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RT430/434
Master, Slave and Grandmaster clocks
PTP Messages
92
In PTP protocol, master clocks send message packets with Sync information, slave
clocks receive and process the Sync messages and grandmaster clocks are the
source of synchronization for the entire network.
IEEE1588 standard specifies the Best Master Clock algorithm (BMC) which selects the
best candidate to be elected the master of the network, used as time source. The
selection is performed from the attributes and attributed priorities to the possible
candidates. The algorithm establishes a search order, and from the results, it
determines which one will be the clock used as time source.
RT430/434 is pre-configured to operate as master of the network and it can be
configured to operate as slave.
In PTP protocol, Sync messages followed by the timestamp messages are sent to the
entire network in multicast mode, in which a PTP message sent by a network port
can be received by all other ports connected to the same network. The advantage of
multicast mode network is that the master clock sends only one packet containing
time information to the network and this packet is received by all slave devices
connected to this network.
Among the messages specified by IEEE1588 standard, the ones that stand out are
related to synchronization, timestamp and propagation delay.
The Announce messages are used to inform the devices connected to the network
about the existence of a master clock available to send Sync messages. The
Announce message includes a packet of values that indicates the time accuracy of
the clock, enabling the BMC algorithm to decide which of the available clocks will be
used as master. The speed the Announce messages are sent influences directly the
frequency the slave will perform the BMC algorithm. Many announce messages can
be transmitted at the same time through the network and the slave clock is
responsible to process these messages. All devices connected to the network that
are able to operate as master should periodically send Announce messages to the
network, becoming candidates to be master of the network.
The clock connected to the network selected as master by the BMC algorithm, should
send Sync messages, and in case it is a two-step clock, it should also send a Follow
Up message, containing a timestamp. The sending interval of the messages is
configurable and its standard value, specified by IEEE1588 standard, is one-
message-per-second. This interval specifies the frequency the slave devices receive
synchronization information, allowing to adjust its internal clocks to use the master
clock as time reference. In the interval between two Sync messages the slave devices
operate free from external time reference and the time stability in this period is
determined by its internal time base, that can be, for example a crystal oscillator. By
choosing the frequency to send Sync messages through the master clock, it is
important to consider the accuracy of the internal clocks of the slave devices that will
be synchronized by it, and also the bandwidth, because the higher the frequency to
send messages is, the higher the network traffic is.
The delay measurement of messages passing through devices is important to reach
the accuracy required by IEEE1588 standard. Especially in E2E networks, the
propagation delay measurement is crucial for the synchronization. In networks with
E2E delay measurement, the frequency the slave devices should measure delay,
which results in sending and receiving messages, should be according to the network
stability regarding the variation of this information.
Chapter 10 - Appendixes
RT430/434
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