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I am going through the datasheet of the P82B715 I2C extender.

The datasheet says "400 kHz when other system delays permit."

What does this mean? (In the Texas Instruments datasheet they do not mention anything like that.)

Will it affect my application?

My application:

  • I need a multidrop configuration with 400kHz.
  • cable cat 5 (50pF per meter)
  • 15 sensor drops, from sensor to sensor 2 feet cat 5 cable
  • total bus capacitance buffered = 15x(2/3)x50 = 500pF
  • total 16 of P82B715 chips (including MCU board), one P82B715 for each sensor drop
  • sensor data are sequentially continuously collected from an MCU at 400KHz

The reason to select this IC is its 2 wire communication. I need a low number of wires for the communication bus.

Updated

one sensor drop is one sensor node sensor node = sensor IC + P82B715 IC

Mcu P82B715 IC--> 2ft cable --> 1st sensor node-> 2ft cable -->2nd sensor node-> 2ft cable --> 3rd sensor node -> 2ft cable -->... --> 15th sensor node

so the 15th sensor node is 30ft away from the MCU board

from data sheet enter image description here

as shown above, other nodes will be connected buffered bus, same as I2C 3, in a parallel way

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  • \$\begingroup\$ "total 15 of P82B715 chips, one P82B715 for each sensor drop" - Can you show or tell us the exact layout (cable lengths and position of MCU, P82B715 chips and sensors on them)? \$\endgroup\$ – Bruce Abbott Apr 20 '20 at 5:41
  • \$\begingroup\$ @Bruce Abbott I have updated \$\endgroup\$ – komto909 Apr 20 '20 at 5:48
  • \$\begingroup\$ So that's a total of 16 P82B715's, right? \$\endgroup\$ – Bruce Abbott Apr 20 '20 at 5:52
  • \$\begingroup\$ @Bruce Abbott yes, will my application workout? \$\endgroup\$ – komto909 Apr 20 '20 at 6:17
  • \$\begingroup\$ What sensors they are? It won't work unless the sensors support setting them to 15 different I2C addresses. They must not be at the same address if they are all on the same bus. \$\endgroup\$ – Justme Apr 20 '20 at 6:52
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I am going through the datasheet of the P82B715 I2C extender.
The datasheet says "400 kHz when other system delays permit."
What does this mean? (In the Texas Instruments datasheet they do not mention anything like that.)
Will it affect my application?

Re "400 kHz when other system delays permit." -> Means: This IC will operate at 400 kHz - in an overall system other components may add delays which drop overall speed to below 400 kHz.
Analogy: A Ferrari in bumper to bumper rush hour traffic is as fast as a Honda Civic and slower than a moped.

The TI datasheet DOES say something like that - but in a different form.

400 kHz = 2.5 uS/clock cycle. That 1.25 uS = 1250 nS/half cycle.

trise and tfall for a "barely just making it" pulse is sometimes taken as t_pulse_width/3
here = about 400 nS.
That is the time allowed for a clocked data level to stabilise.
Longer would be a very good idea.

Note that for this IC the Vin/Vout delay is stated to be 0 ns typical !
This is because there is a 30 Ohm resistor from input to output and the measure of performance is the time for Iout to respond to a change in Vin.

The TI datasheet section 6.6 page 5 says the output CURRENT step for an input VOLTAGE step - measured from the Vin transitioning Vcc/2 point is 250 nS typical.
A maximum value is not stated BUT 250 ns typ is close enough to 400 ns max to say that the two datasheets are saying much the same thing.

How can other components can add a delay

This IC will pass data that arrives properly formed at a 400 kHz rate. Regardless of HOW failure to provide or accept data at this rate is 'not the concern iof the IC designers' - they just tell you what it will do if properly fed.

They say it may be used in star and chain configurations with up to 4000 pF capacitance, and with well under 400 pF per Sx or Sy termination. Your system sounds as if it easily meets these requirements.

We do not know what is accepting or sourcing data on your network. If they are able to do so at 4 kHz clock rate then it should work at 400 kHz. If not then it won't.


TI P82B715 datasheet
Philips P82B715 datasheet

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  • \$\begingroup\$ also, I found that for the total bus capacitance, each slave i2c device adds 10 times of its capacitance \$\endgroup\$ – komto909 Apr 21 '20 at 5:05
  • \$\begingroup\$ @komto909 That statement has very little meaning without a datasheet or other reference and an explanation of what you mean. As stated it sounds like eg "each person adds 10 people" -> it makes no sense in isolation. IF you want our input then more than "one ;liners". your input needs to be n \$\endgroup\$ – Russell McMahon Apr 21 '20 at 8:31

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