Silicon Laboratories SI5375 - Manual (Page 81)

Si53xx-RM
Rev. 1.2 81
The NC5_LS divider uses CKOUT2 as its clock input to derive FS_OUT. The limits for the NC5_LS divider are
NC5_LS = [1, 2, 4, 6, …, 2
19
]
f
CKOUT2
< 710 MHz
Note that when in frame synchronization realignment mode, writes to NC5_LS are controlled by FPW_VALID. See
section “6.8.4. FS_OUT Polarity and Pulse Width Control (Si5368)”.
Common NC5_LS divider settings on FS_OUT are shown in Table 36.
6.8.1. FSYNC Realignment (Si5368)
The FSYNC_ALIGN_PIN bit determines if the realignment will be pin-controlled via the FS_ALIGN pin or register-
controlled via the FSYNC_ALIGN_REG register bit. The active CKIN3 or CKIN4 edge to be used is controlled via
the FSYNC_POL register bit.
In either FSYNC alignment control mode, the resolution of the phase realignment is 1 clock cycle of CKOUT2. If
the realignment control is not active, the NC5 divider will continuously divide down its f
CKOUT2
input. This
guarantees a fixed number of high-frequency clock (CKOUT2) cycles between each FS_OUT cycle.
At power-up, the device automatically performs a realignment of FS_OUT using the currently active sync input.
After this, as long as the PLL remains in lock and a realignment is not requested, FS_OUT will include a fixed
number of high-speed clock cycles, even if input clock switches are performed. If many clock switches are
performed in phase build-out mode, it is possible that the input sync to output sync phase relationship will shift due
to the accumulated residual phase transients of the phase build-out circuitry. The ALIGN_ERR[8:0] status register
reports the deviation of the input-to-output sync phase skew from the desired FSYNC_SKEW[16:0] value in units of
f
CKOUT2
periods. A programmable threshold to trigger the ALIGN_INT alarm can be set via the ALIGN_THR[2:0]
bits, whose settings are given in Table 37. If the sync alignment error exceeds the threshold in either the positive or
negative direction, the alarm becomes active. If it is then desired to reestablish the desired input-to-output sync
phase relationship, a realignment can be performed. A realignment request may cause FS_OUT to
instantaneously shift its output edge location in order to align with the active input sync phase.
Table 36. Common NC5 Divider Settings
CKOUT2 Frequency (MHz) NC5 Divider Setting
2 kHz FS_OUT 8 kHz FS_OUT
19.44 9720 2430
77.76 38880 9720
155.52 77760 19440
622.08 311040 77760
Table 37. Alignment Alarm Trigger Threshold
ALIGN_THR [2:0] Alarm Trigger Threshold (Units of T
CKOUT2
)
000 4
001 8
010 16
011 32
100 48
101 64
110 96
111 128