A single register, SMOD, is used for overall control of the internal clocks within the device.
· SMOD Register
CKS2~CKS0: The system clock selection when HLCLK is ²0²
000: fL (fLXT or fLIRC)
001: fL (fLXT or fLIRC)
These three bits are used to select which clock is used as the system clock source. In addition
to the system clock source, which can be either the LXT or LIRC, a divided version of the high
speed system oscillator can also be chosen as the system clock source.
FSTEN: Fast Wake-up Control (only for HXT)
This is the Fast Wake-up Control bit which determines if the fSUB clock source is initially used
after the device wakes up. When the bit is high, the fSUB clock source can be used as a
temporary system clock to provide a faster wake up time as the fSUB clock is available.
LTO: Low speed system oscillator ready flag
0: Not ready
This is the low speed system oscillator ready flag which indicates when the low speed system
oscillator is stable after power on reset or a wake-up has occurred. The flag will be low when in
the SLEEP0 Mode but after a wake-up has occurred, the flag will change to a high level after
1024 clock cycles if the LXT oscillator is used and 1~2 clock cycles if the LIRC oscillator is used.
HTO: High speed system oscillator ready flag
0: Not ready
This is the high speed system oscillator ready flag which indicates when the high speed system
oscillator is stable. This flag is cleared to ²0² by hardware when the device is powered on and
then changes to a high level after the high speed system oscillator is stable. Therefore this flag
will always be read as ²1² by the application program after device power-on. The flag will be
low when in the SLEEP or IDLE0 Mode but after a wake-up has occurred, the flag will change to
a high level after 1024 clock cycles if the HXT oscillator is used and after 15~16 clock cycles if
the ERC or HIRC oscillator is used.
IDLEN: IDLE Mode control
This is the IDLE Mode Control bit and determines what happens when the HALT instruction is
executed. If this bit is high, when a HALT instruction is executed the device will enter the
IDLE Mode. In the IDLE1 Mode the CPU will stop running but the system clock will continue to
keep the peripheral functions operational, if FSYSON bit is high. If FSYSON bit is low, the CPU
and the system clock will all stop in IDLE0 mode. If the bit is low the device will enter the
SLEEP Mode when a HALT instruction is executed.
HLCLK: system clock selection
0: fH/2 ~ fH/64 or fL
This bit is used to select if the fH clock or the fH/2 ~ fH/64 or fL clock is used as the system
clock. When the bit is high the fH clock will be selected and if low the fH/2 ~ fH/64 or fL clock will
be selected. When system clock switches from the fH clock to the fL clock and the fH clock will
be automatically switched off to conserve power.
April 16, 2010