Transmit Time Stamping Service
BCM5718 Programmer’s Guide
Broadcom
®
January 29, 2016 • 5718-PG108-R
Page 157
Transmit Time Stamping Service
The software is able to mark certain types of PTP packets (Delay Request, Synch etc) and the NIC hardware
captures the time when such a packet's first byte, or SOF, is launched for transmission. Ideally, the time capture
is the instant of the actual SOF launch on the wire, but due to practical limitations, the capture will take place at
the time the SOF appears on GMII interface among EMAC and GPHY. The propagation delay through the GPHY
is virtually fixed, and hence, could easily be accounted for by system software.The captured time is in reference
to the EAV Reference Count. There is only a single 64-bit register, the TX Time Stamp Register, for this purpose.
The precision of this stamp is 1ns. System software may read the value of this register anytime to get the time-
stamp of last sent PTP packet. Reference the following table of send ring SBD for IEEE1588.
Table 45: Send Ring SBD Flags
Bit # Flag Name
Flag Description
0
TCP/UDP Checksum Offload Enable This bit enables calculation of TCP or UDP checksums for IPv4
and IPv6 transmitted packets. The driver will set this bit only if the
packet contained within a buffer is TCP or UDP over IPv4 or IPv6.
1
IP Checksum Offload enable
This bit enables calculation of the IPv4 layer-3 checksum. This bit
will be set only for IPv4 packets. The driver will never be set for
IPv6 packets.
2
Packet End
This bit will be set for the last send buffer in a packet.
3
Jumbo Frame
Driver must set this bit to 1 if the MTU length of the Send Frame is
> 1500B. The MTU length is the Ethernet payload length and
excludes Header length (and Trailer length). All BDs belonging to
a Send Packet must configure this bit identically.
4
HDRLEN[2]
The length of the Ether+IP+TCP Headers to be replicated in each
segment arising out of a Large TCP Segment (LSO).
5
Capture Time Stamp
If this bit is 1, this frame’s launch time shall be captured in the TX
Time-Stamp Register.
6
VLAN TAG
When this bit is set, the NIC will insert a VLAN tag in the Ethernet
header. The value for the inserted tag is taken from the VLAN Tag
field in the send BD.