Programmer’s Guide BCM5722
10/15/07
Broadcom Corporation
Document 5722-PG101-R Transmit Data Path Page 20
Bit 10 in the Flags field of the ISO SBD notifies the transmit MAC to transmit the frame as a time sync packet, which is
described in Figure 7.
Figure 7: Time-Sync Packet
When the ISO SDI block detects that flag bit 10 is set, it logs the 32-bit local network clock time stamp to a register for the
software driver to read later.
The ISO SDI block compares the 16-bit launch time against its local network time clock bits 25:10. The 16-bit launch time
field allows a 1-µs resolution a 67-ms scheduling rollover. If the network clock bit field 25:10 is greater than or equal to the
16-bit launch time, it issues a data DMA request to the RDMA engine to transmit the ISO packet at the proper time.
The software driver can directly program a different value to the local network clock to speed it up or slow it down.
Upon receiving a time-sync packet that matches the 16-bit EtherType and 8-bit Protocol ID (programmed using registers
0x0510 and 0x0514, respectively), the 32-bit local network clock is logged into a register for software to access. The received
time-sync packet is preserved and forwarded to the host just like a normal packet. Based on the time delta between the
global (or “master”) clock and the logged local clock, the host can update the local clock if required by the application.
An application is likely to send out the master clock’s time-sync-logged time value to a slave node as a separate frame after
the actual time-sync frame. This allows a slave node to compare appropriate time clock values (i.e., clock values that were
logged at essentially the same time).
Table 6: ISO SDI Block
31 15 Offset
Host Address – 0x00
––0x04
Len Flags 0x08
Launch Time VLAN tag 0x0C
Destination
Address
Source
Address
Ethertype
Protocol
ID
CRC
48 bits 48 bits 16 bits 8 bits 32 bits
Offset
User-
defined
frame
content
User-
defined
frame
content
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