Error Signalling - Samsung S3C2500B User Manual

ETHERNET CONTROLLER

7.5.4 ERROR SIGNALLING

The error/abnormal operation flags asserted by the MAC are arranged into transmit and receive groups. These
flag groups are located either in the transmit status register (MACTXSTAT) or the receive status register
(MACRXSTAT). A missed frame error counter is included for system network management purposes.
Normally, software does not have enough direct control to examine the status registers directly. Therefore, the
BDMA engine must store the values in system memory so that they can be examined by software.
7.5.4.1 Reporting of Transmission Errors
A transmit operation terminates when the entire frame (preamble, SFD, data, and CRC) has been successfully
transmitted through the MII without a collision. In addition, the transmitter block detects and reports both the
internal and the network errors.
Under the following conditions, the transmit operation will be aborted (in most cases).
Parity error
MTxFIFO Underflow
No Carrier
Excessive collision error
Late collision error
Excessive deferral error
7-48
The 8-bit of data incoming through the BDMA has an optional parity bit.
A parity bit also protects each byte in the MTxFIFO. If a parity error occurs,
the transmission is aborted. A detected parity error sets the TxParErr bit in the
BMTXSTAT register.
The 80-byte MTxFIFO can handle a system latency of 640 bit times.
An underflow of the MTxFIFO during transmission indicates that the system
cannot keep up with the demand of the MAC, and the transmission is aborted.
The carrier sense signal (CrS) is monitored from the beginning of the start
of frame delimiter (SFD) to the last byte transmitted. A NoCarr indicates that
CrS was never present during transmission (a possible network problem),
but the transmission will NOT be aborted. Note that during loop-back mode,
the MAC is disconnected from the network, and a 'No CrS' will not be detected.
Whenever the MAC encounters a collision during transmit, it will back off,
update the 'attempt counter' and retry the transmission later on. When the
attempt counter reaches 16 (16 attempts that all resulted in a collision), the
transmission is aborted. This indicates a network problem.
Normally, the MAC would detect a collision (if one occurs) within the first 64
bytes of data that are transmitted, including the preamble and SFD. If a collision
occurs after this time frame, a possible network problem is indicated. The error
is reported to the transmission state machine, but the transmission is NOT
aborted. Instead, it performs a back-off, as usual.
During the first attempt to send a frame, the MAC may have to defer the
transmission because the network is busy. If this deferral time is longer than
32K-bit times, the transmission is aborted. Excessive deferral errors indicate
a possible network problem.
S3C2500B