Rev. 1.40
38
March 29, 2019
Rev. 1.40
39
March 29, 2019
HT45F23A/HT45F24A
TinyPower
TM
Flash MCU with OPA & Comparators
HT45F23A/HT45F24A
TinyPower
TM
Flash MCU with OPA & Comparators
External Oscillator – EC
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 supplying the clock on pin OSC1. Pin OSC2 should be left floating
if the external oscillator is used. The internal oscillator circuit contains a filter circuit to reduce
the possibility of erratic operation due to noise on the oscillator pin, however as the filter circuit
consumes a certain amount of power, a 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.
Internal RC Oscillator – HIRC
The internal RC oscillator is a fully integrated system oscillator requiring no external components.
The internal RC oscillator has four fixed frequencies of either 910kHz, 2MHz, 4MHz or 8MHz.
Device trimming during the manufacturing process and the inclusion of internal frequency
compensation circuits are used to ensure that the influence of the power supply voltage, temperature
and process variations on the oscillation frequency are minimised. As a result, at a power supply of
either 3V or 5V and at a temperature of 25˚C degrees, the fixed oscillation frequency of 910kHz,
2MHz, 4MHz or 8MHz will have a tolerance within 2%. Note that if this internal system clock
option is selected, as it requires no external pins for its operation, I/O pins PC0 and PC1 are free for
use as normal I/O pins.
External 32.768kHz Crystal Oscillator – LXT
The External 32.768kHz Crystal System Oscillator is one of the low frequency oscillator choices,
which is selected via configuration option. This clock source has a fixed frequency of 32.768kHz
and requires a 32.768kHz crystal to be connected between pins XT1 and XT2. The external resistor
and capacitor components connected to the 32.768kHz crystal are necessary to provide oscillation.
For applications where precise frequencies are essential, these components may be required to
provide frequency compensation due to different crystal manufacturing tolerances. During power-up
there is a time delay associated with the LXT oscillator waiting for it to start-up.
External LXT Oscillator
When the microcontroller enters the SLEEP or IDLE Mode, the system clock is switched off to stop
microcontroller activity and to conserve power. However, in many microcontroller applications
it may be necessary to keep the internal timers operational even when the microcontroller is in
the SLEEP or IDLE Mode. To do this, another clock, independent of the system clock, must be
provided.
However, for some crystals, to ensure oscillation and accurate frequency generation, it is necessary
to add two small value external capacitors, C1 and C2. The exact values of C1 and C2 should be
selected in consultation with the crystal or resonator manufacturer's specification. The external
parallel feedback resistor, Rp, is required.