© Koninklijke Philips Electronics N.V. 2005. All rights reserved.
User manual
Rev. 01 — 15 August 2005
40
Philips Semiconductors
UM10139
Volume 1
Chapter 3: System Control Block
3.11 VPB divider
The VPB Divider determines the relationship between the processor clock (CCLK) and the
clock used by peripheral devices (PCLK). The VPB Divider serves two purposes.
The first is to provides peripherals with desired PCLK via VPB bus so that they can
operate at the speed chosen for the ARM processor. In order to achieve this, the VPB bus
may be slowed down to one half or one fourth of the processor clock rate. Because the
VPB bus must work properly at power up (and its timing cannot be altered if it does not
work since the VPB divider control registers reside on the VPB bus), the default condition
at reset is for the VPB bus to run at one quarter speed.
The second purpose of the VPB 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 VPB Divider relative to the oscillator and the processor clock is
shown in
. Because the VPB 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 VPB Divider.
[1]
Reset value reflects the data stored in used bits only. It does not include reserved bits content.
3.11.2 VPBDIV
register (VPBDIV - 0xE01F C100)
The VPB Divider register contains two bits, allowing three divider values, as shown in
.
2
WDTR
This bit is set when the Watchdog Timer times out and the WDTRESET
bit in the Watchdog Mode Register is 1. It is cleared by any of the other
sources of Reset.
see text
3
BODR
This bit is set when the 3.3 V power reaches a level below 2.6 V. If the
V
DD
voltage dips from 3.3 V to 2.5 V and backs up, the BODR bit will be
set to 1. Also, if the V
DD
voltage rises continuously from below 1 V to a
level above 2.6 V, the BODR will be set to 1, too. This bit is not affected
by External Reset nor Watchdog Reset.
Note: only in case a reset occurs and the bit POR = 0, the BODR bit
indicates if the V
DD
voltage was below 2.6 V or not.
see text
7:4
-
Reserved, user software should not write ones to reserved bits. The
value read from a reserved bit is not defined.
NA
Table 27:
Reset Source identification Register (RSIR - address 0xE01F C180) bit description
Bit
Symbol Description
Reset
value
Table 28:
VPB divider register map
Name
Description
Access Reset
value
Address
VPBDIV
Controls the rate of the VPB clock in relation to
the processor clock.
R/W
0x00
0xE01F C100