WP34s Differential Time and shift

[matthiaspaul]

(02-10-2015 01:58 AM)BarryMead Wrote:  If you reserved a register for "Last Correction Time" (Say R99) then whenever you run your program it could use the difference between R99 and current time and compute the correction factor, then it could update the current time and save the current time into R99 for the next calibration.

Yes, and once the semi-constant calibration factor for a particular unit has been established after a few weeks, it should be stored in flash, thereby freeing up the register again.

I recall that my old CORRTIME utility could optionally maintain two correction factors, depending on if the PC was switched off and on. This was down to my observeration that the clock was off by different factors depending on if the machine was powered or the RTC running on its backup battery, probably due to the slight difference in voltage (constantly high versus variable and lower). However, this required to run the tool before switching off the PC as well.

So, for most accurate results, we might take the battery voltage and the temperature into account as well. Also, a temperature sensor might be a nice addition to the WP 34S in general.

As far as I see, the WP 34S' hardware has enough free I/Os left for use. Since the firmware already includes code for the analog/digital conversion of the battery voltage, an analog temperature sensor like the Texas Instruments TMP20 (http://www.ti.com/product/TMP20/technicaldocuments) appears to be more desirable than using one with digital read-out (via I2C or a similar protocol). The code overhead to read out the sensor should be minimal, and the electrical interface is simple as well: One pin for power (via a port output to reduce power consumption), another for GND, and a third pin for the analog signal. Fully operational the device drains 4 µA, and truely zero ampere if powered off. Basically, anyone who could install the crystal would be able to add the temperature sensor as well.

Greetings,

Matthias