Datasheet

SCEA017
7–294 GTLP in BTL Applications
Table 5 compares slew rates and duty cycles for Figures 8 and 9. The slew rates and duty
cycles were measured at the optimum termination, which is 33 Ω.
Table 5. Waveform Slew Rate and Duty Cycle
LEVEL
MONITOR
POINT
DATA
FREQUENCY
(MHz)
L-H
SLEW RATE
(V/ns)
H-L
SLEW RATE
(V/ns)
DUTY
CYCLE
(%)
GTLP G1 B1 11.5 0.5 0.33 48–52
GTLP G1 B1 25 0.45 0.33 46–54
GTLP G1 B1 43.5 0.48 0.28 45–55
BTL G1 B1 11.5 0.46 0.40 46–54
BTL G1 B1 25 0.52 0.47 46–54
BTL G1 B1 43.5 0.58 0.43 46–54
In Figures 8 and 9, V
OL
is lower than the specification levels of 0.55 V at GTLP levels and
1.1 V at BTL levels. This larger voltage swing can reduce the maximum possible frequency,
increases EMI, and changes the lower end of possible R
TT
values. Table 6 is a comparison of
V
OH
and V
OL
at 11.5 MHz with the termination resistance. The I
OL
was calculated based on
I
OL
= 2 × ∆V/R
TT
. The GTLP data sheet lists a recommended maximum device I
OL
at V
OL
of
0.4 V and 0.55 V. The lowest termination-resistor value that is possible without exceeding these
limits also is calculated, based on R
TT
= 2 × ∆V/I
OL
.
Table 6. V
OH
and V
OL
vs I
OL
LEVEL
MONITOR
POINT
R
TT
(Ω)
V
OH
(V)
V
OL
(V)
∆V
(V)
I
OL
(mA)
MAXIMUM
RECOMMENDED R
TT
(Ω)
POINT
(Ω)
(V)
(V)
(V)
(
m
A)
I
OL
66 mA
AT 0.4 V
I
OL
100 mA
AT 0.55 V
GTLP G1 B1 25 1.46 0.30 1.16 93 35.2 23.2
GTLP G1 B1 33 1.45 0.25 1.20 73 36.4 24.0
GTLP G1 B1 38 1.43 0.20 1.23 65 37.3 24.6
GTLP G1 B1 50 1.42 0.15 1.27 51 38.5 25.4
BTL G1 B1 25 2.00 0.45 1.55 124 50.0 31.0
BTL G1 B1 33 1.93 0.35 1.58 96 47.9 31.6
BTL G1 B1 38 1.90 0.29 1.61 85 48.8 32.2
BTL G1 B1 50 1.86 0.20 1.66 66 50.3 33.2
As R
TT
is reduced, V
OL
increases because the device has a finite capacity for pulling down the
bus voltage. V
OH
on the monitored bit is reduced at higher levels of R
TT
because a larger share
of V
TT
is consumed in the fixed 255-Ω test-monitor-point bit resistor. Without this leakage path,
which is not found on commercial backplanes, V
OH
goes to V
TT
, as shown on the unmonitored
bit waveforms.