HT56R66 Holtek Semiconductor Inc., HT56R66 Datasheet - Page 79

• Manufacturer Part Number: HT56R66
• Description: Tinypower Tm A/d Type With Lcd 8-bit Otp Mcu
• Manufacturer: Holtek Semiconductor Inc.

Internal 32K_INT Oscillator
When microcontrollers enter a power down condition,
their internal clocks are normally switched off to stop
microcontroller activity and to conserve power. How-
ever, in many microcontroller applications it may be nec-
essary to keep some internal functions operational,
such as timers, even when the microcontroller is in the
Power-down mode. To do this, the device has a
32K_INT oscillator, which is a fully integrated free run-
ning RC oscillator with a typical period of 31.2 s at 5V,
requiring no external components. It is selected via con-
figuration option. When the device enters the Power
Down Mode, the system clock will stop running, how-
ever the 32K_INT oscillator will continue to run if se-
lected to keep various internal functions operational.
External 32768Hz Oscillator
With a function similar to the internal 32K-INT 32KHz os-
cillator, that is to keep some device functions opera-
tional during power down, this device also has an
external 32768Hz oscillator. This oscillator also remains
active at all times, even when the microcontroller is in
the Power-down mode. This clock source has a fixed
frequency of 32768Hz and requires a 32768Hz crystal
to be connected between pins OSC3 and OSC4.
The external resistor and capacitor components con-
nected to the 32768Hz crystal are not necessary to pro-
vide oscillation. For applications where precise
frequencies are essential, these components may be
required to provide frequency compensation due to dif-
ferent crystal manufacturing tolerances.
A configuration option selects whether the external
32768Hz oscillator or the internal 32K_INT oscillator is
selected. Selecting low frequency oscillators for use as
a system oscillator is implmented using bits in the
CLKMOD register.
Rev. 1.10
External 32768Hz Oscillator
Internal 32K_INT Oscillator
79
During power-up there is a time delay associated with
the 32768Hz oscillator waiting for it to start-up. To mini-
mise this time delay, bit 4 of the RTCC register, known
as the QOSC bit, is provided to have a quick start-up
function. During a power-up condition, this bit will be
cleared to zero which will initiate the 32768Hz oscillator
quick start-up function. However, as there is additional
power consumption associated with this quick start-up
function, to reduce power consumption after start-up
takes place, it is recommended that the application pro-
gram should set the QOSC bit high for about 2 seconds
after power-on. It should be noted that, no matter what
condition the QOSC bit is set to, the 32768Hz oscillator
will always function normally, only there is more power
consumption associated with the quick start-up function.
External Oscillator
The system clock can also be supplied by an externally
supplied clock giving users a method of synchronising
their external hardware to the microcontroller operation.
This is selected using a configuration option and supply-
ing the clock on pin OSC1. Pin OSC2 should be left
floating if the external oscillator is used. The internal os-
cillator circuit contains a filter circuit to reduce the possi-
bility of erratic operation due to noise on the oscillator
pin, however as the filter circuit consumes a certain
amount of power, a oscillator configuration option exists
to turn this filter off. Not using the internal filter should be
considered in power sensitive applications and where
the externally supplied clock is of a high integrity and
supplied by a low impedance source.
32768 Hz Crystal Recommended Capacitor Values
Crystal Frequency
32768Hz
Note: 1. C3 and C4 values are for guidance only.
32768Hz Oscillator C1 and C2 Values
2. R
P2
=5M~10M is recommended.
HT56R66/HT56R666
8pF
C3
September 8, 2009
10pF
C4