LAN91C111 Datasheet, PDF (26/125 Pages) SMSC Corporation – 10/100 Non-PCI Ethernet Single Chip MAC + PHY

LAN91C111
7.7.7.2 Transmitter - 10 Mbps
The transmitter operation in 10 Mbps mode is much different than the 100 Mbps transmitter. Even so, the transmitter
still consists of a waveform generator and line driver.
The purpose of the waveform generator is to shape the output transmit pulse. The waveform generator consists of a
ROM, DAC, dock generator, and filter. The DAC generates a stair-stepped representation of the desired output wave-
form. The stairstepped DAC output then goes through a low pass filter in order to "smooth' the DAC output and remove
any high frequency components. The DAC values are determined from the ROM outputs; the ROM contents are chosen
to shape the pulse to the desired template and are clocked into the DAC at high speed by the clock generator. In this
way, the waveform generator preshapes the output waveform to be transmitted onto the twisted pair cable to meet the
pulse template requirements outlined in IEEE 802.3 Clause 14 and also shown in Figure 7-4. The waveshaper replaces
and eliminates external filters on the TP transmit output.
The line driver converts the shaped and smoothed waveform to a current output that can drive 100 meters of category
3/4/5 100 Ohm unshielded twisted pair cable or 150 Ohm shielded twisted pair cable tied directly to the TP output pins
without any external filters. During the idle period, no output signal is transmitted on the TP outputs (except link pulse).
FIGURE 7-4:
TP OUTPUT VOLTAGE TEMPLATE - 10 MBPS
1.0
0.8
0.6
0.4
0.2
0.0 _A
-0.2
-0.4
-0.6
-0.8
-1.0
0
B
DE
C
10 20 30
H
I
N
P
O
M
F
Q
J
R
S
LK
U
W
V
G
T
40 50 60 70 80 90 100 10
TIME (ns)
Reference
A
B
C
D
E
F
DS00002276A-page 26
Time (NS) Internal MAU
0
15
15
25
32
39
Voltage (V)
0
1.0
0.4
0.55
0.45
0
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