CHAPTER FOUR
4-6
bc620/627AT Time and Frequency Processor
Symmetricom, Inc.
Two counter modes are supported; 1pps synchronous and asynchronous. It is the responsibility
of the user to select the appropriate mode. No error checking is performed by the bc620/627AT
firmware. The synchronous mode should only be selected if the number of output counts per
second is an integer. If the number of counts per second is not an integer then the asynchronous
mode should be used. The number of counts per second is always of the following form:
N = (10,000,000) / (n1 * n2)
where: N = counts per second
n1 = Counter #1 divide
n2 = Counter #2 divide
The range of values for Counter #1 and #2 is mode dependent as follows.
Asynchronous Mode: 2 to 65535
Synchronous Mode: 3 to 65535
* * * WARNING * * *
Periodic heartbeat pulse/interrupt generation can
not
be guaranteed in synchronous mode when
counter divide values of two are used.
The two modes of operation are accessed using standard INTEL mode identifiers. For
synchronous operation the mode byte must be an ASCII “5.” For asynchronous operation the
mode byte must be an ASCII “2.” The packet format is as follows:
byte 1
SOH.
byte 2
“F.”
byte 3
ASCII “2” (asynch) or ASCII “5” (synch).
byte 4
ASCII “0” - ”F” (n1 bits 12 through 15).
byte 5
ASCII “0” - ”F” (n1 bits 8 through 11).
byte 6
ASCII “0” - ”F” (n1 bits 4 through 7).
byte 7
ASCII “0” - ”F” (n1 bits 0 through 3).
byte 8
ASCII “0” - ”F” (n2 bits 12 through 15).
byte 9
ASCII “0” - ”F” (n2 bits 8 through 11).
byte
10
ASCII “0” - ”F” (n2 bits 4 through 7).
byte 11
ASCII “0” - ”F” (n2 bits 0 through 3).
byte 12
ETB.
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