Datasheet

P82B715_8 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 08 — 9 November 2009 10 of 23
NXP Semiconductors
P82B715
I
2
C-bus extender
8.2 Quick design-in point-to-point/multi-point circuit information for 5 V
bus
With many variables (cable length/capacitance, local capacitive loading on each I
2
C-bus,
bus voltages, and bus speed), optimizing a design can be complex and requires
significant study of the application note information. The following circuit and simplified
approach has been checked to provide adequate performance in the typical 100 kHz
application and can be easily implemented by just using the values and circuit shown for
either point-to-point application, up to 30 meters long, or in multiple point applications if
additional nodes need to be added along the way.
Specific information on this circuit implementation:
• The pull-up on each I
2
C-bus is (V
CC
− 0.4V)/1mA=4.6kΩ, using 4.7 kΩ as the
nearest usual value.
• The net pull-up on the cable bus can be (V
CC
− 0.5 V) / (21 − n) mA where
n = total number of P82B715 modules on the cable. When there are only two
modules, one each end of the cable, the pull-up = (4.5 / 19) = 237 Ω. Make the
pull-ups at each end of the cable equal. Signalling is bidirectional so there is no
advantage optimizing for any one direction. The pull-up at each end will be 474 Ω,
using 470 Ω as the nearest usual value.
• The 100 kHz rise time requirement is 1 µs. Meeting this requires the product of the
bus capacitance and pull-up resistor on each bus section to be less than 1.18 µs. That
provides one capacitance limit. With 4.7 kΩ pull-ups the I
2
C-bus limit is 250 pF each,
while the 235 Ω sets a cable bus limit at 5000 pF.
Remark: Cable bus pull-ups only fitted at the cable ends, not fitted to modules connected along cable.
Fig 9. Quick design-in point-to-point/multi-point circuit for 5 V bus
P82B715
I
2
C-BUS
MASTER
µC
SDA
SCL
4.7 kΩ 4.7 kΩ
Sx
Sy
Lx
Ly
470 Ω 470 Ω
5V1
optional
ESD protection
5 V
5V1
optional
ESD protection
470 Ω 470 Ω
P82B715
Lx
Ly
Sx
Sy
SDA
SCL
4.7 kΩ 4.7 kΩ
I
2
C-BUS
SLAVE
5 V
5V1
P82B715
Lx
Ly
Sx
Sy
SDA
SCL
4.7 kΩ 4.7 kΩ
I
2
C-BUS
SLAVE
5 V
002aad817
optional
ESD
protection
20 meter Cat5e
twisted pair cable