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register 2D are always read as zeros even though the timer itself is stored in twos
complement value. The TRT counter holds the twos complement of the time remaining in
80-ns units. For example, if register 2C held FF3C (196 in decimal) and register 2D held
FFFE (1 in decimal with a weighting of 2 16 ), then the time remaining would be (196 × 80) +
(65536 × 80) = 15.68 + 5242.88 = 5258.56 µs.
1F
1E
0
0
17
16
F
E
7
6
3.6.3 THT Timer, Sent Count Registers (THT_TIMER_A, THT_TIMER_B/
SENT_COUNT)
Since the THT timer is 24-bits wide, the NP may need to read it in two consecutive reads
(only when T_Opr is > = 5.242880 ms). Because this register can (and usually will) change
between the two read operations, care must be taken to get a consistent value.
The least significant 16 bits of the THT timer occupy register address 2E, and the most
significant 8 bits of the THT timer occupy bits 7–0 of register 2F. Bits 15 and 7 are the
least significant, and bit 0 is the most significant in each register. The THT counter holds
the twos complement of the time remaining in 80-ns units.
The upper 8 bits of register 2F holds the 8 MSBs of the 10-bit SENT_COUNT register as
an unsigned integer between 0 and 255 inclusive. The SENT_COUNT is the count of
outstanding frames, up to 1023 frames—i.e., the number of frames transmitted
(incremented two BYTCLK cycles after the frame's FS is sent) minus the number
of frames received (decremented when a frame's T-symbol has been received). The
granularity of this register is four, so the number of outstanding frames is the value of
SENT_COUNT times four. SENT_COUNT is used in BRIDGE_STRIP mode to determine
whether a frame was sent by this station or not. All data frames as well as special void
frames are counted. Claim and beacon frames are not counted. This count is cleared by
the receiver upon receipt of a LOWER_CLAIM, MY_CLAIM, HIGHER_CLAIM,
OTHER_CLAIM, OTHER_BEACON, MY_BEACON, special void frame, or token. lt is also
cleared when RING_OPERATIONAL is false or when the MAC is turned off (MAC_ON =
0).
MOTOROLA
1D
1C
0
0
15
14
TRT_TIMER (MOST SIGNIFICANT)
D
C
5
4
TRT_TIMER (LEAST SIGNIFICANT)
MC68838 USER'S MANUAL
1B
1A
0
0
13
12
B
A
3
2
19
18
0
0
11
10
9
8
1
0
3- 29
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