AM7968 AMD [Advanced Micro Devices], AM7968 Datasheet - Page 68

• Manufacturer Part Number: AM7968
• Description: TAXIchip Integrated Circuits(Transparent Asynchronous Xmitter-Receiver Interface)
• Manufacturer: AMD [Advanced Micro Devices]

Figure 5-3
Figure 5-4
64
AMD
threshold level of the receiver’s differential amplifier. Once the Receiver recognizes the
state change, variations in the falling edge are not significant.
To avoid edge rate variations due to driver turn-off, we must equate the voltage to which
the driver is taken at turn-off with a point in the logic swing which will guarantee that the
Receiver changes state. Since PECL logic swings are 800 mV, we may safely choose a
500 mV change at the driver (100 mV past the midpoint) as a guaranteed state change
at the Receiver. If the driver turns off instantly, we require the voltage divider formed by
RE and Z0 to produce a 500 mV change from V
V
4.1 x RE/(RE + Z0) = 3.6
RE = 7.33 Z0
As a general rule, we may then say that:
186 < RE < 7.33 Z0
Pull-Down and Termination for AC Coupled Link
Serial Link with Output Driver Model
5.4 Baseline Wander and the AC Coupling Capacitor
The 4B/5B and 5B/6B data encoding schemes which are used by the TAXIchip set are
run-length limited to a maximum of 3 consecutive LOW states (non-transitions in NRZI).
This type of encoding ensures that on average there will be less than 10% variation in
the DC component of the encoded data.
When the encoded data is passed through an AC coupled link, the high-pass filtering of
the AC coupling will introduce jitter because of the fluctuating threshold caused by the
variation in DC component. This undesired side-effect of AC coupling is often described
OH
RE/(RE + Z0) = V
Driver (Model)
Pseudo-ECL
Pseudo-ECL
TAXI TX
TAXI TX
V
OH
R0
Driver
SEROUT
SWI
TAXIchip Integrated Circuits Technical Manual
OH
– 0.5
RE
RE
C
C
B
Copper Media
Copper Media
OH
. We can write:
V
CC
R2
R1
R1
R2
V
CC
TAXI RX
TAXI RX
SERIN
SERIN
12330E-10
12330E-11