
IDT SMBus Interfaces
PES32NT24xG2 User Manual
12 - 2
January 30, 2013
Notes
The goal of the reset procedure is to ensure interoperability with serial EEPROM or IO expander devices
that do not have a reset signal input. When the switch is reading from these devices and a fundamental
reset is applied to the switch (e.g., via assertion of the PERSTN input signal), the I
2
C bus may be left in an
unpredictable state that prevents the switch’s master SMBus interface from creating a START condition on
the bus.
This problem occurs when the I
2
C slave device is driving the data signal (MSMBDAT) to a logic 0 at the
time the reset occurs. In this case, after the reset is deasserted and the switch starts booting, the switch’s
master SMBus interface will not be able to drive the data signal to a logic 1 value (since the slave is pulling
the signal in the opposite direction), which is a pre-requisite to create a START condition.
The reset procedure described below ensures that the bus is gracefully returned to a state that allows
the switch’s master SMBus interface to generate a START condition on the bus and re-establish communi-
cation with the slaves.
The I
2
C reset procedure consists of the switch’s master SMBus interface performing the steps shown
below.
1. The master SMBus interface drives the MSMBCLK signal with a maximum of nine clock pulses.
2. At each clock pulse, when the MSMBCLK signal is high, the master SMBus interface samples the
MSMBDAT signal. If the sampled value is a logic 1, then the next step is performed. Otherwise, step
3 is performed during the ninth clock pulse.
This step ensures that a slave who was driving the MSMBDAT signal prior to the reset of the
switch, releases the MSMBDAT signal during the I
2
C acknowledge cycle following the reading
of a byte.
3. The master SMBus interface holds the MSMBCLK signal high. This signal is held high until the
master SMBus interface decides to create a START condition on the I
2
C bus in preparation to
access the Serial EEPROM slave or the IO expanders.
–
The START condition is created by driving the MSMBDAT signal low while the MSMBCLK signal
remains high.
–
Note that the reception of a START (or repeated START) condition on the I
2
C bus causes a slave
to reset its bus logic and anticipate the reception of an address, regardless of the positioning of
the START condition with respect to prior commands.
Serial EEPROM
During a switch fundamental reset, an optional serial EEPROM may be used to initialize any software-
visible register in the device. Serial EEPROM loading occurs if the Switch Mode (SWMODE) signal in the
boot vector selects a mode that performs serial EEPROM initialization. The address used by the SMBus
master interface to access the serial EEPROM is shown in Table 12.1.
Address
Bit
Address Bit Value
1
0
2
0
3
0
4
0
5
1
6
0
7
1
Table 12.1 Serial EEPROM SMBus Address
Содержание PCI Express 89HPES32NT24xG2
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