[41CL] Lockup Recovery Procedures


I have been playing with my new 41CL for a few weeks now and have succeeded in locking it up several times now. Generally, once it locks up, if I remove the batteries for 10 seconds and re-insert, whatever was on the locked-up display reappears when I turn it on, and it is still locked up. If I leave the batteries out for longer, I lose the time and date in the Time Module, a small inconvenience.

Trying the Master Clear sequence does not work and adding the ENTER key to the sequence does not generally help either. This sequence seems to have worked for me the past two lockups and preserved my clock settings.

Remove batteries. After display disappears, reinstall batteries. Press and hold these three keys in order, ENTER, CLx, ON. Release these keys. The display will still be off. Perform a normal Master Clear (hold CLx and press and release ON). This has unlocked the device, performed a Master Clear, and preserved the Time and Date settings.

BTW, I've locked the 41CL up by running a physical HP-41C diagnostic module at Turbo 50 mode (non-Turbo worked pretty well). I've also locked it up by testing the ROM/RAM flag behavior when the 41CL has 'auto-offed'. I also loaded an ES RSU ROM page, and upon turning on the 41CL, it locked up, likely because the ROM was looking for its normally associated RAM, which was not available.


Once the 41 (any kind, from C to CL) locks up, removing the batteries is usually the only remedy. The reason for this is that the keyboard is only polled under software control, so if the software isn't checking the keyboard it can't be interrupted. The keyboard is continuously scanned, but the software needs to look at the result of the scanning to take any action.

The CL contains a capacitor to supply current briefly if the batteries are removed, and the power-on-reset detector does not trip until the internal voltage drops below about 3.9V. Depending on what the software is doing, and the Turbo mode, this might take as long as 30 seconds. Pressing the ON key while the batteries are out will help to shorten the time, because this guarantees that the power supply is on and the FPGA is powered up. Any other keys are not likely to have any effect unless the software actually is polling the keyboard.

And, yes, by the time that the power-on-reset trips the time module will probably have lost its memory.

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