Lpc2000

---- Original Message ----
From: "Digital Systems, Partner" <dsystem@...>
To: <lpc2000@yahoogroups.com>
Sent: Friday, January 20, 2006 6:42 PM
Subject: Re: [lpc2000] Re: changing PLL multiplier on fly - PLLCFG
(MSEL)

> But there is a bit, PLOCK. I am not very clear about application.
>
> Can I do as follows?
> Disable PLL (Not just switch). I neet to conserve power. And while
> enabling again, I check this bit again befor switching to PLL again.

I guess you can.  Other than the temporary PLL disconnecting, I have only 
done the normal PLL enabling at startup:

SYS->PLLCFG  = 0x23;  /* P=2, M=4 */
SYS->PLLCON  = 0x01;  /* Enable PLL */
SYS->PLLFEED = 0xAA;
SYS->PLLFEED = 0x55;
while (!(SYS->PLLSTAT & 0x400));      /* Wait until PLOCK */
SYS->PLLCON  = 0x03;  /* Enable and connect PLL */
SYS->PLLFEED = 0xAA;
SYS->PLLFEED = 0x55;


To conserve power, disconnect and disable PLL, and enter Idle mode as much
as possible.  Powerdown mode would be better, but only an external interrupt
or reset will bring you out of it.

When waking up, enable but not yet connect the PLL.  According to the
datasheet, the PLL takes 100 usec to lock.  Don't trust this number, wait
until the PLOCK bit is set, which tells that it has locked.

While you wait for it to lock, you can continue working at the low clock.
If you have nothing useful to do at the low speed, you'll conserve most
power by enabling the PLL interrupt so that you get an interrupt when PLOCK
is set, and while waiting for the interrupt, you enter Idle mode.  When the
PLL has locked, your PLL interrupt handler is called, and in that you
connect the PLL, and start the real processing.  This means that the CPU is
idle, not active, during the ~100 usec lock time.

Also remember to disable all unused peripherals in PCONP, and don't pull any
P1.x pins low (neither by external circuitry, nor by having them as GPIO
outputs that are low), as there are permanent internal pull-ups on these
pins.

Karl Olsen