P82B715
NXP Semiconductors
I2C-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 I2C-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.
5 V
5 V
4.7 kΩ
4.7 kΩ
470 Ω
470 Ω
470 Ω
470 Ω
4.7 kΩ
4.7 kΩ
20 meter Cat5e
twisted pair cable
SDA
Sx
Lx
Lx
Sx
SDA
2
I C-BUS
MASTER
µC
2
I C-BUS
P82B715
P82B715
SLAVE
Sy
Ly
Ly
Sy
SCL
SCL
5V1
5V1
optional
ESD protection
optional
ESD protection
5 V
optional
ESD
protection
4.7 kΩ
4.7 kΩ
Lx
Sx
Sy
SDA
2
I C-BUS
SLAVE
P82B715
Ly
SCL
5V1
002aad817
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
Specific information on this circuit implementation:
• The pull-up on each I2C-bus is (VCC − 0.4 V) / 1 mA = 4.6 kΩ, using 4.7 kΩ as the
nearest usual value.
• The net pull-up on the cable bus can be (VCC − 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 I2C-bus limit is 250 pF each,
while the 235 Ω sets a cable bus limit at 5000 pF.
P82B715_8
© NXP B.V. 2009. All rights reserved.
Product data sheet
Rev. 08 — 9 November 2009
10 of 23
NXP [ NXP ]
