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参数资料
| 型号： | AD9549A/PCBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 20/76页 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION FOR AD9549A |
| 设计资源： | AD9549 Schematics AD9549 Gerber Files |
| 标准包装： | 1 |
| 主要目的： | 计时，时钟发生器 |
| 嵌入式： | 否 |
| 已用 IC / 零件： | AD9549A |
| 主要属性： | 8 kHz ~ 750 MHz 输入 |
| 次要属性： | 差分输出 |
| 已供物品： | 板 |
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AD9549
Rev. D | Page 27 of 76
The following four values are needed to calculate the correct
values of the reference monitor:
System clock frequency, fS (usually 1 GHz)
Reference input frequency, fR (in Hz)
Error bound, E (1% = 0.01)
Monitor window size (W)
The monitor window size is the difference between the maximum
and minimum number of counts accumulated between adjacent
edges of the reference input. If this window is too small, random
variations cause the OOL detector to indicate incorrectly that
a reference is out of limits. However, the time required to
determine if the reference frequency is valid increases with window
size. A window size of at least 20 is a good starting point.
The four input values mentioned previously are used to calculate
the OOL divider (D) and OOL nominal value (N), which, in
turn, are used to calculate the OOL upper limit (U) and OOL
lower limit (L), according to the following formulas:
×
=
E
W
f
D
S
R
5
1
ceil
,
535
,
65
min
,1
max
4
D
f
N
R
S
×
=
)
floor(
)
floor(
W
N
L
=
)
floor(
)
ceil(
W
N
U
+
=
The timing accuracy is dependent on two factors. The first is
the inherent accuracy of fS because it serves as the time base for
the reference monitor. As such, the accuracy of the reference
monitor can be no better than the accuracy of fS. The second
factor is the value of W, which must be sufficiently large (≥20)
so that the timer resolves the deviation between a nominal value
of fR and a value that is out of limits.
As an example, let fR = 10 MHz, Ε = 0.05%, fS = 1 GHz, and
W = 20. The limits are then
D = 79
Lower Limit = 1980
Upper Limit = 2020
Next, let Ε = 0.0005%. Then the limits are
D = 7999
Lower Limit = 199980
Upper Limit = 200020
Note that the number of counts (and time) required to make
this measurement has increased by 100×. In addition, it is
recommended that D be an odd number.
REFERENCE SWITCHOVER
The AD9549 supports dual input reference clocks. Reference
switchover can be accomplished either automatically or manually
by appropriately programming the automatic selector bit in the
I/O register map (Register 0x01C0, Bit 2).
Transition to a newly selected reference depends on a number
of factors:
State of the REFSELECT pin
State of the REF_AB bit (Register 0x01C1[2])
State of the enable ref input override bit (Register 0x01C1[3])
Holdover status
A functional diagram of the reference switchover and holdover
logic is shown in Figure 33.
06744-
033
1
0
1
0
AUTOREFSEL
OVERRIDE REFPIN
AUTOHOLD
OVERRIDE HLDPIN
1
0
1
0
REF_AB
HLDOVR
DERIVED
REFSEL STATE
DERIVED
HOLDOVER
STATE
REFSELECT
HOLDOVER
STATE
MACHINE
TO
REFERENCE
SWITCHING
CONTROL
LOGIC
TO
HOLDOVER
CONTROL
LOGIC
ACTIVE REFSEL STATE
ACTIVE HOLDOVER STATE
Figure 33. Reference Switchover and Holdover Logic
In manual mode, the active reference is determined by an exter-
nally applied logic level to the REFSELECT pin. In automatic
mode, an internal state machine determines which reference is
active, and the REFSELECT pin becomes an output indicating
which reference the state machine is using.
The user can override the active reference chosen by the internal
state machine via the enable ref input override bit. The REF_AB
bit is then used to select the desired reference. When in override,
it is important to note that the REFSELECT pin does not
indicate the physical reference selected by the REF_AB bit.
Instead, it indicates the reference that the internal state machine
would select if the device were not in the override mode. This
allows the user to force a reference switchover by means of the
programming registers while monitoring the response of the
state machine via the REFSELECT pin.
The same type of operation (manual/automatic and override)
also applies to the holdover function, as shown in the reference
switchover logic diagram (see Figure 33). The dashed arrows in
the diagram indicate that the state machine output is available
to the REFSELECT and HOLDOVER pins when in override mode.
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