83C694 ETC, 83C694 Datasheet - Page 15

• Manufacturer Part Number: 83C694
• Description: TWISTED PAIR INTERFACE AND MACHESTER ENCODER/DECODER
• Manufacturer: ETC

83C694D
12
NUMBER
PIN
16
17
18
19
20
21
22
23
LBK
X1
X2
TXD
TXC
TXE
TPX2-
TPX1-
MNEMONIC
TABLE 3-1. PIN DESCRIPTION cont.
Loopback
Crystal/Ext. Input
Crystal Feedback
Transmit Data
Transmit Clock
Transmit Enable
TwPr Transmit
TwPr Transmit
SIGNAL NAME
I/O
O
O
O
O
I
I
I
I
A high level enables loopback of TXD to
RXD/RXC. A low level enables normal trans-
mit/receive operation. The LBK pin includes
an internal pull-down resistor, so it may be left
open if unused.
X1 is driven by an external clock frequency
source or is connected to one terminal of the
20MHz crystal.
The IEEE 802.3 standard requires 0.01% ab-
solute accuracy on the transmitted signal fre-
quency. Stray capacitance can shift the
crystal’s frequency out of range, causing it to
exceed the 0.01% tolerance. To remedy this,
extra load capacitance may be added.
To determine the amount of capacitance to
add, measure the board capacitance and the
capacitance between the X1 and X2 pins.
Then add these values together, and subtract
them from the crystal’s required load capaci-
tance. (Refer to Figure 2-1.)
This output is connected to the other terminal
of the 20MHz crystal. If X1 is driven with an
external source, X2 must be left open.
TXD is sampled on the rising edge of TXC
when TXE is high. The NRZ data input here
is encoded and transmitted on TX+/TX- or
TPX+/TPX- as a differential signal.
This is a 10MHz clock signal derived from the
internal 20MHz oscillator. It is enabled except
when RES is low and MPE is high.
TXE enables encoding and transmission of
the data input via TXD. It is sampled on the
rising edge of TXC.
TPX2- is used for 10BaseT only. It is the low
current negative output pin. See TPX1+ for
details.
TPX1- is used for 10BaseT only. It is the high
current negative output pin. See TPX1+ for
details.
DESCRIPTION
PIN DESCRIPTION