AT91SAM_13 Datasheet, PDF (189/775 Pages) ATMEL Corporation – ARM-based Flash MCU

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.
24.4
Divider and PLL Block
The PLL embeds an input divider to increase the accuracy of the resulting clock signals. However, the user must
respect the PLL minimum input frequency when programming the divider.
Figure 24-3 shows the block diagram of the divider and PLL block.
Figure 24-3. Divider and PLL Block Diagram
DIV
MUL OUT
MAINCK
Divider
PLL
PLLCK
SLCK
PLLRC
PLLCOUNT
PLL
Counter
LOCK
24.4.1 PLL Filter
The PLL requires connection to an external second-order filter through the PLLRC pin. Figure 24-4 shows a sche-
matic of these filters.
Figure 24-4. PLL Capacitors and Resistors
PLLRC
PLL
R
C2
C1
GND
Values of R, C1 and C2 to be connected to the PLLRC pin must be calculated as a function of the PLL input fre-
quency, the PLL output frequency and the phase margin. A trade-off has to be found between output signal
overshoot and startup time.
24.4.2 Divider and Phase Lock Loop Programming
The divider can be set between 1 and 255 in steps of 1. When a divider field (DIV) is set to 0, the output of the cor-
responding divider and the PLL output is a continuous signal at level 0. On reset, each DIV field is set to 0, thus the
corresponding PLL input clock is set to 0.
The PLL allows multiplication of the divider’s outputs. The PLL clock signal has a frequency that depends on the
respective source signal frequency and on the parameters DIV and MUL. The factor applied to the source signal
SAM7S Series [DATASHEET]
6175M–ATARM–26-Oct-12
189