Processor Clock speed change abd RS232

I am looking for a microcontroller with UART that will run at 1Mhz
normally and 8Mhz when more power is available. My problem is the
receive on the RS232 when changing the processor clock speed. I think
this will cause an upset in any receive characters coming in at the
change over. Has any body got any ideas for a low power processor
(0.5mA at 1MHz) that can change processor clock speed without
upsetting the receive side of the RS232.

Thanks

sjones@scannex.co.uk wrote:
> I am looking for a microcontroller with UART that will run at 1Mhz
> normally and 8Mhz when more power is available. My problem is the
> receive on the RS232 when changing the processor clock speed. I think
> this will cause an upset in any receive characters coming in at the
> change over. Has any body got any ideas for a low power processor
> (0.5mA at 1MHz) that can change processor clock speed without
> upsetting the receive side of the RS232.

Rather than switching speeds, can you just use a sleep mode
to reduce power consumption?

-Mike

sjones@scannex.co.uk wrote:
> I am looking for a microcontroller with UART that will run at 1Mhz
> normally and 8Mhz when more power is available. My problem is the

Either:
1. change the clock divisor when you switch speeds. Look at maybe an
MSP430, where you can reprogram the DCO according to your current power
mode.

2. use a micro that can accept an external clock for the UART. However
this will inherently waste power.

Maybe, but just wondered if possible to switch clock speed without
upsetting RS232.

On 5 May 2006 03:51:50 -0700, sjones@scannex.co.uk wrote:

>I am looking for a microcontroller with UART that will run at 1Mhz
>normally and 8Mhz when more power is available. 

How do you change the speed ? Do you have a separate crystals for 1
MHz or 8 MHz, or are you using an 8 MHz oscillator and an external
CMOS divide by 8 divider and a data selector to either select 1 or 8
MHz.

>My problem is the
>receive on the RS232 when changing the processor clock speed. I think
>this will cause an upset in any receive characters coming in at the
>change over. 

Use a separate crystal for the UART running at constant frequency with
a suitable fixed divider. 

If there are no requirements for accurate CPU frequency, why not just
use an RC oscillator and switch in extra capacitors/resistors to alter
the frequency.

>Has any body got any ideas for a low power processor
>(0.5mA at 1MHz) that can change processor clock speed without
>upsetting the receive side of the RS232.

One approach would be to change the CPU speed and divider settings in
the RxFull interrupt between characters. Of course you would have to
suspend transmitting new characters between the speed change request
and the RxFull interrupt, assuming constant incoming bytes. 

Paul

On 2006-05-05, sjones@scannex.co.uk <sjones@scannex.co.uk> wrote:
> I am looking for a microcontroller with UART that will run at 1Mhz
> normally and 8Mhz when more power is available. My problem is the
> receive on the RS232 when changing the processor clock speed. I think
> this will cause an upset in any receive characters coming in at the
> change over. Has any body got any ideas for a low power processor
> (0.5mA at 1MHz) that can change processor clock speed without
> upsetting the receive side of the RS232.

Pick a processor like the MSP430 where the UART and CPU can run
from different clocks.  Most modern microcontrollers are set up
to do that.

-- 
Grant Edwards                   grante             Yow!  The LOGARITHM of
                                  at               an ISOCELES TRIANGLE is
                               visi.com            TUESDAY WELD!!

larwe wrote:
> sjones@scannex.co.uk wrote:
> 
>> I am looking for a microcontroller with UART that will run at 1Mhz
>> normally and 8Mhz when more power is available. My problem is the
> 
> Either:
> 1. change the clock divisor when you switch speeds. Look at maybe an
> MSP430, where you can reprogram the DCO according to your current
> power mode.
> 
> 2. use a micro that can accept an external clock for the UART.
> However this will inherently waste power.

You could run the clock from a VCO controlled by a phase lock loop,
and control the slew rate so that clocks are continuous, rather
than discrete jumps.

The fixed crystal runs the UART proper, and provides the reference
for the phase locked loop.  You might even be able to use the cpu
and the analog of delta-modulation to control the VCO with minimum
hardware.  Getting started could be a problem.

-- 
"If you want to post a followup via groups.google.com, don't use
 the broken "Reply" link at the bottom of the article.  Click on 
 "show options" at the top of the article, then click on the 
 "Reply" at the bottom of the article headers." - Keith Thompson
More details at: <http://cfaj.freeshell.org/google/>
Also see <http://www.safalra.com/special/googlegroupsreply/>

sjones@scannex.co.uk wrote:
> I am looking for a microcontroller with UART that will run at 1Mhz
> normally and 8Mhz when more power is available. My problem is the
> receive on the RS232 when changing the processor clock speed. I think
> this will cause an upset in any receive characters coming in at the
> change over. Has any body got any ideas for a low power processor
> (0.5mA at 1MHz) that can change processor clock speed without
> upsetting the receive side of the RS232.

You are right, a surprising number of uC scale the Peripheral clock, 
when they scale the core clock. Seems the wrong thing to do, and
the power delta must be tiny, relative to the OscBuffer ?
Might be forced on them by flag handshake handling ?

One device that looks to do this properly, is the Silabs C8051Fxx series

This has a choice of SYSCLK or  PreScaledClk into the Baud timers.
Thus you can freely scale the CPU, and not mess up the peripherals.

The C8051F41x specs core at 300uA/1Mhz active, and 150uA/1Mhz idle,
so you might get the whole device inside 500uA/1MHz.

It also has an on chip regulator, so Icc does not change with Vcc,
and covers 2.1-5.5V

-jg