Datasheet
P82B715_8 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 08 — 9 November 2009 9 of 23
NXP Semiconductors
P82B715
I
2
C-bus extender
Figure 8 shows P82B715 in an analog radial IPMB shelf application.
In this example the total system capacitance is 2800 pF, but it is distributed over 18
different bus sections and no section has a capacitance greater than 200 pF.
If every individual bus section is designed to rise at least as fast as the IPMB requirement,
then when any driver releases the bus, all bus sections will rise together and no amplifiers
in the P82B715s will be activated or, if one is activated, it can only slow the system bus
rise to its own rate and that has been designed to meet the requirement.
It is then only necessary to calculate the equivalent static bus pull-up loading and to
ensure that it exceeds the specification requirement. The calculated loadings meet the
requirements.
Note that in this example only one of the four IPMB lines is shown and the usual switching
arrangements for isolating or cross-connecting bus lines are not shown. The typical offset
(increase in the bus LOW level) measured between any two Sx points in this system is
below 100 mV.
Calculation of static loading at ShMM buffer and each FRU:
Loading on ShMM buffer = R1 || {10 (R2 || R3/16)} = 3.5 kΩ
Loading on each FRU = R3 || {10 (R1 || R2 || R3/15)} = 3.76 kΩ
Fig 8. Typical arrangement and calculations for an IPMB analog radial shelf
002aad708
LxSx
R1
3.3 V
effective capacitance
at ShMM buffer
ShMM buffer pull-up
ShMM buffer
strays
P82B715
total capacitance C1
10 pF
20 pF
10 pF
40 pF
R1 = = 25 kΩ
1 µs
40 pF
effective capacitance
at common Lx node
Lx common pull-up
17 × P82B715
trace capacitance
total capacitance C2
170 pF
30 pF
200 pF
R2 = = 5 kΩ
1 µs
200 pF
effective capacitance
at average radial trace
radial trace pull-up
1 × FRU
radial trace/connector
P82B715
total capacitance C3
25 pF
125 pF
10 pF
160 pF
R3 = = 6.2 kΩ
1 µs
160 pF
Calculations to ensure rise time is met on each bus section:
BUFFERShMM
C1
R2
C2
Lx Sx
1
R3
C3
Lx Sx
2
R3
C3
radial traces
Lx Sx
16
R3
C3
BUFFER µC
FRU 1
BUFFER µC
FRU 2
BUFFER µC
FRU 16
common Lx node