DS3105
20
Table 7-1. Input Clock Capabilities
INPUT CLOCK
SIGNAL FORMATS
FREQUENCIES (MHz)
DEFAULT FREQUENCY
IC3
CMOS/TTL
8kHz
IC4
CMOS/TTL
8kHz
IC5
LVDS/LVPECL or CMOS/TTL
Up to 156.25
(2)
19.44MHz
IC6
LVDS/LVPECL or CMOS/TTL
Up to 156.25
(2)
19.44MHz
IC9
CMOS/TTL
19.44MHz
Note 1: Available frequencies for CMOS/TTL input clocks are: 2kHz, 4kHz, 8kHz, 1.544MHz (SONET mode), 2.048MHz (SDH mode),
6.312MHz, 6.48MHz, 19.44MHz, 25.0MHz, 25.92MHz, 38.88MHz, 51.84MHz, 62.5MHz, 77.76MHz, and any multiple of 2kHz up to 125MHz.
Note 2: Available frequencies for LVDS/LVPECL input clocks include all CMOS/TTL frequencies in Note 1 plus any multiple of 8kHz up to
155.52MHz and 156.25MHz.
7.4.2 Frequency Configuration
Input clock frequencies are configured in the FREQ field of the
ICR registers. The DIVN and LOCK8K bits of these
same registers specify the locking frequency mode, as shown in
Table 7-2.Table 7-2. Locking Frequency Modes
DIVN
LOCK8K
LOCKING FREQUENCY
MODE
0
Direct Lock
0
1
LOCK8K
1
0
DIVN
1
Alternate Direct Lock
7.4.2.1 Direct Lock Mode
In direct lock mode, the DPLLs lock to the selected reference at the frequency specified in the corresponding
ICRregister. Direct lock mode can only be used for input clocks with these specific frequencies: 2kHz, 4kHz, 8kHz,
1.544MHz, 2.048MHz, 5MHz, 6.312MHz, 6.48MHz, 19.44MHz, 25.92MHz, 31.25MHz, 38.88MHz, 51.84MHz,
77.76MHz, and 155.52MHz. For the 155.52MHz case, the input clock is internally divided by two, and the DPLL
direct-locks at 77.76MHz. The DIVN mode can be used to divide an input down to any of these frequencies except
155.52MHz.
MTIE figures may be marginally better in direct lock mode because the higher frequencies allow more frequent
phase updates.
7.4.2.2 Alternate Direct Lock Mode
Alternate direct lock mode is the same as direct lock mode except an alternate list of direct lock frequencies is used
(see the FREQ field definition in the
ICR register description). The alternate frequencies are included to support
clock rates found in Ethernet, CMTS, wireless, and GPS applications. The alternate frequencies are: 10MHz,
25MHz, 62.5MHz, 125MHz, and 156.25MHz. The frequencies 62.5MHz, 125MHz, and 156.25MHz are internally
divided down to 31.25MHz, while 10MHz and 25MHz are internally divided down to 5MHz.
7.4.2.3 LOCK8K Mode
In LOCK8K mode, an internal divider is configured to divide the selected reference down to 8kHz. The DPLL locks
to the 8kHz output of the divider. LOCK8K mode can only be used for input clocks with the standard direct lock
frequencies: 8kHz, 1.544MHz, 2.048MHz, 5MHz, 6.312MHz, 6.48MHz, 19.44MHz, 25.0MHz, 25.92MHz,
31.25MHz, 38.88MHz, 51.84MHz, 62.5MHz, 77.76MHz, and 155.52MHz. LOCK8K mode is enabled for a particular
input clock by setting the LOCK8K bit in the corresponding
ICR register.