Kelly Murray wrote:
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,
some capacitors have a rather high leak current, so choose your part
carefully. an other trick is to load the cap to a higher voltage. i.e
you place the cap before the voltage regulator and use a low drop/low
quiscent currrent regulator (e.g. the Motorola LC30 uses only around 1uA)
lets say your input voltage is 5V then you have 2V (or over 3V if you
allow it to go down to 1.8V vcc) that means you can stay much longer
alive, c*u=i*t :-)
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 1.8 volts are the minimum guarateed value over the full temp range.
the typical value at room temp will be lower.
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 ?
according to the datasheet, your using it out of specs
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.
as mentioned before the cap is important. then take a look at the
peripherals of the msp430.
LPM4 just switches off clocks, it does not switch off the comparator or
the adc, or the flash controller, or the timers (external clock...) or
the uart or ...
make sure you shut down these peripherals before going into the lowpower
mode.
if the circuit connected to the msp430 is switched off, make sure that
no ouput pins are on high, keep attention to input protection diodes of
connected ICs, nice way to power an IC from the msp ;-). consider
switching those pins to low or configure them as input.
and make a current measurement, see what it's drawing in the low power
mode. the msp itself can go as low as <1uA (0.1uA typicaly) while still
be able to keep the RAM contents and wakeup on interrupts.
how do you wake up every second? do you run a timer from a crystal? if
so, try to divide the clocks as early as possible so that as much as
possible of the silicium runs as slow as possible. use a watch crystal
and not a HF one. in any case, a running timer/watchdog timer and
crystal quickly uses more than 10uA...
chris
