SmartWatch Dead Battery
- The following article appeared in issue #120, March 2012, of the "Z-100 LifeLine".
- Some real electronics experience and soldering skills are necessary.
- Click on the drawings to magnify the image.
The Case of the Dead SmartWatch
By now, you have probably noticed that your SmartWatch no longer keeps time! Actually, this should be no surprise, because the lithium battery was only predicted to have a life of about 10 years. As we are nearly 30 years since the introduction of the Z-100, I was surprised the clocks lasted this long.
So what can we do with the remaining carcass?
It would be a real shame to dump the SmartWatch because of a bad battery, so I did some research and played around with one of mine.
First, the bad news. These lithium cells are dangerous, hence the following warnings:
WARNING: Do NOT go poking around a lithium cell or try to dismantle the SmartWatch. These battery cells have been known to short out and explode. I measured the voltage remaining in one of my SmartWatches and found that the voltage was still about .7 to .8 volts.
WARNING: Do NOT attempt to recharge these lithium cells either. Recharging could cause the cells to explode.
So, what to do? First, let's review some SmartWatch specifications:
The DS1216E SmartWatch is an assembly containing non-volatile (NV) RAM with a built-in CMOS watch function and two lithium cells contained in a 28-pin DIP socket that mounts under the monitor ROM in the Z-100.
The SmartWatch ROM sockets use the embedded lithium source to maintain the time and date only. The SmartWatch monitors the computer's +5 Vdc power at pin 28 of the ROM chip and when the voltage drops to below the battery's voltage, the internal lithium energy source is automatically switched on and write protection is unconditionally enabled to prevent loss of watch and NV-RAM data.
The SmartWatch is based on the Maxim DS1315 clock controller chip with two energy cells, one attached to pin 4 and the other attached to pin 14 of the DS1315 chip. As the energy cells are ONLY attached to the clock chip,the predicted energy drain was such that the cells would last about 10 years.
Each DS1216 was shipped from Maxim with its lithium energy source disconnected, ensuring full energy capacity when finally used in the computer. When computer power is first applied at a level greater than the lithium energy source, this energy source is enabled for battery-backup operation. During my research, I found no means to turn the cells off.
NOTE: The DS1216E has been discontinued and is no longer available from Maxim, but according to the Internet, some of these still appear to be out there for sale. So, a word of caution. As you have no way of knowing whether a SmartWatch has been activated (used), the SmartWatch that you receive may already be on its last legs, if not already dead.
Finally, the lithium cells are isolated from computer power, so there is supposedly no danger of charging the cells under normal use.
Further information on operation of the clock can be found on the Internet from Maxim.
Ok, enough preliminaries.
The ideal means of powering the SmartWatch would be to disconnect the energy cells from the DS1315 clock controller by cutting pins 4 and 14, then attach our own external battery to the pin stubs. However, upon looking more closely at the SmartWatch socket, there is no chance of disconnecting the energy cells from the DS1315 chip without causing extreme damage to the socket, which surrounds the DS1315 chip quite tightly.
So, our only recourse is to simulate computer power to the SmartWatch. However, the downside of this method is that we are not only powering the SmartWatch, but also the ZROM pins and other computer circuitry. This drains any battery much faster than if we were just powering the clock oscillator. Instead of lasting years, the new battery backup may only last hours.
On the positive side, I don't use my Z-100's all that much, and when I do, I'll work on any one of several for several days, with multiple power-ups each day as I test various components and configurations. Then I move on to something else and it may be weeks or months before I work on that computer again. Any battery would be long dead before I used the computer again. I just hate resetting theclock several times over the course of my work.
Rethinking the issue, I reasoned that if I could get the clock to last for about an hour, long enough to change a part or configuration, I'd be happy. This would be an ideal use of a supercapacitor! I had seen this as an option for the Scottie Board clock.
The new circuit is simple. We want to minimize the power drain, so we need to disconnect the SmartWatch socket's pin 28 from the computer's +5 Vdc Vcc and feed that pin separately - from our supercapacitor.
Likewise, to reduce the power drain of the Monitor ROM, or ZROM, we need to disconnect pin 28 of the ZROM and feed it separately from any other +5 Vdc source - another nearby integrated circuit.
We need to insert a diode in the capacitor's charging circuit to prevent powering the computer's entire +5 Vdc bus from the freshly charged capacitor when the computer is shutdown. The internal resistance of the diode also controls the charging current.
1 - 1 Farad, 5.5v Supercapacitor
1 - Diode, such as the 1N4003
4" - Black hook-up wire
20" - Red hook-up wire
1/2" - 1/8" shrink tubing
1" - 1/4" shrink tubing
1" - doublestick or electrical tape
Tools Needed: (In addition to those needed to dismantle the computer)
IC remover or thin flat-bladed screwdriver
Solder gun or iron w/solder
[ ] Disassemble the computer down to the motherboard.
[ ] Remove the three screws holding the video board and fold the board back up against the card cage.
[ ] Referring to Figure 1, remove the ZROM from the SmartWatch at U190,
[ ] Remove the SmartWatch from the U190 socket
[ ] Bend pin 28 out on the SmartWatch socket and reinstall it into the U190 socket
[ ] Likewise, bend pin 28 out on the ZROM, then reinstall it into the SmartWatch socket.
Figure 1. SmartWatch Removal
Construct the following circuit using a 1 Farad supercapacitor, available from Mouser and Digi-Key for less than $5.00 and pretty much any diode. I used a 1N4003 from my junk box, but nearly any diode will do. Just check that the diode's reverse resistance is as high as possible to reduce current drain.
[ ] Using doublestick tape or doubled over electrical tape, mount the supercap to the left side of the computer's metal bottom case in front of the power supply.
[ ] Mount the diode directly to the positive lead of the supercap. Double check that you have the polarity of the diode and capacitor correct. If the diode is reversed, the capacitor will not charge and the SmartWatch cannot receive power.
CAUTION: Don't touch the leads of a charged supercapacitor. While we are only playing with 5 volts dc, the current can be high enough to spark and give you quite the jolt!
[ ] Slide about 1/2" of 1/8" shrink tubing over a 4" length of black wire and solder a spade lug to one end. Attach this end to one of the two base mounting screws of the power supply.
[ ] Solder the other end to the negative lead of the supercap. Slide the shrink tubing over this lead.
Since I didn't wish to cause problems by drawing charging power from one of the integrated circuits of the motherboard, I used one of the 5 volt lines at the power supply connector for the motherboard. It is easy to get to with the soldering iron without removing the motherboard.
[ ] Solder one end of a 7" length of red hookup wire to one of these 5 volt terminals.
[ ] Slide a 1" length of 1/4" shrink tubing over the other end of this wire and solder this end of the wire to the diode. Don't slide the shrink tubing over this yet.
[ ] Slide one end of a 10" length of red hookup wire through the shrink tubing and solder this end to the positive lead of the supercapacitor. Check to ensure the diode is still soldered in place here also.
[ ] Slide the shrink tubing over this lead of the supercap and warm both pieces of tubing with the tip of the soldering iron. They will encase and protect both leads of the supercap and the entire diode.
[ ] Solder the other end of this 10" red wire to pin 28 of the SmartWatch socket.
[ ] Using a small screwdriver, pry up the left end of the integrated circuit at socket U208 to provide better access to pin 28 of the IC.
[ ] Tack solder a 2" length of red wire to the top of pin 28 of this socket. Press this IC back into its socket. This will be the source of needed power to our ZROM.
[ ] Solder the other end to the extended pin 28 of the ZROM.
[ ] Review all of the previous steps. There should be no loose wire ends nor missing IC's from their sockets.
[ ] Leave the video board leaning against the card cage, but ensure it is not shorted by the card cage. Maybe use a sheet of paper between the video board and the card cage.
[ ] Apply power to the computer and measure the voltage at the SmartWatch socket, pin 28. It should be about +4.7 Vdc, reduced slightly from the normal 5 Vdc because of the voltage drop across the diode. If you are fast, it may be lower and increasing as the supercap is charging.
[ ] Measure the voltage to the ZROM, pin 28. It should be the computer's normal Vcc, about +5 Vdc.
[ ] Power down. If successful, the installation is complete. If not, correct the error (diode? or cold solder joint) and repeat.
[ ] Reassemble the computer and boot-up. Run your clock software as normal, then note the time and shutdown.
[ ] Leave the computer off for about two hours, then reboot and note the time.
I found that the SmartWatch's clock will remain running for about 1 hour and 45 minutes after power down. Not enough time? Add one or two more supercaps to double or triple the time. Remember to connect them all in parallel, positive to positive and negative to negative.
Call or e-mail if you have any difficulty. Good Luck.
Steven Vagts, Editor, "Z-100 LifeLine"