ADLINK Technology NuDAQ ACL-8454, Руководство пользователя

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Applications
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4.2 Frequency Generator
Example 1 : To generate a 250 K Hz Square Wave.
step 1: To use fixed clock source because the output is a fixed
frequency.
step 2: I nternal 2M Hz is suitable to generate 250K Hz frequency.
Use Counter #4 for this application.
∵
250 K Hz = 2 M Hz / 8
step 3: Set jumper ‘CK4’. The clock source is coming from internal
‘CLOCK1’. Note that the CLOCK1 must be set as 2 M Hz.
However, the CLOCK1 is shared with other counters. You
must consider the clock source of other counters which
using CLOCK1 in the same time.
step 4 ~ 6 : Skip these steps.
step 7: The gate source is enable always. Note th e jumper ‘G4’
should be removed and no external gate is used.
step 8: Connect the counter output to external device. The following
block diagram illustrates the application
step 9: Write the control program. Please refer the ‘DEMO1.C’
source code.
2 M Hz
COUT4
Counter #4
8254 Chip #2
C
G
O 'H'
Jumper
CK4
Jumper
CLOCK1
Figure 4.2.1 Example of frequency generator (1)
Example 2 : To generate a very low frequency of 1 pulse / 1 hour
step 1: To use fixed clock source because the output is a fixed
frequency.
step 2: Because the desired frequency (1/3600sec=0.000278Hz)
is too slow to use one counter to generate. Even if using
cascaded counter architecture, the clock frequency of the
first stage counter must be as low as possible. At first, try to
use
internal clock. Assume the clock of the first stage is
100 K Hz. then the frequency divider value of the two
cascaded counters is:
100 K Hz / 0.000278 Hz = 360,000,000