Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!mit-eddie!uw-beaver!milton!ogicse!orstcs!usenet!ECE.ORST.EDU!daver From: daver@ECE.ORST.EDU (Dave Rabinowitz) Newsgroups: comp.sys.handhelds Subject: Re: HP28 power study Message-ID: <1990Dec01.224400.11957@scion.CS.ORST.EDU> Date: 1 Dec 90 22:44:00 GMT References: <3017@gmuvax2.gmu.edu> Sender: @scion.CS.ORST.EDU Organization: Oregon State University -- Electrical & Computer Engineering Lines: 76 Nntp-Posting-Host: ece.orst.edu In article <3017@gmuvax2.gmu.edu> peraino@gmuvax2.gmu.edu (Bob Peraino) writes: > A few people have asked how much power can be saved if the operating speed >were set to 0, and the display were turned off. We can now calculate this. >From the above power chart, we see that speed 7 draws 2399 uAMPS and speed >0 draws 1633. Combined with disabling the display at minimum contrast, this >is a savings of (2399-1633)+243 = 1009, or about 42% of the normal operating >current. There are two ways of measuring operating power. Bob considered energy per second, which is applicable when a program is running all the time but not doing much, as in the example he gave (copied at the end of this posting). In most applications a more useful measurement would be energy per computation, since normally the calculator remains in the high-current operating state only while it's computing. We use Bob's numbers to compute the approximate energy (measured in milliamp-seconds; multiply by the battery voltage - not given in his article - to get actual energy) needed to complete his test program by multiplying the run time by the current used during that time. Running mAMPS sec ma-sec Speed -------------------------------- 0 1.6330 2.2593 3.69 1 1.6340 2.2551 3.68 2 1.6990 1.5370 2.61 3 1.8490 1.1171 2.07 4 1.9950 0.8773 1.75 5 2.1370 0.7211 1.54 6 2.2700 0.6112 1.39 7 2.3990 0.5304 1.27 8 2.5230 0.4672 1.18 9 2.6420 0.4187 1.11 10 2.7530 0.3787 1.04 11 2.8610 0.3447 0.99 12 2.9650 0.3174 0.94 13 3.0660 0.2933 0.90 14 3.1610 0.2725 0.86 15 3.2550 0.2544 0.83 Clearly it takes less total energy to execute the whole program at maximum speed, so for normal operation you'd want to run at the maximum speed that's safe given your battery condition. This is not a surprising result. The hp28s is implemented in CMOS, so for digital operations there is no DC power (there is DC power in the analog circuits which generate the display voltages, generate the CPU clock and check the battery condition, etc.). Each operation requires charging and discharging capacitances, and the same capacitances go through the same charge and discharge cycles to execute the same operations regardless of operating speed. Therefore, you can think in terms of capacitance charges per operation. It turns out that the power dissipated in charging a capacitor from a given voltage to another voltage is the same regardless of how fast the capacitor is charged, so each digital operation takes the same amount of energy regardless of how fast the operation is performed (obviously this breaks down if you go too fast or too slow, but this is accurate within the available speeds). Running the calculator faster takes the same amount of energy to do the digital operations as running it slower, but since less total time is taken, there is less overhead energy lost to the analog circuits so the total energy consumption is less. In the following example the calculator is running in a continuous loop but no one cares how many computations it accomplishes: > You may wonder why anyone would want to do this, since operating speed >would be three times slower. One application written by Bob (but not yet >released) is an alarm system for the hp28s called CRON. If you need to use >your 28s for an alarm system, or to run programs at a set time/date, then you >can use CRON. CRON allows you to set repeating or one-time alarms. CRON >accomplishes this by constantly checking the date, time, and setting of >all alarms. Obviously, CRON does not need to run at full power. When CRON >is in "sleep" mode, the display is turned off, and the speed is set >to 0, saving 42% of normal operating power. When it is time to take >action on an alarm, CRON enables the display and restores the speed to 7. >This is a perfect application for reduced operating power. Dave Rabinowitz