The Citrine Citadel

M48T59Y-70PC1 NVRAM Battery Replacement

The Sun Ultra 60 workstation uses an ST M48T59Y-70PC1 battery-backed SRAM with real-time clock. When the embedded lithium primary cell inevitably dies, the chip is essentially useless. The most noticeable negative effect of a dead chip is that the workstation always powers up in a default configuration with diag-switch? true, running several minutes of self tests every time, with bogus MAC and host ID values.

You don’t even know you are already dead.

This device was branded obsolescent by the manufacturer in April of 2008 and has presumably been out of production since around that time. Similar chips like the M48T58Y are still made but they are expensive (around $40) and slight differences may result in compatibility problems. It appears that NOS parts can be found fairly cheap online but these must be almost 15 years old by now and who knows what state the embedded batteries are in. No thanks.

One option could be to find the SOIC version of the M48T59Y for which NOS also seems to be available, although somewhat less readily than the DIP modules, and then construct an adapter board to fit into the DIP socket on the motherboard. The SOIC package does not include the battery and crystal; instead, it has contacts on the top to attach a separate “SNAPHAT” package which are still made. This would be complicated and somewhat expensive.

Fortunately, a straightforward (if time-consuming) repair is possible. This is a literal hackjob: we can dig through the potting to expose the battery terminals, disconnect the internal battery, and connect a normal battery clip. This enables the use of readily-available CR2032 lithium primary cells that can be replaced as needed, just like a normal PC. Since we already have the chip we need, it is not necessary to source any obscure or expensive components.

The battery contacts are located on the end opposite pin 1, between pins 14 and 15. Looking underneath the chip there is a small well at either end filled with potting compound. The battery connections are inside one of these. Using a knife, cut back the plastic outer shell on the battery side, then begin scraping away potting compound until the terminals are exposed. You can use a suitable DIP socket to help protect the pins from accidental damage.

Once the terminals are exposed, the internal battery must be disconnected. The thick protruding tabs near the bottom of the module are what we want to solder to. The vertical strips going up and into the module are the internal battery terminals. These can be desoldered from the tabs and lifted out of the way, or simply cut off.

With the time-consuming parts completed it is a straightforward matter to connect a normal battery clip to the exposed terminals. As the Ultra 60 has no clearance issues I simply glued it to the top of the chip. Since this would obscure the barcode sticker which is useful to restore the original MAC address and host ID, I moved this sticker to the side of the chip.

Finally all that remains is to put everything back together and power up the system. The system will run the full self-diagnostic tests again and if all goes well, the “NVRAM Battery Detect Test” should no longer display any errors. At the ok prompt, the memory can now be initialized. Where “AA BB CC” are the six hexadecimal digits from the orange barcode sticker:

set-defaults
setenv diag-switch? false
8 0 20 AA BB CC AABBCC mkpl
<Ctrl-D><Ctrl-R>
.idprom
reset