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Trt Time Remaining Register (T_Neg_A, T_Neg_B); Information Field Register (Info_Reg_A); Void Time Counter Register (Void_Time) - Motorola MC68838 User Manual

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3.6.4 TRT Time Remaining Register (T_NEG_A, T_NEG_B)

TRT time remaining register is a 24-bit read-only register that the NP will need to read in
two consecutive reads. Because this register can (but usually will not) change between the
two read operations, care must be taken to get a consistent value.
The least significant 16 bits of the TRT time remaining register occupy register address 26,
and the most significant 8 bits occupy bits 7–0 of register 27. The upper 8 bits of register
27 are always read as zeros. The TRT time remaining register holds the twos complement
of the time remaining in 80-ns units. It is up to SMT software to compare this value against
T_MIN and T_MAX.
3.6.5 Information Field Register (INFO_REG_A, INFO_REG_B)
The information field register is a 32-bit register that the NP may need to read out in two
consecutive reads. This register holds the first 4 octets of the INFO field of the last MAC
frame received (normally a claim or beacon frame). Therefore this register holds the last
received TTRT bid when the last MAC frame received was a claim frame, and holds the
beacon type if the last MAC frame received was a beacon frame. The most significant 16
bits of this register (corresponding to the first four INFO field symbols received) have
register address 28, and the least significant 16 bits of this register (corresponding to the
5–8 INFO field symbols received) have register address 29. This register is Ioaded based
upon the FC (i.e., FC = MAC frame) before the FCS is checked, so can contain the
information field of a frame with an incorrect CRC.

3.6.6 Void Time Counter Register (VOID_TIME)

This 16-bit register holds the value of the lost time void count. This count is a count of 80
ns units between the end of the MAC transmitted void frame (TE symbol pair) and the end
of reception of a valid void frame with SA = MA.
Any void frame created and sent by the MAC chip will be timed, and after the completion
of the timing the proper count will be loaded in the void time register. The timer will start to
count when the MAC has transmitted the TE symbol pair and will stop counting when My
Own Valid Void Frame is received. This count is in BYTCLK increments. In bridgestrip or
purger mode, this timer will time the latency of the first void frame transmitted. If a second
void frame is transmitted before receiving the previously transmitted void frame (i.e., the
timer is still counting) the second void frame will not be timed and will have no effect on
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THT_TIMER (MOST SIGNIFICANT)
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THT_TIMER
THT_TIMER (LEAST SIGNIFICANT)
MC68838 USER'S MANUAL
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MOTOROLA

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