© Koninklijke Philips Electronics N.V. 2006. All rights reserved.
User manual
Rev. 01 — 12 January 2006
36
Philips Semiconductors
UM10161
Volume 1
Chapter 3: System control block
3.11 APB divider
The APB Divider determines the relationship between the processor clock (CCLK) and the
clock used by peripheral devices (PCLK). The APB Divider serves two purposes.
The first is to provides peripherals with desired PCLK via APB bus so that they can
operate at the speed chosen for the ARM processor. In order to achieve this, the APB bus
may be slowed down to one half or one fourth of the processor clock rate. Because the
APB bus must work properly at power up (and its timing cannot be altered if it does not
work since the APB divider control registers reside on the APB bus), the default condition
at reset is for the APB bus to run at one quarter speed.
The second purpose of the APB Divider is to allow power savings when an application
does not require any peripherals to run at the full processor rate.
The connection of the APB Divider relative to the oscillator and the processor clock is
shown in
. Because the APB Divider is connected to the PLL output, the PLL
remains active (if it was running) during Idle mode.
3.11.1 Register
description
Only one register is used to control the APB Divider.
[1]
Reset value reflects the data stored in used bits only. It does not include reserved bits content.
3.11.2 APBDIV
register (APBDIV - 0xE01F C100)
The APB Divider register contains two bits, allowing three divider values, as shown in
Table 28:
APB divider register map
Name
Description
Access Reset
value
Address
APBDIV
Controls the rate of the APB clock in relation to
the processor clock.
R/W
0x00
0xE01F C100
Table 29:
APB Divider register (APBDIV - address 0xE01F C100) bit description
Bit
Symbol Value
Description
Reset
value
1:0
APBDIV 00
APB bus clock is one fourth of the processor clock.
00
01
APB bus clock is the same as the processor clock.
10
APB bus clock is one half of the processor clock.
11
Reserved. If this value is written to the APBDIV register, it
has no effect (the previous setting is retained).
7:2
-
-
Reserved, user software should not write ones to reserved
bits. The value read from a reserved bit is not defined.
NA