Kris, There is one possible way you could do it, without the voltage drop! The trick is based on the fact that normally your module is off and NOT drawing any current. During that time, you could be charging a capacitor with the VCC's 3.3V. The voltage at the cap will eventually reach a voltage very close to 3.3V. Then, when you turn on for 100mS, both the VCC and your capacitor will be providing current to your module. The VCC will still supply it through the 330 ohm resistor, but your cap will act like a low-impedance source! Assuming a linear discharge of the capacitor (big assumption, I know), and that VCC does not help you in any way (worst case), you can estimate the minimum capacitor value this way: C = (current draw) / (voltage drop in 100mS) So let's say you can not tolerate more than 0.3V drop (i.e. 3V minimum voltage) during your 100mS operation, and that you draw 3mA: minimum C = (0.003) / ( 0.3 / 0.1 ) = 0.001F (=1000 microFarads). 1000 uF is quite big (bad news), even for a low voltage (10V?) capacitor. But you get the idea of the tradeoff between voltage drop and capacitor value. The good news is that you have VCC helping you during that time, so you will need a much smaller cap to hold the same drop! To determine the best value, you will need to experiment, because your current load is very likely not linear, and because you have VCC helping you. Hope this helps! Jose >>> <[EMAIL PROTECTED]> 05/10/00 04:23PM >>> Yes, that's dumb of me. I'll have to think of something else... Kris At 06:32 PM 5/10/00, you wrote: >Kris, > > If the 3.3V VCC output has a 330 ohm series resistance, then you can > not get much current out of it without a significant voltage drop. > > The VCC you would get is > Vcc = 3.3V - (current draw)*330 > So for 3mA the VCC output is 2.31V. > > Also, I am not sure if the internal 3.3V regulator of the Palm is > designed to output that much current, or if the 3.3V internal voltage > would be affected. > >Jose > > >>> <[EMAIL PROTECTED]> 05/10/00 02:08PM >>> >I've asked a question about this some time in the past, in a different >context. The answers back then do not quite address my current situation. > >I have designed a small peripheral which communicates w/ the palm III >series serially via the hotsync connector. The device needs 3.0-3.5 V >supply, and usually draws only about 5 uA (standby mode). When the Palm >wants some data from the device, its serial driver comes to life, a short >burst of data gets dumped to the palm, and everything shuts back down to >the standby condition. The busy-time is about 100ms with a current draw of >maybe 2-3 mA (certainly less than 5 mA). This acquisition burst occurs >every two to ten seconds. > >Now as I noticed before, Palm formally suggests not taking any power at all >from the hotsync port. In my application, however, size is critical, and >adding even a small coin cell is difficult due to final size limitations. > >Anyway, my question is: if I want to power my peripheral from the hotsync >port, which pin might be the best to use? I see pin 2 is VCC at 3.3 V, and >pin 1 is DTR which is 3-6 V (-ish) depending on whether the serial driver >is on. Each pin is connected through a 330 Ohm R, according to the specs >(so taking as much as 10 mA from the VCC pin should be possible?). From the >looks of it, the VCC pin should be able to supply more than enough for my >application. Right? > >This type of arrangement can and has been done: TaleLight LED flashers, >GoType Keyboards, Palm's fold-out keyboard -- hey, Palm, haven't you read >your own "guidelines"? :) > >Any pitfalls(s) you can think of, apart from reducing battery life very >slightly? > >Would appreciate any words of wisdom, > >Kris Wilk >ReefNet Software >www.reefnet.on.ca > > >-- >For information on using the Palm Developer Forums, or to unsubscribe, >please see http://www.palmos.com/dev/tech/support/forums/ > > > >-- >For information on using the Palm Developer Forums, or to unsubscribe, >please see http://www.palmos.com/dev/tech/support/forums/ -- For information on using the Palm Developer Forums, or to unsubscribe, please see http://www.palmos.com/dev/tech/support/forums/ -- For information on using the Palm Developer Forums, or to unsubscribe, please see http://www.palmos.com/dev/tech/support/forums/
