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參數(shù)資料
| 型號： | AN460 |
| 廠商： | NXP Semiconductors N.V. |
| 英文描述： | Using the P82B96 for bus interface |
| 中文描述： | 使用總線接口P82B96 |
| 文件頁數(shù)： | 4/8頁 |
| 文件大?。?/td> | 66K |
| 代理商： | AN460 |
Philips Semiconductors
Application note
AN460
Using the P82B96 for bus interface
2001 Feb 14
4
LINK VIA A DIFFERENTIAL BUS
Figure 6 shows an SDA signal interfaced via P82B96 and the
PCA82C250 to a balanced twisted pair bus capable of supporting
over 100 nodes. In this way over 100 separate I
2
C nodes can be
interconnected, retaining the conventional I
2
C protocol. Operation
over short distances (e.g., 50 meters) at 100 kHz is possible. To
meet the I
2
C timing requirements for the ‘a(chǎn)cknowledge’ bit it is
necessary to select a lower clock speed when working with very
long busses. As a minimum, the ‘round trip’ signal propagation
delay, from any one node to another node and back again, must be
less than the clock ‘low’ period. Signals will typically travel 1 meter in
5ns.
Note that in this example, the CAN bus hardware is only a transport
medium for carrying I
2
C signals. The I
2
C signal format is not being
converted to CAN bus software data format by the PCA82C250.
TECHNICAL ISSUES
There are a number of issues which may need to be considered in
the application of the P82B96. These are discussed below.
Transient Signal Protection
The table of maximum ratings of the data sheet specifies the voltage
which may be applied to any pin with respect to GND as being from
-0.3 to +18 volts.
No ESD protection diodes have been applied between the input and
output pins and V
CC
as these would prevent the very useful feature
of being able to operate inputs and outputs above the V
CC
rail, up to
15 volts.
There is an internal diode from each pin to V
EE
. This diode will
conduct if negative voltages are applied to any pin. The -0.3 V rating
is applied to ensure these diodes never conduct. Currents in these
diodes can cause unpredictable behavior.
In applications where these voltage ratings could be exceeded, e.g.
by transient signals induced into long buffered bus lines, Schottky
diodes and 15 V zener diode clamps should be fitted between GND
and the buffered bus pins as shown in Figure 7.
I
2
C
SDA
+5 V
+V
CC
1/2 PB2B96
Rx
(SDA)
Tx
(SDA)
TxD
RxD
PCA82C250
I
2
C
SDA
+5 V
+V
CC
1/2 PB2B96
Rx
(SDA)
Tx
(SDA)
TxD
RxD
PCA82C250
I
2
C
SDA
+5 V
+V
CC
1/2 PB2B96
Rx
(SDA)
Tx
(SDA)
TxD
RxD
PCA82C250
LINE TERMINATION
canh
canl
SU01017
Example using CAN hardware allows linking
Hardware for carrying SCL signals is identical to SDA (shown here)
100 nodes
Figure 6.
Linking I
2
C nodes via a differential bus
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