AMD
P R E L I M I N A R Y
82
Am79C970A
A system that wants to stop the clock during Magic
Packet mode should use one of the LED pins as an indi-
cator of Magic Packet frame detection. It should also
stop the clock after enabling Magic Packet mode, other-
wise PCI bus activity, including accessing CSR5 to set
MPMODE and possibly MPEN to a ONE, could be af-
fected. The clock should be restarted before Magic
Packet mode is disabled if MPEN is being cleared or the
clock must be restarted right after magic packet mode is
disabled if
SLEEP
is being deasserted. Otherwise, the
receive FIFO may overflow if new frames arrive. The
network clock (XTAL1) must continue running at all
times while in Magic Packet mode.
MANCHESTER ENCODER/DECODER
The
integrated
Manchester
(MENDEC) provides the PLS (Physical Layer Signaling)
Encoder/Decoder
functions required for a fully compliant ISO 8802-3
(IEEE/ANSI 802.3) station. The MENDEC provides the
encoding function for data to be transmitted on the net-
work using the high accuracy on-board oscillator, driven
by either the crystal oscillator or an external CMOS level
compatible clock. The MENDEC also provides the de-
coding function from data received from the network.
The MENDEC contains a Power On Reset (POR) cir-
cuit, which ensures that all analog portions of the
PCnet-PCI II controller are forced into their correct state
during power up, and prevents erroneous data trans-
mission and/or reception during this time.
External Crystal Characteristics
When using a crystal to drive the oscillator, the following
crystal specification may be used to ensure less than
±
0.5 ns jitter at DO
±
:
Table 7. Crystal Characteristics
Parameter
Min
Nom
Max
Units
1. Parallel Resonant Frequency
20
MHz
2. Resonant Frequency Error
–50
+50
PPM
3. Change in Resonant Frequency
With Respect To Temperature (0 – 70 C)*
–40
+40
PPM
4. Crystal Load Capacitance
20
50
pF
5. Motional Crystal Capacitance (C1)
0.022
pF
6. Series Resistance
35
ohm
7. Shunt Capacitance
7
pF
8. Drive Level
TBD
mW
* Requires trimming specification, not trim is 50 PPM total.
External Clock Drive Characteristics
When driving the oscillator from a CMOS level
external clock source, XTAL2 must be left floating
(unconnected). An external clock having the following
characteristics must be used to ensure less than
±
0.5 ns
jitter at DO
±
.
Table 8. External Clock Source Characteristics
Clock Frequency:
20 MHz
±
0.01%
<= 6 ns from 0.5 V to VDD –0.5 V
Rise/Fall Time (tR/tF):
XTAL1 HIGH/LOW Time (tHIGH/tLOW):
20 ns min.
<
±
0.2 ns at 2.5 V input (VDD/2)
XTAL1 Falling Edge to Falling Edge Jitter:
MENDEC Transmit Path
The transmit section encodes separate clock and NRZ
data input signals into a standard Manchester encoded
serial bit stream. The transmit outputs (DO
±
) are de-
signed to operate into terminated transmission lines.
When operating into a 78
terminated transmission
line, the transmit signaling meets the required output