Is it possible to run the RTC from vBatt while Vdd is sitting at 0V?
There seem to be mixed opinions of this on the forum and the data
sheet is anything but specific. There are details about how many uA
the RTC draws from vBatt when the LPC is in power-down mode, but
nothing about the current draw when the only thing being powered is
the RTC with power removed from everything else.

Can this be done or are we doing to need an external RTC? Are there
any dirt-cheap SPI RTCs around?

Thanks! Start a New Thread

Hi,

You can run the LPC2138 RTC from Vbatt with Vdd = 0volts.

The only draw back I see is the size off battery to keep the RTC alive
for a long period of time when Vdd = 0. I have not been able to
find current comsumption numbers for the RTC. It would
be interesting however to compare them with the current draw
of an external RTC solution.

I can only get about 4months before may RTC battery goes flat.
The battery I use is Renata CR1025FV-LF 30mAh.

I was thinking of going to a external RTC solution but it is so
handy from a programming and manufacturing stand point to have
the RTC in the Micro.

Grant
NZ

-----Original Message-----
From: l... [mailto:l...]
On Behalf Of Darcy Williams
Sent: Wednesday, 2 May 2007 8:34 a.m.
To: l...
Subject: [lpc2000] LPC2138 and RTC running from vBatt

Is it possible to run the RTC from vBatt while Vdd is sitting at
0V?
There seem to be mixed opinions of this on the forum and the
data
sheet is anything but specific. There are details about how many
uA
the RTC draws from vBatt when the LPC is in power-down mode, but
nothing about the current draw when the only thing being powered
is
the RTC with power removed from everything else.

Can this be done or are we doing to need an external RTC? Are
there
any dirt-cheap SPI RTCs around?

Thanks! Start a New Thread

Did anybody ever figure out anything regarding battery life and
the RTC ?? In my LPC2103 application, I'm seeing the battery voltage
coming down 0.2 volts in just a few days. 3.1 something to around
2.7 something today.

I wonder also, if maybe if there is a particular 32.769 kHz xtal that
might be recommended ? I can certainly see the RTC operating off
of the normal 3.3 volt supply instead of battery, but I would
think there would be either diodes and/or an extra external transistor
to switch from one supply to the other (battery) when non-coin cell
power is removed. Maybe a little SOT23 FET gating on from 5 volts
on the board. Hmmmm.... Might be a solution I suppose.

boB

--- In l..., "Grant Sullivan" wrote:
>
> Hi,
>
> You can run the LPC2138 RTC from Vbatt with Vdd = 0volts.
>
> The only draw back I see is the size off battery to keep the RTC alive
> for a long period of time when Vdd = 0. I have not been able to
> find current comsumption numbers for the RTC. It would
> be interesting however to compare them with the current draw
> of an external RTC solution.
>
> I can only get about 4months before may RTC battery goes flat.
> The battery I use is Renata CR1025FV-LF 30mAh.
>
> I was thinking of going to a external RTC solution but it is so
> handy from a programming and manufacturing stand point to have
> the RTC in the Micro.
>
> Grant
> NZ
>
>
>
> -----Original Message-----
> From: l... [mailto:l...]
> On Behalf Of Darcy Williams
> Sent: Wednesday, 2 May 2007 8:34 a.m.
> To: l...
> Subject: [lpc2000] LPC2138 and RTC running from vBatt
>
>
>
> Is it possible to run the RTC from vBatt while Vdd is sitting at
> 0V?
> There seem to be mixed opinions of this on the forum and the
> data
> sheet is anything but specific. There are details about how many
> uA
> the RTC draws from vBatt when the LPC is in power-down mode, but
> nothing about the current draw when the only thing being powered
> is
> the RTC with power removed from everything else.
>
> Can this be done or are we doing to need an external RTC? Are
> there
> any dirt-cheap SPI RTCs around?
>
> Thanks!
>
>
>
>
>
>
> Start a New Thread