So this evening, I got into a discussion with good sir BrandonW on our IRC/SAX channel #cemetech about the CR1620 backup battery used in TI-83, TI-83+/SE, and TI-84+/SE graphing calculators.
My hypothesis is that the running time with no AAAs is on the order of one to two seconds because the calculator is not directly running off the backup battery, instead drawing power from something like a capacitor charged up by the backup battery. I whipped out the dead motherboard from one of the TI-83+s I lobotomized to figure out the partial pinout of the mysterious TI REF 9815455GAII integrated circuit, and started tracing out, well, traces. I was mostly able to find my way around, and it looks like the battery is charging a 100uF capacitor on the left underside of the TI-83+ PCB (hardware revision XR303, anyway) right underneath the Flash chip. I checked one final fact with BrandonW, to make sure that his 2 seconds of battery life didn't leave the backup completely drained (ie, that the 2 seconds were using only a tiny fraction of the CR1620's capacity):
Discuss.
#Cemetech on EfNet wrote:
+KermM_: I bet the calculator could run for quite a few minutes on the backup battery
+KermM_: I'd be interested in testing that one day
+BrandonW: Two seconds at most, I already tested it with OS2.
+KermM_: ah
+KermM_: really, only two seconds? that doesn't make any sense to me
+KermM_: it's a 75mAh battery
+BrandonW: It's also 3V, and the calculator needs twice that.
+KermM_: hold on, hold on
+BrandonW: Either 1.5V or 3V, I forget.
+KermM_: an AAA is about 1250 mAh
+KermM_: four AAAs are therefore 5000mAh
+KermM_: in parallel.
+KermM_: We have them in series, so we get 1250mAh at 6V
+KermM_: =1250*10^-3*6 = 7.5Wh
+KermM_: the backup battery provides 3V at 75mAh
+KermM_: so 0.075*3 = 0.225Wh
+KermM_: assuming that the circuit they use to step up the power is really really bad, say 60% efficient
+KermM_: that's 0.135Wh at 6V
+KermM_: if 0.135Wh lasts 2 seconds, then the AAAs at 7Wh would last...
+BrandonW: In an OS that doesn't check bit 0 of port 2, it lasts for a couple of seconds and then the LCD fades away.
+KermM_: 103.7 seconds maximum
+KermM_: ie, the AAAs are 50 times as powerful, but if you consider that a TI calculator can operate for, what, at least 8 continuous hours on a set of AAAs?
+KermM_: they're certainly not (8*3600)/2 = 14400 times as powerful
+KermM_: so either something odd is happening on the hardware level
+KermM_: or TI did a particularly poor job implementing the power conversion
+KermM_: consider that a much smaller battery can power a watch LCD and "processor" and crystal for months!
+KermM_: sure, it's a giant LCD by comparison, and an immense processor
+KermM_: but no where near 5 million times as power-hungry, if you say a watch battery lasts 4 months
+BrandonW: Write a small program that resets the flag, pull the battery, and see what happens.
+KermM_: I totally believe you
+KermM_: I'm just very curious now how exactly the schematics for this thing look
@tifreak: I am guessing it might have something to do with not wanting to wear out that battery? I mean, they probably don't want people to have to worry about replacing it often at all
+KermM_: tifreak, BrandonW, a proposition.
+KermM_: perhaps the backup battery keeps a capacitor topped off
+BrandonW: The flag you actually want to reset is 7,(iy+0Fh).
+KermM_: and when you pull the main batts, it's actually powering itself off said capacitor, not the backup battery directly?
+BrandonW: It makes good sense to want to prevent it from running without the AAA batteries for more than a few seconds, because all they need to do is execute that small bit of code.
+KermM_: I have a calc motherboard right here, lemme take a look
+KermM_: I'd be interested in testing that one day
+BrandonW: Two seconds at most, I already tested it with OS2.
+KermM_: ah
+KermM_: really, only two seconds? that doesn't make any sense to me
+KermM_: it's a 75mAh battery
+BrandonW: It's also 3V, and the calculator needs twice that.
+KermM_: hold on, hold on
+BrandonW: Either 1.5V or 3V, I forget.
+KermM_: an AAA is about 1250 mAh
+KermM_: four AAAs are therefore 5000mAh
+KermM_: in parallel.
+KermM_: We have them in series, so we get 1250mAh at 6V
+KermM_: =1250*10^-3*6 = 7.5Wh
+KermM_: the backup battery provides 3V at 75mAh
+KermM_: so 0.075*3 = 0.225Wh
+KermM_: assuming that the circuit they use to step up the power is really really bad, say 60% efficient
+KermM_: that's 0.135Wh at 6V
+KermM_: if 0.135Wh lasts 2 seconds, then the AAAs at 7Wh would last...
+BrandonW: In an OS that doesn't check bit 0 of port 2, it lasts for a couple of seconds and then the LCD fades away.
+KermM_: 103.7 seconds maximum
+KermM_: ie, the AAAs are 50 times as powerful, but if you consider that a TI calculator can operate for, what, at least 8 continuous hours on a set of AAAs?
+KermM_: they're certainly not (8*3600)/2 = 14400 times as powerful
+KermM_: so either something odd is happening on the hardware level
+KermM_: or TI did a particularly poor job implementing the power conversion
+KermM_: consider that a much smaller battery can power a watch LCD and "processor" and crystal for months!
+KermM_: sure, it's a giant LCD by comparison, and an immense processor
+KermM_: but no where near 5 million times as power-hungry, if you say a watch battery lasts 4 months
+BrandonW: Write a small program that resets the flag, pull the battery, and see what happens.
+KermM_: I totally believe you
+KermM_: I'm just very curious now how exactly the schematics for this thing look
@tifreak: I am guessing it might have something to do with not wanting to wear out that battery? I mean, they probably don't want people to have to worry about replacing it often at all
+KermM_: tifreak, BrandonW, a proposition.
+KermM_: perhaps the backup battery keeps a capacitor topped off
+BrandonW: The flag you actually want to reset is 7,(iy+0Fh).
+KermM_: and when you pull the main batts, it's actually powering itself off said capacitor, not the backup battery directly?
+BrandonW: It makes good sense to want to prevent it from running without the AAA batteries for more than a few seconds, because all they need to do is execute that small bit of code.
+KermM_: I have a calc motherboard right here, lemme take a look
My hypothesis is that the running time with no AAAs is on the order of one to two seconds because the calculator is not directly running off the backup battery, instead drawing power from something like a capacitor charged up by the backup battery. I whipped out the dead motherboard from one of the TI-83+s I lobotomized to figure out the partial pinout of the mysterious TI REF 9815455GAII integrated circuit, and started tracing out, well, traces. I was mostly able to find my way around, and it looks like the battery is charging a 100uF capacitor on the left underside of the TI-83+ PCB (hardware revision XR303, anyway) right underneath the Flash chip. I checked one final fact with BrandonW, to make sure that his 2 seconds of battery life didn't leave the backup completely drained (ie, that the 2 seconds were using only a tiny fraction of the CR1620's capacity):
#Cemetech on EfNet wrote:
+KermM_: were you able to repeatedly do that [run the calc from the backup battery for 2 seconds] with OS2?
+BrandonW: Yes.
+KermM_: If you can repeatedly run it for two seconds or so off the backup battery, then my hypothesis is correct
+KermM_: excellent, thanks
+BrandonW: Yes.
+KermM_: If you can repeatedly run it for two seconds or so off the backup battery, then my hypothesis is correct
+KermM_: excellent, thanks
Discuss.
They are a hardware development company, after all. It's just their software that's crap. 
