UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
52 of 362
NXP Semiconductors
UM10208
Chapter 7: LPC2800 CGU
The following points bear noting about
:
•
Use of the USB interface constrains the fast oscillator frequency to 12 MHz.
•
Use of the Real Time Clock, including its battery backup capability, requires use of the
slow oscillator with a 32.768 kHz crystal. If this capability is not needed, ground the
X32I pin.
contains the elements shown in
.
The selection stages select among the main clocks, although they are more complex than
simple selectors in order to avoid glitches when they are being dynamically switched
between main clocks. The outputs of the selection stages are called “base clocks”. Some
selection stages and base clocks are dedicated to a particular spreading stage and
Fig 15. Clock generation unit block diagram
FAST
PLL
MAIN
PLL
HP0
LP0
FAST
OSCILLATOR
ffast
SLOW
OSCILLATOR
fslow
MCLK
BCKI
WSI
SWITCHBOX
X32I
X32O
XTALI
XTALO
clocks
66
resets
31
Fig 16. Switchbox block diagram
main clocks
SELECTION
STAGES
(11)
7
FRACTIONAL
DIVIDERS
(17)
6
17
base clocks
11
SPREADING
STAGES
(66)
module clocks
66
RESET
LOGIC
resets
31