BS2824 Holtek Semiconductor Inc., BS2824 Datasheet - Page 29

• Manufacturer Part Number: BS2824
• Description: Touch Key Mcu
• Manufacturer: Holtek Semiconductor Inc.

Rev. 1.00
Internal RC Oscillator - HIRC
External 32768Hz Crystal Oscillator - LXT
The internal RC oscillator is a fully integrated system oscillator requiring no external components.
The internal RC oscillator has three fixed frequencies of either 4MHz, 8MHz or 12MHz. 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 degrees, the fixed oscillation frequency of 4MHz, 8MHz or 12MHz
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 PA5 and PA6 are free for use as normal I/O pins.
When the microcontroller enters the Idle/Sleep 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
Power-down Mode. To do this, another clock, independent of the system clock, must be provided.
To do this a configuration option exists to allow a high speed oscillator to be used in conjunction
with a a low speed oscillator, known as the LXT oscillator. The LXT oscillator is implemented using
a 32768Hz crystal connected to pins OSC1/OSC2 for the BS2854 or connected to pins XT1/XT2 for
the BS2868. 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. For the BS2854 devices the LXT oscillator must
be used together with the HIRC oscillator. For the BS2868 devices the LXT oscillator must be used
together with either the HXT, ERC or HIRC register.
32768 Hz Crystal Recommended Capacitor Values
32768Hz
Note: 1. C1 and C2 values are for guidance only.
Crystal Frequency
LXT Oscillator C1 and C2 Values
2. R
External LXT Oscillator - HXT
Internal RC Oscillator - HIRC
P
=5M~10M is recommended.
29
BS2824/BS2854/BS2868
8pF
C1
Touch Key MCU
10pF
C2
January 27, 2010