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Receive Crc Checker; Sent Count; Counters; Receive Finite State Machine - Motorola MC68838 User Manual

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2.2.2 Receive CRC Checker

This logic block checks the FCS field of received frames and operates a byte at a time,
using the data symbol pair currently on RCDATx. The CRC checker receives a signal
from the receiver FSM telling it when to initialize for another CRC check and receives a
strobe from the receiver FSM for each symbol pair to be included in the CRC check. This
chip provides a valid CRC signal to the receiver FSM. This signal should only be
examined after the CRC checker has processed the last byte of the incoming frame's
FCS field.

2.2.3 Sent Count

Sent count is used in bridgestrip mode to determine if a frame was sent by this station.
All data frames, as well as special void frames, are counted.

2.2.4 Counters

This block holds the following counters:
• Frame_Ct is a 16-bit unsigned count of the number of frames (good or bad) that
have been received since the last time this counter was read and reset or the chip
was reset.
• Error_Ct is a 6-bit unsigned count of the number of error frames (i.e., frames
containing a bad CRC or an invalid data length) that were detected by this station,
but by no previous station, since the last time this counter was read and reset or the
chip was reset.
• Lost_Ct is a 6-bit unsigned count of the number of frame format errors that have
occurred since the last time this counter was read and reset or the chip was reset.
• Token_Ct is a 16-bit unsigned integer.
These counters do not stick at their largest value, but always wrap around to zero. These
registers receive their increment signals from the receive FSM.

2.2.5 Receive Finite State Machine

This FSM implements the receiver process as described in the FDDI MAC standard—
i.e., it controls all aspects of parsing and validating frames and tokens, determines
whether a frame should be received or stripped, detects ring errors, and notifies the
transmit FSM of any relevant events. This block also decodes and forwards the received
FC field, decodes the current symbol pair on the receive data path, and parses and
forwards the received frame status field. This block interprets all the status flags
described in the MAC standard: E_FLAG, A_FLAG, C_FLAG, N_FLAG, and R_FLAG.
MOTOROLA
MC68838 USER'S MANUAL
2-3

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