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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
23 of 808
NXP Semiconductors
UM10360
Chapter 3: LPC17xx System control
7.
Other system controls and status flags
Some aspects of controlling LPC17xx operation that do not fit into peripheral or other
registers are grouped here.
7.1 System Controls and Status register (SCS - 0x400F C1A0)
The SCS register contains several control/status bits related to the main oscillator. Since
chip operation always begins using the Internal RC Oscillator, and the main oscillator may
not be used at all in some applications, it will only be started by software request. This is
accomplished by setting the OSCEN bit in the SCS register, as described in Table 3-13.
The main oscillator provides a status flag (the OSCSTAT bit in the SCS register) so that
software can determine when the oscillator is running and stable. At that point, software
can control switching to the main oscillator as a clock source. Prior to starting the main
oscillator, a frequency range must be selected by configuring the
OSCRANGE bit in the SCS register.
2
EXTPOLAR2 0
EINT2 is low-active or falling-edge sensitive (depending on
EXTMODE2).
0
1
EINT2 is high-active or rising-edge sensitive (depending on
EXTMODE2).
3
EXTPOLAR3 0
EINT3 is low-active or falling-edge sensitive (depending on
EXTMODE3).
0
1
EINT3 is high-active or rising-edge sensitive (depending on
EXTMODE3).
7:4 -
-
Reserved, user software should not write ones to reserved
bits. The value read from a reserved bit is not defined.
NA
Table 12.
External Interrupt Polarity register (EXTPOLAR - address 0x400F C14C) bit
description
Bit Symbol
Value Description
Reset
value
Table 13.
System Controls and Status register (SCS - address 0x400F C1A0) bit description
Bit
Symbol
Value Description
Access Reset
value
3:0
-
-
Reserved. User software should not write ones to reserved bits. The value
read from a reserved bit is not defined.
-
NA
4
OSCRANGE
Main oscillator range select.
R/W
0
0
The frequency range of the main oscillator is 1 MHz to 20 MHz.
1
The frequency range of the main oscillator is 15 MHz to 25 MHz.
5
OSCEN
Main oscillator enable.
R/W
0
0
The main oscillator is disabled.
1
The main oscillator is enabled, and will start up if the correct external
circuitry is connected to the XTAL1 and XTAL2 pins.