Lpc2000

--- In lpc2000@yahoogroups.com, "microbit" <microbit@c...> wrote:

> Look at PLLCFG register.
> If you for example are after 14.7 MHz * 4 = 58.8 MHz,
> then you would set the Multiplier(M) to 4, and the presaler (P)
> could for example be 2 - thus :
> Fcco = Fosc * M * 2 * P = 14.7 * 4 * 2 * 2 = 235.2 MHz.
> cclk = Fosc * M  = 58.8 MHz.

The demo code supplied by Rowley has P set to 1 and M set to 4 
(PLLCFG = 0x24) in their example giving 59Mhz with a 14.7Mhz 
crystal.  This gives a Fcco of 117Mhz which should be illegal.

Second, the data sheet shows "P" is a 2 bit value (bits 5 and 6 of 
PLLCFG) , yet it talks about values for P in the range of 2 - 16!  
Which is it Philips?

Third, Toggling an I/O pin in a while(1) loop I get periods of:

PLL OFF: 1150ns (should be 14.7Mhz)
PLLCFG = 0x61 (P=3, M=1): 577ns (should be 14.7Mhz but is 29Mhz?)
PLLCFG = 0x21 (P=1, M=1): 577ns (as above)
PLLCFG = 0x42 (P=2, M=2): 384ns (Should be 29.4Mhz but is 44Mhz?)
PLLCFG = 0x43 (P=2, M=3): 288ns (Should be 44Mhz but is 59Mhz?)
PLLCFG = 0x44 (P=2, M=4): 231ns (Should be 59Mhz but is 73.5Mhz?)
PLLCFG = 0x23 (P=1, M=4): 231ns (as above)

The above numbers are running from RAM with VPBDIV = 1 (full speed 
peripherals).  Because of the way I/O works on the Philips parts, you 
don't get a 1:1 correlation between clock rate and I/O speed but the 
above still show something is wrong?  What's going on here?


> You need to change the "mov R1, # 0x24" instruction to 
> change the prescaler ("divider") value.
> For example, 0x25 sets P to 2 instead of etc.

Not as I read the User Manual. 0x24 is P=1, M=4.  0x25 would be P=1, 
M=5.