W5500 Datasheet, PDF (38/65 Pages) List of Unclassifed Manufacturers – Supports 8 independent sockets simultaneously

RTR (Retry Time-value Register) [R/W] [0x0019 – 0x001A] [0x07D0]
RTR configures the retransmission timeout period. The unit of timeout period is
100us and the default of RTR is „0x07D0‟ or „2000‟. And so the default timeout period
is 200ms(100us X 2000).
During the time configured by RTR, W5500 waits for the peer response to the
packet that is transmitted by Sn_CR(CONNECT, DISCON, CLOSE, SEND, SEND_MAC,
SEND_KEEP command). If the peer does not respond within the RTR time, W5500
retransmits the packet or issues timeout.
Ex) When timeout-period is set as 400ms, RTR = (400ms / 1ms) X 10 = 4000(0x0FA0)
0x0019
0x001A
0x0F
0xA0
RCR (Retry Count Register) [R/W] [0x001B] [0x08]
RCR configures the number of time of retransmission. When retransmission occurs
as many as „RCR+1‟, Timeout interrupt is issued (Sn_IR[TIMEOUT] = „1‟).
Ex) RCR = 0x0007
0x001B
0x07
The timeout of W5500 can be configurable with RTR and RCR. W5500 has two kind
timeout such as Address Resolution Protocol (ARP) and TCP retransmission.
At the ARP (Refer to RFC 826, http://www.ietf.org/rfc.html) retransmission
timeout, W5500 automatically sends ARP-request to the peer‟s IP address in order to
acquire MAC address information (used for communication of IP, UDP, or TCP). While
waiting for ARP-response from the peer, if there is no response during the configured
RTR time, a temporary timeout is occurred and ARP-request is retransmitted. It is
repeated as many as „RCR + 1‟ times. Even after the ARP-request retransmissions are
repeated as „RCR+1‟ and there is no response to the ARP-request, the final timeout is
occurred and Sn_IR(TIMEOUT) becomes „1‟. The time of final timeout (ARP_TO) of ARP-
request is as below.
𝐴𝑅𝑃_{𝑇𝑂} = (𝑅𝑇𝑅 × 0.1𝑚𝑠) × (𝑅𝐶𝑅 + 1)
At the TCP packet retransmission timeout, W5500 transmits TCP packets (SYN, FIN,
RST, DATA packets) and waits for the acknowledgement (ACK) during the configured
RTR time and RCR. If there is no ACK from the peer, a temporary timeout occurs and
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W5500 Datasheet Version1.0 (August 2013)