EDM01-36v10 DAG_9.2X2_Card_User_Guide - Synchronizing clock time
©2010 - 2012 Endace Technology Ltd. Confidential - Version 10 - May 2012
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Timestamps
ERF files contain a hardware generated timestamp of each packet’s arrival.
The format of this timestamp is a single little-endian 64-bit fixed point number, representing the number
of seconds since midnight on the 1
st
January 1970.
The high 32-bits contain the integer number of seconds, while the lower 32-bits contain the binary
fraction of the second. This allows an ultimate resolution of 2
-32
seconds, or approximately 233
picoseconds.
Different DAG cards have different actual resolutions. This is accommodated by the lower most bits that
are not active being set to zero. In this way the interpretation of the timestamp does not need to change
when higher resolution clock hardware is available. The DAG 9.2X2 implements the 28 most significant
bits which provides a time resolution of 7.5 nanoseconds.
The ERF timestamp allows you to find the difference between two timestamps using a single 64-bit
subtraction. You do not need to check for overflows between the two halves of the structure as you
would need to do when comparing Unix time structures.
Example
Below is example code showing how a 64-bit ERF timestamp (erfts) can be converted into a
struct
timeval
representation (tv):
unsigned long long lts;
struct timeval tv;
lts = erfts;
tv.tv_sec = lts >> 32;
lts = ((lts & 0xffffffffULL) * 1000 * 1000);
lts += (lts & 0x80000000ULL) << 1; /* rounding */
tv.tv_usec = lts >> 32;
if(tv.tv_usec >= 1000000) {
tv.tv_usec -= 1000000;
tv.= 1;
}
