UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
62 of 362
NXP Semiconductors
UM10208
Chapter 7: LPC2800 CGU
3.7 High Speed PLL Programming and Operation
3.7.1 Power-down procedure
Setting up the high speed PLL involves the following steps:
1. If the PLL is in operation:
a. write 0 to the SCRs of any selection stages that use the PLL, to disable use of the
PLL’s output.
b. write 0x004 to HPMODE to power it down
2. If necessary, write a new value to HPFIN. (The reset value selects the 12 MHz
oscillator, which is the most commonly used input clock.)
3. Determine the values corresponding to the desired multiplication and division factors
by one of the methods described in
, and write them to the HPNDEC,
HPMDEC, HPPDEC, HPSELR, HPSELI, and HPSELP registers,
4. Write 0x001, 0x009, 0x011, or 0x019 to HPMODE, to start the PLL.
5. Read HPSTAT periodically until the LOCK bit is 1, indicating that the high speed PLL
has achieved synchronization lock. Subject this waiting to a time-out as described in
6. Program one or more selection stages to use the high speed PLL as their clock input.
3.7.2 Handshake procedure
The steps above are simple enough to serve for reprogramming, but there is an
alternative that allows software to make rate changes more quickly than waiting for a
complete power-up:
1. Write 0 to the SCRs of any selection stages that use the PLL, to disable use of the
PLL’s output.
2. For each of HPNSEL, (HPMSEL, HPSELR, HPSELI, HPSELP), and HPPSEL that
need to be changed:
a. determine the new value(s) as described in
b. write the value(s) to the appropriate register(s),
Table 52.
I Bandwidth Register (HPSELI - 0x8000 4CDC)
Bit
Symbol
Description
Reset
value
3:0
SELI
The value to be written to this field depends on the multiplication factor,
and can be determined as described in
.
0
31:4
-
Reserved, user software should not write ones to reserved bits. The
value read from a reserved bit is not defined.
-
Table 53.
P Bandwidth Register (HPSELP - 0x8000 4CE0)
Bit
Symbol
Description
Reset
value
4:0
SELP
The value to be written to this field depends on the multiplication factor,
and can be determined as described in
.
0
31:5
-
Reserved, user software should not write ones to reserved bits. The
value read from a reserved bit is not defined.
-