16
derived from the 14MHz clock. The CLK32_PRSN bit is set to ‘1’. In this case, the range on the
LED blink rates is as indicated above.
If bits[1:0] are set to ‘00’ then the 32kHz clock is always used for the functions described above. In
this case, the ring oscillator is turned off. The CLK32_PRSN bit is set to ‘0’.
Note: It is recommended that bits[1:0] be reprogrammed to ‘00’ upon power-up if the 32kHz input is to
be used so that the ring oscillator is turned off.
Bit 0 of the CLOCKI32 Configuration Register (CLK32_PRSN) determines whether an external 32kHz
clock is connected to the part. If the external 32kHz clock is not connected to the part (bit 0 of the
CLOCKI32 Configuration Register is ‘1’) then the bits defined above must be ‘01’ for the internal ring
oscillator to be used for these functions described above. That is, if the external 32kHz clock is not
connected to the part then the internal ring oscillator replaces the 32kHz clock to all functions when
VCC=0 if bits[1:0] are set to ‘01’. Note that if the 32kHz clock is not used then the 32kHz clock signal
is derived from the 14MHz clock when VCC is active.
See the Intrusion/Oscillator Select Register for a description of the bits to be used to determine
whether the 32kHz trickle clock is used or the Ring Oscillator is used. This register is located at the
offset of 0x5E from the base I/O address in logical device A. See the Runtime Registers section for
description.
The 32kHz clock signal from the external 32kHz clock input is available/unavailable for these features
in one of two cases:
Case 1: External 32kHz clock input is active when VCC is active (S0, S1) and when VCC=0 (S3-
S5 states) i.e., the 32kHz clock is connected and always active. In this case, the internal ring
oscillator would not be used and bits[1:0] of the Oscillator Select Register should be programmed
to ‘00’. The CLK32_PRSN bit is ‘0’.
Case 2: External 32kHz clock is not connected. The internal ring oscillator is used when VCC=0
with bits[1:0] of the Oscillator Select Register = ‘01’. Note that in this case, the internal ring
oscillator is used for the LED blink and wake on specific key in the S3 state as well as in the
S4/S5 state. When VCC is active (S0, S1) the 32kHz clock is derived from the 14MHz clock. The
CLK32_PRSN bit is ‘1’.
When the LPC47B34x switches from the ring oscillator to the 32kHz clock, edge detection on the
internal 32kHz clock source is used to insure that it is active before the switching takes place.
Switching between the ring oscillator and the 32kHz clock derived from the 14MHz clock input (case 2)
is performed by the part as follows:
When VCC goes inactive, use the ring oscillator.
When VCC goes active and an edge is detected on the 32kHz clock source to the Clock Select
Logic, switch to the 32kHz clock.
The “Wake on Specific Key” option and LED blink will run off of the ring oscillator when the external
32kHz clock and 14MHz clock are not available.
The Ring Filter and WDT options do not function when both external 32kHz input clock signal and
14MHz input clock signal are not available. These features do not run off the ring oscillator.