W5300 datasheet(82/128 Pages) WIZNET

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Sn_MSSR (SOCKETn Maximum Segment Size Register) [R/W]

[0x08218+0x40n/0x218+0x40n] [0x0000]

It sets MTU(Maximum Transfer Unit) of SOCKETn or notifies MTU that is already set.

If the host does not set the Sn_MSSR, it is set as default MTU.

It just supports

TCP or UDP mode. When using PPPoE (MR(PPPoE)=‘1’), the MTU of TCP or

UDP mode is assigned in the range of MTU of PPPoE.

At the IPRAW or MACRAW, MTU is not processed internally, but default MTU is used. Therefore,

when transmitting the data bigger than default MTU, the host should manually divide the data

into the unit of default MTU.

At the TCP or UDP mode, if transmitting data is bigger than MTU, W5300 automatically divides

the data into the unit of MTU.

MTU is called as MSS at the TCP mode. By selecting from Host-Written-Value and peer's MSS,

MSS is set as smaller value through TCP connection process.

At the UDP mode, there is no connection-process of TCP mode, and Host-Written-Value is just

used. When communicating with the peer having different MTU, W5300 is able to receive

ICMP(Fragment MTU) packet. In this case, IR(FMTU) becomes '1', and the host can acquire the

fragment MTU and destination IP address through FMTUR and UIPR respectively. In case of

IR(FMTU)='1', the UDP communication with the peer, is not possible. So, you should close the

SOCKET, set FMTU as Sn_MSSR and retry the communication with OPEN command.

Mode

Normal

(MR(PPPoE)=‘0’)

PPPoE

(MR(PPPoE)=‘1’)

Default MTU

Range

Default MTU

Range

TCP

1460

1 ~ 1460

1452

1 ~ 1452

UDP

1472

1 ~ 1472

1464

1 ~ 1464

IPRAW

1480

1472

MACRAW

1514

Ex) Sn_MSSR = 1460 (0x05B4)

Sn_MSSR(0x08218+0x40n/0x218+0x040n)

Sn_MSSR0(0x08218+0x40n/0x218+0x040n)

Sn_MSSR1(0x08219+0x40n/0x219+0x040n)

0x05

0xB4

Sn_KPALVTR(SOCKETn Keep Alive Time Register)[R/W]

[0x0821A+0x40n/0x21A+0x40n][0x00]

It is 1 byte register that sets transmitting timer of KEEP ALIVE(KA) packet of SOCKETn. It is

valid only in TCP mode, and ignored in other modes. The unit is 5s.

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