Download Print this page

Motorola MC68838 User Manual page 36

Media access controller
Hide thumbs


Table of Contents
This bit controls the state of the field sequence machine that is responsible for creating
tokens, beacon, claim, and special void frames; for adding the preamble, JK, and frame
status; and for controlling the addition of the FCS to frames that the FSI passes to the
MAC for transmission.
0000 = Pre_State—Transmit preamble (idles)
0001 = Post_State—Transmit postamble (idles)
0010 = Data_FC_State—Transmit FC for data frame
0011 = Data_DA_State—Transmit DA for data frame
0100 = SD_State—Transmit JK for token/all frames
0101 = CRC_State—Transmit FCS (all required frames)
0110 = Ed_State—Transmit TT (token) or TR (otherwise)
0111 = Data_State—Transmit INFO field for data frame
1000 = Data_SA_State—Transmit SA for data frame
1001 = FS_State—Transmit RR + any ExtraFS required
1010 = Unused
1011 = Unused
1100 = FC_State—Transmit FC for all but data frames
1101 = Info_State—Transmit INFO (claim/beacon frame)
1110 = DA_State—Transmit DA (claim/beacon/void)
1111 = SA_State—Transmit SA (claim/beacon/void)
FLD_CNT_STATE—Field Count State
These bits indicate the state of the field count machine that counts down to determine
when each of the various fields have ended and when the field sequence state machine
should proceed to its next state. ln every case below, when a field has x more bytes
(really symbol pairs) to go, that count of x includes the current byte transfer. Hence,
there are x–1 transfers after this cycle (and subsequent state change).
000 = Field ends after this byte
110 = Field has 8 more bytes
111 = Field has 7 more bytes
001 = Field has 6 more bytes
010 = Field has 5 more bytes
011 = Field has 4 more bytes
100 = Field has 3 more bytes
101 = Field has 2 more bytes
LATE_CT—Current Value of LATE_CT
This 3-bit counter holds the current value of LATE_CT, which is simply the number of
times that the TRT timer has expired since the receiver has seen and/or created a token
(except for the second rotation of the token). This counter does not wrap around—i.e., if
TRT expires when LATE_CT is seven (111), LATE_CT will continue to be seven until a
clear LATE_CT (LATE_CT = 1) action is performed by the transmitter.
3- 16


Table of Contents

  Related Manuals for Motorola MC68838