6.0 Hardware User Information (Continued)
Symptoms:
specifies the differential output voltage to be between 2.2V
to 2.8V (zero to peak).
The COL signal could go low up to 87ns after TX_EN goes
low.
Symptoms:
Solution/Workaround:
National Semiconductor believes that there will be no
system ramifications with higher than specified Vod
voltages. The worse case scenario would be a slight
increase in cross-talk in between the twisted pair cables.
There is no current workaround to this test mode function.
In a network environment, this will not be a problem.
Solution/Workaround:
6.6 Synchronization of Idle
Problem:
If desired, the Vod voltage can be modified by connecting
resistors from RTX (pin 28) and REQ (pin 29) to ground.
Figure 25 illustrates the recommended connection of
external circuitry which determines the Vod amplitude and
shape. This will cause the Vod voltage to be lowered.
Based on the data taken from the laboratory, National
In 100 Mb/s operation, descrambler violates the TP-PMD
specification of 12 idle times or 60 cipher bits.
Description:
Semiconductor recommends starting with
a 100kΩ
The DP83840A will acquire descrambler synchronization in
13 symbols. However, due to latency and pipeline delays,
the DP83840A can not provide valid decoded data less
than 16 Idle symbols.
resistors for both the RTX and REQ pins. The Vod will be in
the range in between 2.2V to 2.9V when the 100kΩ
resistors are used. The value of resistor you choose may
be different based on your board layout and selection of
components such as pulse transformers.
Symptoms:
If the Idle stream is between 13 and 16 symbols, the
descrambler will synchronize to the transmit data, yet the
Start of Frame Delimiter (SFD) will be lost resulting in the
detection of a Bad_SSD. The subsequent packet following
the next idle stream will be lost.
6.8 10Base-T Transmit Differential Output
Impedance
Problem:
Solution/Workaround:
Based on the characterization data across fabrication
process, voltage, and temperature, the DP83840A does
not meet the letter of tIEEE 802.3 10BaseT specification on
the 10 Mb/s Transmit Differential Output Impedance (which
is measured as return loss).
The minimum number of consecutive idles required for a
proper network operation is sixteen. There is no current
workaround to this problem. Since the minimum IPG in a
network system is greater than 16 symbols, there is no real
system impact for this problem.
Description:
The IEEE 802.3 10BaseT standard requires a return loss
of 15dB or better at all times (during transmit state or idle
state) over the frequency range of 5 MHz to 10 MHz with a
cable impedance range from 85Ω to 111Ω. The DP83840A
characterization data shows that while the device meets
the 15dB return loss specification during transmit and
during idle on average, some devices may have return loss
of less than 15dB during idle.
6.7 10 Mb/s Differential Output Voltage
Problem:
Based on the characterization data across fab process,
voltage, and temperature, the DP83840A is on the high
side of the Vod specification.
Description:
The Vod measured without resistors connected to the RTX
and REQ pins, ranges from 2.4V to 3.1V. IEEE 802.3
Pin 28 (RTX)
100 kΩ
100 kΩ
Pin 29 (REQ)
DP83840A
Figure 25. Recommended External Circuitry to Control the Amplitude and Shape of Vod
Version A
National Semiconductor
63
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