Forums

Re: [lpc2100] Real time clock

Started by Igor Janjatovic February 2, 2004
> This means that the contents of the clock are a bit fragile, really -
> suggesting it will be better used as a local 'cache' of an external
> seperately-clocked real time clock unit that's slow to access (I2C or
> similar), loaded from it during startup or return from power down mode
> after setting up the PLL and the RTC divider registers. Unless you have
> a setup that's rarely reset and never put into power down mode, it's not
> all that useful, as I see it.

I have this low power design that I'm working on and I'm using PCF8593 I2C
RTC with 32.768kHz clock. PCF will take only 1uA so I planned to put MCU in
Power Down most of the time. When running, MCU will take time and date from
PCF and use it with its own integrated RTC just as you suggested. PCF will
be updated only when time or date is changed by user.

> Anyone else had any thoughts about it? It would have been nice if they
> provided an external 32,768Hz clock input pin to run the RTC off of when
> the MHz oscillator was down!

Looking at LPC spec for the first time that's what I expected. Later,
looking at User Manual was very... well... disappointing. Who needs RTC with
20mA current consumption (when in Idle mode)?????

> On the other hand, it occurs to me that the PLL seems amply capable of
> producing processor clock frequencies far in advance of the rated 60MHz
> - overclocking! How soon before we see huge copper heatsinks with fans
> for LPC2100s, eh? ;-)

Few days ago someone said that they used overclocked LPC without any
problems...

Regards,
Igor



An Engineer's Guide to the LPC2100 Series


Looking at the LPC210x manual, the real time clock:

1) Needs to be set up for the crystal frequency / PLL settings

2) Will not tick during Power Down mode

This means that the contents of the clock are a bit fragile, really -
suggesting it will be better used as a local 'cache' of an external
seperately-clocked real time clock unit that's slow to access (I2C or
similar), loaded from it during startup or return from power down mode
after setting up the PLL and the RTC divider registers. Unless you have
a setup that's rarely reset and never put into power down mode, it's not
all that useful, as I see it.

Anyone else had any thoughts about it? It would have been nice if they
provided an external 32,768Hz clock input pin to run the RTC off of when
the MHz oscillator was down!

On the other hand, it occurs to me that the PLL seems amply capable of
producing processor clock frequencies far in advance of the rated 60MHz
- overclocking! How soon before we see huge copper heatsinks with fans
for LPC2100s, eh? ;-)

ABS




----- Original Message -----
From: "Alaric B Snell" <>
To: <>
Sent: Wednesday, February 04, 2004 1:56 PM
Subject: [lpc2100] Real time clock >
> Looking at the LPC210x manual, the real time clock:
>
> 1) Needs to be set up for the crystal frequency / PLL settings
>
> 2) Will not tick during Power Down mode
>
> This means that the contents of the clock are a bit fragile, really -
> suggesting it will be better used as a local 'cache' of an external
> seperately-clocked real time clock unit that's slow to access (I2C or
> similar), loaded from it during startup or return from power down mode
> after setting up the PLL and the RTC divider registers. Unless you have
> a setup that's rarely reset and never put into power down mode, it's not
> all that useful, as I see it.
>
> Anyone else had any thoughts about it? It would have been nice if they
> provided an external 32,768Hz clock input pin to run the RTC off of when
> the MHz oscillator was down!
>
> On the other hand, it occurs to me that the PLL seems amply capable of
> producing processor clock frequencies far in advance of the rated 60MHz
> - overclocking! How soon before we see huge copper heatsinks with fans
> for LPC2100s, eh? ;-)

Someone posted their experiences with overclocking a few weeks ago.

Leon
--
Leon Heller, G1HSM
Email:
My low-cost Philips LPC210x ARM development system:
http://webspace.webring.com/people/jl/leon_heller//lpc2104.html



--- In , "Igor Janjatovic" <kodrat@p...> wrote:
> > This means that the contents of the clock are a bit fragile, really -
> > suggesting it will be better used as a local 'cache' of an external
> > seperately-clocked real time clock unit that's slow to access (I2C or
> > similar), loaded from it during startup or return from power down mode
> > after setting up the PLL and the RTC divider registers. Unless you
have
> > a setup that's rarely reset and never put into power down mode,
it's not
> > all that useful, as I see it.
>
> I have this low power design that I'm working on and I'm using
PCF8593 I2C
> RTC with 32.768kHz clock. PCF will take only 1uA so I planned to put
MCU in
> Power Down most of the time. When running, MCU will take time and
date from
> PCF and use it with its own integrated RTC just as you suggested.
PCF will
> be updated only when time or date is changed by user.
>
> > Anyone else had any thoughts about it? It would have been nice if they
> > provided an external 32,768Hz clock input pin to run the RTC off
of when
> > the MHz oscillator was down!
>
> Looking at LPC spec for the first time that's what I expected. Later,
> looking at User Manual was very... well... disappointing. Who needs
RTC with
> 20mA current consumption (when in Idle mode)?????
>
> > On the other hand, it occurs to me that the PLL seems amply capable of
> > producing processor clock frequencies far in advance of the rated
60MHz
> > - overclocking! How soon before we see huge copper heatsinks with fans
> > for LPC2100s, eh? ;-)
>
> Few days ago someone said that they used overclocked LPC without any
> problems...
>
> Regards,
> Igor

Hi,

I wondered as well why there was no 32 kHz input for the LPC. This
question was also asked at the Philips training that I participated.
The answer was along the lines of "there are applications that are
always connected to AC and they need to track time" well I guess that
is true. "Also the are other applications that need to go into power
down for minimum power and still should track time with some uAs of
power. Unfortunately this can not be done with the current
implementation of the LPC." Guess that is true as well. The proposed
solution was to use a small LPC900 device, e.g. an 8-pin micro that
would have a RTC / System Clock. Looked into this a little and it can
definitely be done. Providing 32 kHz to such an LPC900 device and
using the 1k Flash to generate a real time-of-day timer is not hard to
do. According to my information the LPC900 devices would be lower cost
than the PCF 8593 and more flexible but on the downside it would
require some 10s of uAs instead of 1 uA. If battery driven that can
make all the difference.

You may come up with your own conclusions ;-)

Cheers, Bob



On Thu, 05 Feb 2004 00:19:53 -0000, lpc2100_fan wrote: Hi,

I wondered as well why there was no 32 kHz input for the LPC. This
question was also asked at the Philips training that I participated.
The answer was along the lines of "there are applications that are
always connected to AC and they need to track time" well I guess that
is true. "Also the are other applications that need to go into power
down for minimum power and still should track time with some uAs of
power. Unfortunately this can not be done with the current
implementation of the LPC." Guess that is true as well. The proposed
solution was to use a small LPC900 device, e.g. an 8-pin micro that
would have a RTC / System Clock. Looked into this a little and it can
definitely be done. Providing 32 kHz to such an LPC900 device and
using the 1k Flash to generate a real time-of-day timer is not hard to
do. According to my information the LPC900 devices would be lower cost
than the PCF 8593 and more flexible but on the downside it would
require some 10s of uAs instead of 1 uA. If battery driven that can
make all the difference.

You may come up with your own conclusions ;-)

======================================================================
Interesting comments Bob. You got me thinking. I've got a board in
fab that uses a X1226 RTC because I didn't feel the LPC's RTC would
do the job I needed. The cost from Digikey is $4.09 and it's back-ordered.
One the next turn of the board I may replace it with a MSP430F1101 which
is $2.48 from Digikey and is suppose to be in stock. It will run directly
from the same battery and crystal as the 1226 part. A bit-banged serial
interface should be no problem.

Regards
-Bill Knight
R O SoftWare