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Chapter 9
Precise Timing Protocol Packet Buffers
This chapter considers two of the logical components which are partly responsible for the AVB timing synchronization protocol.
•“Tx PTP Packet Buffer”
•“Rx PTP Packet Buffer”
These are both described in this chapter as they are closely related.
Tx PTP Packet Buffer
The Tx PTP packet buffer is illustrated in Figure
The PTP packet buffer is implemented in
TEMAC.
The Tx PTP Packet Buffer is divided into eight identical buffer sections as illustrated. Each section contains 256 bytes, which are formatted as follows:
•the first byte, at address zero, contains a frame length field. This indicates how many bytes make up the PTP frame that is to be transmitted from this particular PTP buffer.
•The next seven bytes, from address 1 to 7, are reserved for future use.
•The PTP frame data itself is stored from address 8 onwards. The amount of addresses used is dependent on the indicated frame length field, which will be different for each PTP frame type. Each PTP buffer provides a maximum of 244 bytes (more than that required for the largest PTP frame). Each PTP frame holds the entire MAC frame (with the exception of any required MAC padding or CRC - these will automatically be inserted by the TEMAC) from the Destination Address field onwards.
•The top four addresses of each buffer, from address 0xFC to 0xFF are reserved for a time stamp field. At the beginning of PTP frame transmission from any of the eight buffers, the “Time Stamping Logic” will sample the “Real Time Clock”. Following the end of PTP frame transmission, this captured timestamp will automatically be written into this location to accompany the frame for which it was taken.
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UG492 July 23, 2010