Bit[0] controls the source of the 32kHz (nominal) clock for the RTC, 40 MHz clock output, fan tachometer logic, the
LED blink logic, the WDT, and the “wake on specific key” logic. When an external 32kHz clock source is connected,
that will be the source for the RTC, 40 MHz clock output, fan tachometer, LED, WDT and “wake on specific key” logic.
When an external 32kHz clock source is not connected, an internal 32kHz clock source will be derived from the
14MHz clock for the RTC, 40MHz clock output, fan tachometer, LED, WDT and “wake on specific key” logic.
The following functions will not work under VTR power (VCC removed) if an external 32kHz clock source is not
connected. These functions will work under VCC power even if an external 32kHz clock source is not connected.
Fan Tachometer
Wake on specific key
LED blink
WDT
40MHz clock output
RTC
5.6 XOSEL
The XOSEL signal is used to differentiate between a single-input 32kHz signal driven on the CLKI32 pin or an
oscillator connected between XTAL1 and XTAL2. If a single-input 32kHz signal is connected to the CLKI32 pin, the
XOSEL pin should be pulled up to VTR. (A 10kΩ pull-up resistor is suggested.) If a crystal oscillator is connected
across XTAL1 and XTAL2 then the XOSEL pin should be left unconnected. There is an internal pulldown on this pin
that will keep this pin low when not in use. If no external clock source is supplied, then Bit[0] of the Clock Select
register found in Logical Device A at offset 0xF0 should be set to ‘1’. This bit, referred to as CLK32_PRSN,
determines if the 32kHz clock source is supplied externally or if it is derived from the 14MHz clock input. The
following table summarizes the state of the 32 kHz clock inputs and the XOSEL pin.
BIT[0] (See Note)
(CLK32_PRSN)
32KHZ CLOCK INPUT
XOSEL
No external 32kHz clock input or XTAL
No Connect
(see Note1)
1
The 32kHz clock source derived internally
from 14MHz input.
No Connect
(see Note1)
32kHz oscillator connected between
XTAL1 and XTAL2 (default) (see Note1)
0
0
Single-input, external 32kHz clock input
driven on the CLKI32 pin
Pull-up to VTR
No external 32kHz clock input or XTAL
Pull-up to VTR
1
The 32kHz clock source derived internally
from 14MHz input.
Note: Bit[0] is located in the Clock Select register found in Logical Device A at offset 0xF0.
Note 1: When the XOSEL pin is left unconnected (i.e., No Connect), the XTAL2/CLKI32 pin is an output and must
not be grounded.
5.7 Trickle Power Functionality
When the LPC47S45x is running under VTR only, the PME wakeup events are active and (if enabled) able to assert
the IO_PME# pin active low. The following lists the wakeup events:
UART 1 Ring Indicator
UART 2 Ring Indicator
Keyboard data
Mouse data
Wake on Specific Key Logic
Fan Tachometer (Note)
nPB_IN
RTC Alarm
SMSC DS – LPC47S45x
Page 21 of 259
Rev. 07/09/2001
DATASHEET
SMSC [ SMSC CORPORATION ]
