A/D conversions take quite a bit of power (.65 to 1.6mA for the converter and
.5 to .8mA
for Vref for the F44x series per conversion for the duration of the conversion
{according to SLAS344A}). Perhaps you need to add this to the power
requirements when
calculating how long the MSP should take. Sure, in shutdown, it takes 1.8uA or
so, but
the A/D would be "on top" of that. Also, A/D conversions can be made to happen
in the
background, so that's a consideration also.
If it were me, I'd suggest a supercap of something like 1F or something. That
should
keep the MSP430 on for quite a long time.
Simple calculation (not necessarily complete):
.65 + .5 = 1.1mA per conversion during conversion. Let's just say the
conversion takes
300us (completely made up). So, that gives us:
1.1mA * .0003 = 3.3x10-7 A-Seconds. Or, 5.5x10-9 A-Minutes Or: 9.167x10-11 A-H.
If we take that a capacitor discharges according to the formula:
Vt=Vo(e^(-t/RC))
Where
Vt=voltage at time t
Vo=Voltage at time 0
t=time in seconds
And R and C in ohms and farads
AND we characterize the above A-s values to an equivalent parallel RC circuit
(major
generalizations), then, we can calculate for t (using a 10M ohm resistor which
gives
roughly 3.3x10-7A-s at 3.3v).
Using Excel, and calculating to V=1.8v (spec of MSP), then I get roughly 6s for
a 1uF
cap. Not very long at all. And of course this is assuming a perfect cap w/ no
ESR,
etc.
Of course, this also doesn't take into account that the voltage is decaying,
and so
would the current in the RC circuit (I'm not sure what would happen in the
430), so that
would have an impact also.
Hope this helps.
-Mark
-----Original Message-----
From: Kelly Murray [mailto:k...@netnimble.com]
Sent: Tuesday, May 04, 2004 12:55 PM
To: mspgcc-users@lists.sourceforge.net
Subject: [Mspgcc-users] sleep mode power consumption
Hello,
I've got a system where I use the MSP430 to turn off the power if the
system is
idle for some time, and then wake it back up if there is any user input.
Of course, this is not possible, if the power is off, the MSP430 can't run!
But I use a .22F 3v capacitor that is charged up while power is on, and
then runs
the MSP when the power is off.
The strange thing is that when I monitor the
voltage after power-down, the cap is loosing voltage rather quickly,
about .01 v per second,
until it hits 1.66V and then stays there. This should be less than the
voltage needed to run the MSP?
I think the specs say 1.8v. However, it does still seem to be running,
because it is still monitoring
the input and turns the power back on.
My guess is that perhaps this behavior is "just lucky", that at 1.66v
the MSP is barely operating.
Alternatively, I suspect that because the 1.66v is also used as a logic
pin to shutdown the power,
it may be dipping below an "off" value for a tiny fraction of time,
just enough to give the cap
some juice enough to run the MSP, but not detectable by my voltmeter.
Anyone have any ideas or suggestions? Isn't 1.66v too low to operate
the MSP4301232 ?
Any why is cap dropping so quickly from 3v, in shutdown mode, it should
draw very little power,
just enough to wake up every second and do a A/D reading.
thanks for any input,
Kelly Murray
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