WP34s low battery shut down?

[Harald]

I assume the 34s has a low battery shut down. How is the battery voltage measuring done? I have not found anything on the schematic. And when does it shut down?

I am thinking about equipping my 34s with a USB interface for communication and reflashing. Also I would like to use the USB to recharge an internal LiPo battery. The reson for this choice of battery is, that I can't find any NiMH cells that are small enough to fit insde the calculator.
The LiPo needs to be protected against undervoltage to avoid damage to it. What I could do is use a voltage regulator that shuts itself down when the battery voltage drops too low. But that would cause the calulator to shut down without saving data and I don't like that idea.
Also I have read that the calculator runs at a slower speed when the battery voltage drops below a certain point (was it 2.8V?). So it might not be the best option to just drop the difference between the lowest permited battery voltage and the calculator shut down voltage accross a diode either. On second thoughts this might be a simple modification to the software.

Anyone any thoughts on this?

[Marcus von Cube, Germany]

I'm not at home at the moment so I try to answer the question by heart. For any details, consult the file main.c in the trunk directory on SVN.

IIRC, shutdown voltage is 2.2V. The chip has a brownout detector and the software tries to guess the current voltage from it (it's just a single bit and a programmable threshold value. Measurements are done during interrupts.)

[Harald]

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IIRC, shutdown voltage is 2.2V. The chip has a brownout detector and the software tries to guess the current voltage from it (it's just a single bit and a programmable threshold value. Measurements are done during interrupts.)

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Been trying to get Eclipse to work, but no luck installing the SVN client. I'll try again tomorrow. Once I get that to work it should be easy enough to change theses lines:

103 #define APD_VOLTAGE SUPC_BOD_2_2V

104 #define LOW_VOLTAGE SUPC_BOD_2_5V

Cheers,
Harald

[Marcus von Cube, Germany]

It's not Eclipse, it's Yagarto.de you'll need. Together with a whole bunch of Unix like utilities (I'm using MinGW for this). Why do you want to increase this value?

[Harald]

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It's not Eclipse, it's Yagarto.de you'll need.

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Oh, ok, thanks.

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Together with a whole bunch of Unix like utilities

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I was hoping to avoid that by using Ubuntu. But it doesn't look like Yagarto is available for linux.

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Why do you want to increase this value?

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Because I will have to use a circuit that disconnects the battery if the voltage drops too low in order to avoid deep discharge (which would damage the battery). And I want this to only happen if the calculator has been left with a discharged battery for a long time, because inevitably this will result in data loss. So the calculator needs to shut down before the battery gets down to that point.

[Marcus von Cube, Germany]

Yagarto.de is a port of the GGC ARM tool chain which should be available for Linux. I have yet to locate it but have to admit that I didn't do any serious search.

Your shutdown idea will not work because the battery backed RAM in the processor will fail if you disconnect power. If you plan to let the calculator sit on the shelf for any longer period of time, just do an ON+STO which will create a copy of the RAM in flash. You can then remove power without any loss (except for the built-in RTC).

[Harald]

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Your shutdown idea will not work because the battery backed RAM in the processor will fail if you disconnect power.

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I think you didn't quite get what I was trying to say (unless I am going wrong somewhere here). Disconnecting the battery is ONLY supposed to happen if the calculator is left unattended with a flat battery and not rechard for a long period of time. This is to protect the battery under those circumstances.
That is also precisely why under normal circumstances I would like the calculator to shut down BEFORE the battery gets to the level where it needs to be disconnected. Thus preventing memory loss, as the battery stays connected.

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If you plan to let the calculator sit on the shelf for any longer period of time, just do an ON+STO which will create a copy of the RAM in flash. You can then remove power without any loss (except for the built-in RTC).

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Yes, so that is what I would do if I wanted to leave the calculator for a longer period of time without recharging it. Because in that case the battery might disconnect itsef.

I hope this time my idea was presented in a slightly more understandable way. :)

Cheers,
Harald

PS: I think I figured out how to do this. Next I will need to build and test the circuit and find a way to integrate this into the calculator. I am thinking the space above the display (under the HP logo) is suitable. I'll keep you updated.

Edited: 2 Mar 2012, 3:33 p.m.

[Hans-Peter Bieger]

What's the maximum voltage-level for the calculator? You know, the level of a full loaded cell is 4.2V. I think, that's about 1.0V to high. And the undervoltage-protection should be at 3.2V. That's my RC-Plane experience from the last 5 years. If you have a solution, i would be intrested:-)

Best Regards
Hans-Peter

[Harald]

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What's the maximum voltage-level for the calculator? You know, the level of a full loaded cell is 4.2V. I think, that's about 1.0V to high. And the undervoltage-protection should be at 3.2V. That's my RC-Plane experience from the last 5 years. If you have a solution, i would be intrested:-)

Best Regards
Hans-Peter

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According to the Atmel datasheet the maximum Voltage is 3.6V. It is not a problem to get down to there from the 4.2V. I am more concerned about protecting the battery against undervoltage. I have not found a good solution to disconnect the battery from the rest of the circuit in such a way that no current is drawn from it if the voltage is below 3.2V.
I'll let you know if I come up with something.

[Jim Horn]

Many LiPo cells include their own protection circuits such as documented at:

http://www.sii-ic.com/en/product1.jsp?subcatID=5&productID=379

The cell ("battery") in Palm Tungstens and many other devices have a tiny circuit board with this sort of system on it in them. Depending on the source of the cell you use in your calculator modification, this may already be included. If not, it would be easy to add and is probably available from your cell vendor on request.