PCIe-24DSI32
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General Standards Corporation Ph:(256)880-8787 FAX:(256)880-8788 Email: [email protected]
3-14
3.6.2.2 Legacy Rate Generator
For boards equipped with the Legacy Rate Generator option, the frequency
Fgen
of an internal
rate generator is controlled over a frequency range of 25.6 -51.2 MHz by a variable
Nrate
(Table 3.6.2.2-1) as:
Fgen = 25.6 * (1 +Nrate/511),
(3-5)
where
Fgen
is in Megahertz, and
Nrate
has a range from zero through 511.
Table 3.6.2.2-1. Legacy Rate Control Register
Offset: 0008h
Default: 0000 0000h *
BIT FIELD
MODE
DESIGNATION
FUNCTION
D[16..00]
R/W
RATE CONTROL
(Nrate)
Frequency control factor; 0-511
D[31..17]
R/W
(Reserved)
---
* Default frequency is 25.6MHz.
Determining the optimum values for
Nrate
and
Ndiv
is usually an iterative process that starts by
selecting a maximum value for
DIVISOR
(from
Ndiv
) in Equation 3-1, and then selects
successively smaller values of
Ndiv
until valid values for both
Ndiv
and
Nrate
produce a value
for
Fsamp
that is as close as possible to the required value.
3.6.3 Direct External Clocking
If the rate assignment selection (Table 3.6.1.2-2) is "Direct External Sample Clock," the signal
at the external clock input is routed directly to the ADC's without modification. This
configuration eliminates the effect of the
Nrate
control variable, and gives the external clock
source direct control of the ADC's.
Although the variable
Nrate
has no effect in this configuration, the clocking mode of the ADC's
must be controlled by
Ndiv
as shown in Figure 3.6.1.1. Table 3.6.1.6 shows the relationships
between the sample rate
Fsamp
, the external clock frequency, and the required values for
Ndiv
.
Table 3.6.3. Direct External Clocking
Sample Rate
Fsamp (KSPS)
External Clock
Frequency
Divisor Integer
Ndiv
2-50
256 * Fsamp
2 thru 25
50-100
128 * Fsamp
1
100-200
64 * Fsamp
0
