UM10429
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© NXP B.V. 2010. All rights reserved.
User manual
Rev. 1 — 20 October 2010
14 of 258
NXP Semiconductors
UM10429
Chapter 3: LPC1102 System configuration
3.5.6 Watchdog oscillator control register
This register configures the watchdog oscillator. The oscillator consists of an analog and a
digital part. The analog part contains the oscillator function and generates an analog clock
(Fclkana). With the digital part, the analog output clock (Fclkana) can be divided to the
required output clock frequency wdt_osc_clk. The analog output frequency (Fclkana) can
be adjusted with the FREQSEL bits between 500 kHz and 3.4 MHz. With the digital part
Fclkana will be divided (divider ratios = 2, 4,...,64) to wdt_osc_clk using the DIVSEL bits.
The output clock frequency of the watchdog oscillator can be calculated as
wdt_osc_clk =
Fclkana
⁄
(2
×
(1 + DIVSEL))
= 7.8 kHz to 1.7 MHz (nominal values).
Remark:
Any setting of the FREQSEL bits will yield a Fclkana value within
±
40% of the
listed frequency value. The watchdog oscillator is the clock source with the lowest power
consumption. If accurate timing is required, use the IRC or system oscillator.
Remark:
The frequency of the watchdog oscillator is undefined after reset. The watchdog
oscillator frequency must be programmed by writing to the WDTOSCCTRL register before
using the watchdog oscillator.
1
FREQRANGE
Determines frequency range for Low-power
oscillator.
0x0
0
1 - 20 MHz frequency range.
1
15 - 25 MHz frequency range
31:2
-
-
Reserved
0x00
Table 10.
System oscillator control register (SYSOSCCTRL, address 0x4004 8020) bit
description
Bit
Symbol
Value
Description
Reset
value
Table 11.
Watchdog oscillator control register (WDTOSCCTRL, address 0x4004 8024) bit
description
Bit
Symbol
Value
Description
Reset
value
4:0
DIVSEL
Select divider for Fclkana.
wdt_osc_clk = Fclkana/ (2
×
(1 + DIVSEL))
00000: 2
×
(1 + DIVSEL) = 2
00001: 2
×
(1 + DIVSEL) = 4
to
11111: 2
×
(1 + DIVSEL) = 64
0x00