace with a tiny bit of wire or a solder blob to put it back as it was if this doesn't work as expected.

after making that cut there should be no battery voltage anywhere you previously found voltage, except at the isolated A pad. if that's the case, hook your new battery + to chip pin A or the stub of the cut trace (NOT the pad).

remember we don't want to hook the new outboard battery to the old battery. the new battery would attempt to charge the old battery, at best greatly reducing the life of the new battery, and at worst making the old battery leak or catch fire(!)

ARGH!

you should have tried cutting the trace leaving pad A first!!!

well, we're trying to isolate the built-in battery's + terminal from everything else. it's quite likely the A pad (with the highest voltage reading) is it. cutting the trace coming up from the A pad would tells us. if after the cut the A pad is the only place you have voltage, then you're done and can hook your new outboard battery to the side of the cut away from the A pad.