S i 5 3 4 1 / 4 0
22
Preliminary Rev. 0.9
4.1.1. Initialization and Reset
Once power is applied, the device begins an
initialization period where it downloads default register
values and configuration data from NVM and performs
other initialization tasks. Communicating with the device
through the serial interface is possible once this
initialization period is complete. No clocks will be
generated until the initialization is done. There are two
types of resets available. A hard reset is functionally
similar to a device power-up. All registers will be
restored to the values stored in NVM, and all circuits will
be restored to their initial state including the serial
interface. A hard reset is initiated using the RST pin or
by asserting the hard reset bit. A soft reset bypasses the
NVM download. It is simply used to initiate register
configuration changes.
4.1.2. Freerun Mode
The Si5341/40 will enter the free-run mode if the a
crystal input (XA/XB) is selected as its input source.
Output frequencies will be generated with a frequency
accuracy determined by the external crystal connected
to the XA/XB pins. Any change or drift of the crystal
frequency will be tracked by the output clocks. If the XA/
XB input is not selected as the device input, or if a XTAL
is not connected to the XA/XB pins, the Si5341/40 will
not enter the free-run mode and no output clocks will be
generated.
4.1.3. Synchronous Mode
If one of the input pins (IN0-IN2) is selected, the Si5341/
40 will operate in synchronize mode if there is a valid
clock at the selected input. Once lock is achieved, the
output clocks will be phase locked to the input clock. If
the selected clock fails the output clocks will stop until
an alternate input clock is manually selected.
4.2. Frequency Configuration
The phase-locked loop is fully contained and does not
require external loop filter components to operate. Its
function is to phase lock to the selected input and
provide a common synchronous reference to the
MultiSynth high-performance fractional dividers.
A crosspoint mux connects any of the MultiSynth
divided frequencies to any of the outputs drivers.
Additional output integer dividers provides further
frequency division if required. The frequency
configuration of the device is programmed by setting the
input dividers (P), the PLL feedback fractional divider
(Mn/Md), the MultiSynth fractional dividers (Nn/Nd), and
the output integer dividers (R). Silicon Labs’
ClockbuilderPro
TM
configuration utility determines the
optimum divider values for any desired input and output
frequency plan.
4.3. Inputs
The Si5341/40 requires either an external crystal at its
XA/XB pins for free-run operation or an external input
clock (IN0-IN2) for synchronous operation. An external
crystal is not required in synchronous mode.
4.3.1. External Reference Input (XA/XB)
An external crystal (XTAL) is used in combination with
the internal oscillator (OSC) to produce a low jitter
reference for the PLL when operating in the free-run
mode. A crystal frequency of 25 MHz can be used
although crystals in the frequency range of 48 MHz to
54 MHz are recommended for best jitter performance.
Frequency offsets due to C
L
mismatch can be adjusted
using the frequency adjustment feature which allows
frequency adjustments of ±1000 ppm. The Si5341/40
Family Reference Manual provides additional
information on PCB layout recommendations for the
crystal to ensure optimum jitter performance. Refer to
Table 11 for crystal specifications.
The Si5341/40 can also accommodate an external
reference clock (REFCLK) instead of a crystal. This
allows the use of crystal oscillator (XO) instead of a
XTAL. Selection between the external XTAL or REFCLK
is controlled by register configuration. The internal
crystal load capacitors (C
L
) are disabled in the REFCLK
mode. Refer to Table 3 for REFCLK requirements. Both
a single-ended or a differential REFCLK can be
connected to the XA/XB pins as shown in Figure 7. A
P
REF
divider is available to accommodate external clock
frequencies higher than 54 MHz. Frequencies in the
range of 48 MHz to 54 MHz will achieve the best output
jitter performance.