LAN9353 Datasheet, PDF (410/523 Pages) Microchip Technology – Interfaces at up to 200Mbps via Turbo MII

LAN9353
It is assumed that the original FCS is valid and is not checked.
• If the frame has a transmit symbol error(s), a transmit symbol error indication will be propagated at the same nib-
ble location(s)
Note: The FCS and IPv6/UDP checksum are updated and the reserved byte cleared only if the frame was actually
modified.
The IPv4/UDP checksum is cleared as indicated above and could be the only modification in the message.
If the IPv4/UDP checksum is cleared, the FCS is recomputed.
If no modifications are done, the existing FCS, checksums and reserved bytes are left unchanged.
15.3 1588 Clock
The tunable 1588 Clock is the time source for all PTP related functions of the device. The block diagram is shown in
Figure 15-1.
FIGURE 15-1:
1588 CLOCK BLOCK DIAGRAM
IEEE 1588 Clock
host
32 Bit Seconds
step
value
inc
carry
+/-
30 Bit NanoSeconds
+
+ 9, 10, 11 ^carry
32 Bit SubNanoSeconds
30 Bit Rate
Adjustment
+
30 Bit Temp Rate
Adjustment
32 Bit Temp Rate
Duration
The 1588 Clock consists of a 32-bit wide seconds portion and a 30-bit wide nanoseconds portion. Running at a nominal
reference frequency of 100MHz, the nanoseconds portion is normally incremented by a value of 10 every reference
clock period. Upon reaching or exceeding its maximum value of 10^9, the nanoseconds portion rolls over to or past zero
and the seconds portion is incremented.
The 1588 Clock can be read by setting the Clock Read (1588_CLOCK_READ) bit in the 1588 Command and Control
Register (1588_CMD_CTL). This saves the current value of the 1588 Clock into the 1588 Clock Seconds Register
(1588_CLOCK_SEC), 1588 Clock NanoSeconds Register (1588_CLOCK_NS) and 1588 Clock Sub-NanoSeconds
Register (1588_CLOCK_SUBNS) where it can be read.
Although the IEEE 1588-2008 specification calls for a 48-bit seconds counter, the hardware only supports 32 bits. For
purposes of event timestamping, residence time correction or other comparisons, the 136 year rollover time of 32 bits
is sufficient. Rollover can be detected and corrected by comparing the two values of interest. To support one-step oper-
ations, the device can insert the Egress Timestamp into the origin Timestamp field of Sync messages. However, the
DS00001925A-page 410
 2015 Microchip Technology Inc.