IDT VersaClock 6E Series, Руководство по описанию регистров и программированию

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27 ©2018 Integrated Device Technology, Inc. August 30, 2018
VersaClock
®
6E Family Register Descriptions and Programming Guide
To calculate the spread spectrum registers (fractional portion), first determine the value in decimal of the FOD output divider, N, for the
nominal output frequency without spread spectrum. The VCO frequency is divided by two to account for a fixed divide by 2 between the
VCO output and the input to the FOD. Convert the integer portion into hex to define ODx_intdiv.
If no spread is to be applied to FODx (ODx_ssce = 0) then ODx_period and ODx_step registers are don't Care and it is permissible to skip
to
Eq.9 . Convert FRAC(N) to 30 bits as per Eq.10 .
When the ODx_period and ODx_step registers are calculated below, ODx_period and ODx_step are explicitly set to 0 if ODx_ssce will
always be 0. This is done for reasons of style, it reinforces the fact that there is no spread spectrum invoked when ODx_ssce = 0. If down
spread is to be turned on by just setting ODx_ssce = 1, then ODx _period and ODx_step must be calculated and registered. See
Eq.9 to
see why changing only ODx_ssce works only for down spread.
Consider one cycle of down spread triangular modulation; the output divider, N, is ramped up linearly from the non-spread value of N
followed by a linear ramp back down to the non-spread value of N. N is always greater than or equal to the non-spread value of N,
therefore the output frequency is always less than or equal to the non-spread frequency.
Table 43. Spread Spectrum Variables in FODx
Name Function
RAM Register
(see Table 23 )
Register
Length Note
ODx_ssce Spread spectrum control enable. 0x25,0x35,0x45,0x55 1
If ODx_ssce = 0, contents of ODx_period and
ODx_step are Don't Care.
ODx_intdiv
Integer portion of the FODx
divider, N. 0x2D, 0x2E,0x3D,0x3E,
0x4D, 0x4E,0x5D,0x5E
12
ODx_period
Spread spectrum modulation
period.
0x29,0x2A,0x39,0x3A,
0x49,0x4A,0x59,0x5A
13
Defined as half the reciprocal of the modulation
frequency and measured in cycles of the FODx
output frequency.
See equation below.
ODx_step Modulation step size.
0x26,0x27,0x28,0x36,
0x37,0x38,0x46,0x47,
0x48,0x56,0x57,0x58
24
Sets the time rate of change or time slope of the
output clock frequency.
ODx_offset
Spread spectrum modulation
offset, which defines down
spread or center spread and is
the fractional portion of the
divider.
0x22,0x23,0x24,0x25,
0x32,0x33,0x34,0x35,
0x42,0x43,0x44,0x45,
0x52,0x53,0x54,0x55
30
ODx_offset is the actual spread spectrum offset
subtracted from the Fractional portion of the
FODx divider N. It is the fractional portion of the
FODx divider and accounts for the fact that there
is no ODx_frcdiv in the Memory map.