UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
65 of 362
NXP Semiconductors
UM10208
Chapter 7: LPC2800 CGU
3.9 Selection stage programming
Operationally, each selection stage selects among the 7 main clocks of the CGU, but it is
more complex than a simple selector to allow software to switch the selection without
producing a glitch on the stage’s output (base clock). To switch a selection stage from one
main clock to another, software should:
1. Read the SSR to determine which side of the stage is currently enabled.
2. Write FSR1 or FSR2, whichever is not enabled, with the select code for the new main
clock.
3. AND the value from step 1 with 3, then XOR it with 3, then write the result to the SCR
to switch to the opposite side.
After software completes step 3, the selection stage first disables the old main clock
during its low time, then waits one stage of the new main clock before driving its output
from the new main clock. This process prevents glitches (minimum high or low time
violations) on the output/base clock.
3.10 Fractional divider registers
Each of the 17 fractional dividers in the CGU includes the registers described below.
Table 59.
Base Control Registers (SYSBCR-DAIOBCR; 0x8000 43F0-43F8)
Bit
Symbol
Description
Reset
value
0
FDRUN
Write a 0 to this bit to disable operation of all the Fractional Dividers
connected to this selection stage, overriding their individual RUN bits.
After all fractional dividers and other CGU registers have been
programmed as desired, write a 1 back to this register to start all of the
FDs simultaneously.
1
31:1
-
Reserved, user software should not write ones to reserved bits. The
value read from a reserved bit is not defined.
-