|Re: I guess the whole point of this question is...|
Message #12 Posted by Tom (UK) on 8 Nov 2000, 1:15 p.m.,
in response to message #11 by Mike Davis
With my HP67 I think the non '0.00' display appears because of switch bounce on power up. What the calculator's CPU does when the power goes on-off-on in a fraction of a second will be fairly random.
I guess what is happening is that the CPU gets confused and starts executing code at a random place until that routeen finishes. Depending on what's in the CPU's scratch registers at that time and the point in the code that the CPU starts to execute will dictate what appears in the display. In my case this is constant figures that are cleared with 'Clx', other calc's may look as though they are executing a program.
As the HP67/65 has a fair ammount of program memory, I wonder if this initialy powers up with a random sequence? Does anyone know what the HP65/67 does on power-up? i.e. does the CPU go through each program location and clear it or is the design of the memory such that the memory always powers up with 'R/S' commands???
More modern designs will have a power on reset that makes the CPU do nothing until the power stabalises. I guess the older classic models either don't have this or the switches get so 'noisey' on power-up that it can't cope.
The later calc's with constant memory have the voltage supplied to the CPU all the time so there is near zero risk of it powering up with a non '0.00' display - even if the switch bounces a great deal on power up.
Each calculator will have different parasitic (i.e. stray) capacitance on the circuit board and within each CPU (silicon devices - especially older types - can vary a great deal) and this may control what happens on power-up. i.e. this is why every model of the same calc may do different things on power-up.
I don't think this should change the value of the calculator as they will work perfectly after a very short power up time. However it may be an early indication that the power switch's silicone grease is getting a bit thin. In fact this behavior makes the classic series of calculators even more endearing to me as they seem to have much more of a personality than the more modern designs. Please note I am NOT a collector, just a avid user, I use my HP67 the most even though I have: HP32/48/49.