Either the internal VCO or the CLK can be selected as the
source for the direct-to-output signal routing. To connect the
LVPECL outputs directly to the internal VCO or CLK, the user
must select the VCO divider as the source to the distribution
section, even if no channel uses it.
Table 32. Routing VCO Divider Input Directly to the Outputs
Register Setting Selection
0x1E1[1:0] = 00b CLK is the source; VCO divider selected
0x1E1[1:0] = 10b VCO is the source; VCO divider selected
0x192 = 1b
Direct-to-output OUT0, OUT1, OUT2
0x195 = 1b
Direct-to-output OUT3, OUT4, OUT5
0x198 = 1b
Direct-to-output OUT6, OUT7, OUT8
0x19B = 1b
Direct-to-output OUT9, OUT10, OUT11
Clock Frequency Division
The total frequency division is a combination of the VCO
divider (when used) and the channel divider. When the VCO
divider is used, the total division from the VCO or CLK to the
output is the product of the VCO divider (1, 2, 3, 4, 5, and 6)
and the division of the channel divider. Table 33 indicates how the
frequency division for a channel is set.
However, when the VCO divider is set to 1, none of the channel
output dividers can be bypassed.
A channel divider drives each group of three LVPECL outputs.
There are four channel dividers (0, 1, 2, and 3) driving twelve
LVPECL outputs (OUT0 to OUT11). Table 34 gives the register
locations used for setting the division and other functions of
these dividers. The division is set by the values of M and N. The
divider can be bypassed (equivalent to divide-by-1, divider circuit
is powered down) by setting the bypass bit. The duty-cycle
correction can be enabled or disabled according to the setting of
the disable div DCC bits.
Table 34. Setting DX for the Output Dividers
Divider Cycles M
0x190[7:4] 0x190[3:0] 0x191
0x193[7:4] 0x193[3:0] 0x194
0x196[7:4] 0x196[3:0] 0x197
0x199[7:4] 0x199[3:0] 0x19A
Channel Frequency Division (0, 1, 2, and 3)
Table 33. Frequency Division
CLK or VCO
CLK or VCO 1 to 6
CLK or VCO 1 to 6
2 to 32
CLK or VCO 2 to 6
CLK or VCO 1
CLK (internal VCO Divider Bypass
CLK (internal VCO Divider 2 to 32
(1 to 6) ×
(2 to 32)
(2 to 6) × (1)
2 to 32
1 The bypass VCO divider (0x1E1 = 1) is not the same as VCO divider = 1.
The channel dividers feeding the output drivers contain one 2-
to-32 frequency divider. This divider provides for division-by-1
to division-by-32. Division-by-1 is accomplished by bypassing
the divider. The dividers also provide for a programmable duty
cycle, with optional duty-cycle correction when the divide ratio
is odd. A phase offset or delay in increments of the input clock
cycle is selectable. The channel dividers operate with a signal at
their inputs up to 1600 MHz. The features and settings of the
dividers are selected by programming the appropriate setup
and control registers (see Table 49 through Table 60).
The VCO divider provides frequency division between the
internal VCO or the external CLK input and the clock
distribution channel dividers. The VCO divider can be set
to divide by 1, 2, 3, 4, 5, or 6 (see Table 56, 0x1E0[2:0]).
For each channel (where the channel number is x: 0, 1, 2, or 3),
the frequency division, DX, is set by the values of M and N
(four bits each, representing Decimal 0 to Decimal 15), where
Number of Low Cycles = M + 1
Number of High Cycles = N + 1
The cycles are cycles of the clock signal currently routed to the
input of the channel dividers (VCO divider out or CLK).
When a divider is bypassed, DX = 1.
Otherwise, DX = (N + 1) + (M + 1) = N + M + 2. This allows
each channel divider to divide by any integer from 1 to 32.
Duty Cycle and Duty-Cycle Correction
The duty cycle of the clock signal at the output of a channel is a
result of some or all of the following conditions:
• The M and N values for the channel
• DCC enabled/disabled
• VCO divider enabled/bypassed
• The CLK input duty cycle (note that the internal VCO has a
50% duty cycle)
The DCC function is enabled by default for each channel divider.
However, the DCC function can be disabled individually for
each channel divider by setting the disable divider DCC bit for
Certain M and N values for a channel divider result in a non-
50% duty cycle. A non-50% duty cycle can also result with an
even division, if M ≠ N. The duty-cycle correction function
automatically corrects non-50% duty cycles at the channel
divider output to 50% duty cycle.
Rev. 0 | Page 45 of 84