FIFO DATA PACKETS
Symmetricom, Inc.
bc620/627AT Time and Frequency Processor
4-3
4.1.2 PACKET “B” - SET MAJOR TIME
TFP Modes 1 and 2 require the major time to be input using this data packet. Modes 0, 3 and 4
acquire major time from their respective time sources. Set time in Mode 3 with Packet “L.”
Packet “B” can be used to load time in any mode when the TFP is flywheeling. The data field of
this packet consists of nine ASCII time digits in the order of seconds through days as follows:
byte 1
SOH.
byte 2
“B.”
byte 3
Seconds
Units.
byte 4
Seconds
Tens.
byte 5
Minutes
Units.
byte 6
Minutes
Tens.
byte 7
Hours
Units.
byte 8
Hours
Tens.
byte 9
Days Units (JAN 1 = DAY 001).
byte 10
Days
Tens.
byte 11 Days
Hundreds.
byte 12
ETB.
The following paragraphs are about timing. This information should be well understood to avoid
problems loading the major time.
The TFP firmware partitions each one second epoch into 30 periods plus a fractional period.
Each period is 65536 counts of a 2 MHz clock. If the 2 MHz clock was perfect and the one
second epoch was perfect then the number of counts in the one second epoch would be exactly
two million. There would be thirty periods plus a remainder of 33920 counts.
For now it is important to know that the bc620/627AT increments, and then transfers major time,
from a software buffer to a set of hardware latches on the 29th period of the internal counter.
The time loaded into the latches will be the major time used for the next one second epoch (i.e.
the next 1pps pulse rising edge transfers the time loaded at the 29th period to a second set of
latches that are used for the major time reference). This double buffering approach ensures that
the major and minor times are completely coherent, and that no ambiguities occur during the
1pps transition. What does all this mean? When loading the major time observe the following
rules:
•
If time is loaded before 29/30 into the second then the loaded time should reference the
current epoch. (Recall the bc620 increments major time before downloading to the hardware
latches.)
•
If time is loaded after 29/30 into the second then the loaded time should reference the
following epoch.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
