Datasheet

 2017 Microchip Technology Inc. DS60001516A-page 265
SAM9G20
24.4.1 Main Oscillator Connections
The Clock Generator integrates a Main Oscillator that is designed for a 3 to 20 MHz fundamental crystal. The typical crystal connection is
illustrated in Figure 24-3. For further details on the electrical characteristics of the Main Oscillator, see Section 40.2 “DC Characteristics”.
Figure 24-3: Typical Crystal Connection
24.4.2 Main Oscillator Startup Time
The startup time of the Main Oscillator is given in Section 40.2 “DC Characteristics”. The startup time depends on the crystal frequency
and decreases when the frequency rises.
24.4.3 Main Oscillator Control
To minimize the power required to start up the system, the main oscillator is disabled after reset and slow clock is selected.
The software enables or disables the main oscillator so as to reduce power consumption by clearing the MOSCEN bit in the Main Oscillator
Register (CKGR_MOR).
When disabling the main oscillator by clearing the MOSCEN bit in CKGR_MOR, the MOSCS bit in PMC_SR is automatically cleared, indi-
cating the main clock is off.
When enabling the main oscillator, the user must initiate the main oscillator counter with a value corresponding to the startup time of the
oscillator. This startup time depends on the crystal frequency connected to the main oscillator.
When the MOSCEN bit and the OSCOUNT are written in CKGR_MOR to enable the main oscillator, the MOSCS bit in PMC_SR (Status
Register) is cleared and the counter starts counting down on the slow clock divided by 8 from the OSCOUNT value. Since the OSCOUNT
value is coded with 8 bits, the maximum startup time is about 62 ms.
When the counter reaches 0, the MOSCS bit is set, indicating that the main clock is valid. Setting the MOSCS bit in PMC_IMR can trigger
an interrupt to the processor.
24.4.4 Main Clock Frequency Counter
The Main Oscillator features a Main Clock frequency counter that provides the quartz frequency connected to the Main Oscillator. Gener-
ally, this value is known by the system designer; however, it is useful for the boot program to configure the device with the correct clock
speed, independently of the application.
The Main Clock frequency counter starts incrementing at the Main Clock speed after the next rising edge of the Slow Clock as soon as
the Main Oscillator is stable, i.e., as soon as the MOSCS bit is set. Then, at the 16th falling edge of Slow Clock, the MAINRDY bit in
CKGR_MCFR (Main Clock Frequency Register) is set and the counter stops counting. Its value can be read in the MAINF field of
CKGR_MCFR and gives the number of Main Clock cycles during 16 periods of Slow Clock, so that the frequency of the crystal connected
on the Main Oscillator can be determined.
24.4.5 Main Oscillator Bypass
The user can input a clock on the device instead of connecting a crystal. In this case, the user has to provide the external clock signal on
the XIN pin. The input characteristics of the XIN pin under these conditions are given in the product electrical characteristics section. The
programmer has to be sure to set the OSCBYPASS bit to 1 and the MOSCEN bit to 0 in the Main OSC register (CKGR_MOR) for the
external clock to operate properly.
1K
XIN XOUT GND
AT91 Microcontroller