ST7263BHX_09 Datasheet, PDF (35/186 Pages) STMicroelectronics – Low speed USB 8-bit MCU family with up to 32 KB Flash/ROM, DFU capability, 8-bit ADC, WDG, timer, SCI and I²C

ST7263Bxx
Reset and clock management
6.2
6.2.1
6.2.2
Clock system
General description
The MCU accepts either a crystal or ceramic resonator, or an external clock signal to drive
the internal oscillator. The internal clock (f_CPU) is derived from the external oscillator
frequency (f_OSC), which is divided by 3 (and by 2 or 4 for USB, depending on the external
clock used). The internal clock is further divided by 2 by setting the SMS bit in the
miscellaneous register.
Using the OSC24/12 bit in the option byte, a 12 MHz or a 24 MHz external clock can be
used to provide an internal frequency of either 2, 4 or 8 MHz while maintaining a 6 MHz for
the USB (refer to Figure 18).
The internal clock signal (f_CPU) is also routed to the on-chip peripherals. The CPU clock
signal consists of a square wave with a duty cycle of 50%.
The internal oscillator is designed to operate with an AT-cut parallel resonant quartz or
ceramic resonator in the frequency range specified for f_osc. The circuit shown in Figure 17 is
recommended when using a crystal, and Table 8 lists the recommended capacitance. The
crystal and associated components should be mounted as close as possible to the input
pins in order to minimize output distortion and start-up stabilization time.
Table 8. Recommended Values for 24 MHz crystal resonator
Recommended capacitance and resistance
R_SMAX(1)
20 Ω
25 Ω
C_OSCIN
56pF
47pF
C_OSCOUT
56pF
47pF
R_P
1-10 MΩ
1-10 MΩ
1. R_SMAX is the equivalent serial resistor of the crystal (see crystal specification).
70 Ω
22pF
22pF
1-10 MΩ
External clock
An external clock may be applied to the OSCIN input with the OSCOUT pin not connected,
as shown on Figure 16. The t_OXOV specifications do not apply when using an external clock
input. The equivalent specification of the external clock source should be used instead of
t_OXOV (see Table 62: Control timing characteristics).
Figure 16. External clock source connections
OSCIN
OSCOUT
NC
EXTERNAL
CLOCK
Doc ID 7516 Rev 8
35/186