GR740-UM-DS, Nov 2017, Version 1.7
364
www.cobham.com/gaisler
GR740
of this system providing SpaceWire Time-Codes, CCSDS Time Codes, datation, time-stamping of
distributed interrupts, support for transmission of CCSDS Time Codes through RMAP and support
for latency measurement and correction. In this implementation the CCSDS Time Codes carried
between the SpaceWire network is based on CCSDS Unsegmented Code format (CUC) which is
explained below [CCSDS]. The table below shows an example Preamble Field (P-Field) which corre-
sponds to 40 bits of coarse time and 24 bits of fine time.
31.3.1 CCSDS Unsegmented Code: Preamble Field (P-Field)
Table 463.
CCSDS Unsegmented Code P-Field definition
31.3.2 CCSDS Unsegmented Code: Time Field (T-Field)
For the unsegmented binary time codes described herein, the T-Field consists of a selected number of
contiguous time elements, each element being one octet in length. An element represents the state of 8
consecutive bits of a binary counter, cascaded with adjacent counters, which rolls over at a modulo of
256.
The basic time unit is the second. The T-Field coarse time (seconds) can be maximum 56 bits and
minimum 8 bits. The T-Field fine time (sub seconds) can be maximum 80 bits and minimum of 0 bits.
The number of bits representing coarse and fine time implemented in this core can be obtained by
reading the DPF bits of Datation Preamble Field register.
The coarse time code elements are a count of the number of seconds elapsed from the initial time
value. This code is not UTC-based and leap second corrections do not apply according to CCSDS.
31.3.3 Time generation
The core consist of time generator which is the source for time in this system. The core may act as ini-
tiator or a target but both have their respective time generator. The Elapsed Time (ET) counter is
implemented complying with the CUC T-Field. The number of bits representing coarse and fine time
of a ET counter implemented in a design can be obtained by reading the DPF bits of Datation Pream-
ble Field register.
Bit
Value
Interpretation
0
“1”
Extension flag, P-Field extended with 2nd octet
1-3
“010”
Agency-defined epoch (Level 2)
Time code identification
4 - 5
“11”
(number of octets of coarse time) + 1
Detail bits for information on the code
6 - 7
“11”
(number of octets of fine time)
8
“0”
Extension flag, P-Field not extended with 3rd octet
9-10
“01”
Number of additional octets of the
coarse time.
added to octet 1
11-13
“000”
Number of additional octets of the fine
time.
added to octet 1
14-15
RESERVED
Table 464.
Example CCSDS Unsegmented Code T-Field with 32 bit coarse and 24 bit fine time
CCSDS Unsegmented Code
Preamble
Field
Time Field
Coarse time
Fine time
-
2
31
2
24
2
23
2
16
2
15
2
8
2
7
2
0
2
-1
2
-8
2
-9
2
-15
2
-16
2
-24
0:15
0
31
32
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