SAM7X128_14 Datasheet, PDF (169/662 Pages) ATMEL Corporation – ARM-based Flash MCU

Figure 24-2. Typical Crystal Connection
AT91SAM7X Microcontroller
XIN
XOUT
GND
1K
24.3.2 Main Oscillator Startup Time
The startup time of the Main Oscillator is given in the DC Characteristics section of the product datasheet. The startup
time depends on the crystal frequency and decreases when the frequency rises.
24.3.3 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, indicating 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.
24.3.4 Main Clock Frequency Counter
The Main Oscillator features a Main Clock frequency counter that provides the quartz frequency connected to the Main
Oscillator. Generally, 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.
24.3.5 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.
SAM7X Series [DATASHEET]
6120K–ATARM–11-Feb-14
169