power consumption

F

fred_b

Member
I have a circuit I have been working on that has a picaxe 18X, a ds1307 clock, an LCD, an LCD driver (a PIC microcontroller). The LCD, LCD driver and the ds1307 only need supply voltage during the first minute or so of operation. After that, the picaxe switches off the supply to those parts to save power.

(They are powered through 18X output pins).

My question is: Will the various resistors that are pulled high by a connection to the main power rail still cause some power drain or is it sufficient to only switch off the supply voltage?

I will do some testing but thought I would ask here also.

Thank you!
 
Last edited:
A

Andrew Cowan

Senior Member
I'm not sure, but I don't know if the chips would like having some inputs high while no power is present. However, it will still draw a little power - how much is up to you to test.

A
 
D

Dippy

Moderator
I think it would be best to post the precise schematic for a precise answer.

I had exactly that issue (pullups leaking) when using a.n.other RTC.

Will you be using a battery for RTC backup?
 
B

BeanieBots

Moderator
Yes, they will but with clever design you can eliminate them.
For example, if your LCD has a pull-up, make sure that it is connected to the line that you bring down when it powered down.

For the DS1307, the pull-ups are on the I2C bus so that cannot be done.
However, from memory, the inactive current draw is < 1uA. Will that have any real impact on your overall current draw?

In situations where an input remains high (eg pull-ups), consider releasing the 0v pin rather than the supply pin.
 
F

fred_b

Member
That didn't work!

The current draw actually doubled for some reason (up to 12 mA) when I pulled the supply low for those three devices.

So I tried switching the entire power rail off to cut the supply as well as anything "pulled high" on it.

I found out the clock didn't want to work properly so I ended up powering it and the associated pull-ups off of the main supply.

So:
pcaxe and clock and related pull ups are powered all the time.

lcd driver and lcd and related pull ups are switched off.

power use went from 4.2 to 1.2 mA. That works for me!

(A lot less when picaxe is in sleep)

Thanks for the help!
 
You must log in or register to reply here.
Top