IDT SMBus Interfaces
PES24T3G2 User Manual
5 - 7
February 22, 2012
Notes
SMBus errors may occur when accessing the serial EEPROM. If an error occurs, then it is reported in
the SMBus Status (SMBUSSTS) register. Software should check for errors before and after each serial
EEPROM access.
I/O Expanders
The PES24T3G2 utilizes external SMBus/I
2
C-bus I/O expanders connected to the master SMBus inter-
face for hot-plug and port status signals. The PES24T3G2 is designed to work with Phillips PCA9555
compatible I/O expanders (i.e., PCA9555, PCA9535, and PCA9539). See the Phillips PCA9555 data sheet
for details on the operation of this device. An external SMBus I/O expander provides 16 bit I/O pins that
may be configured as inputs or outputs.
The PES24T3G2 supports up to three external I/O expanders. Table 5.4 summarizes the allocation of
functions to I/O expanders. I/O expanders zero through three are used to provide hot-plug I/O signals while
I/O expander four is used to provide link status and activity LED control. I/O expander signals associated
with LED control (i.e., link status and activity) are active low (i.e., driven low when an LED should be turned
on). I/O expander signals associated with hot-plug signals are not inverted.
During the PES24T3G2 initialization process, the SMBus/I
2
C-bus address allocated to each I/O
expander used in that system configuration should be written to the corresponding IO Expander Address
(IOE[0,2,4]ADDR) field.
Hot-plug outputs and I/O expanders may be initialized via serial EEPROM. Since the I/O expanders and
serial EEPROM both utilize the master SMBus, no I/O expander transactions are initiated until serial
EEPROM initialization completes.
–
Since no I/O expander transactions are initiated until serial EEPROM initialization completes, it is
not possible to toggle a hot-plug output through serial EEPROM initialization (i.e., it is not possible
to cause a 0 -> 1 -> 0 transition or a 1 -> 0 -> 1 transition).
Whenever the value of an IOEXPADDR field is written, SMBus write transactions are issued to the
corresponding I/O expander by the PES24T3G2 to configure the device. This configuration initializes the
direction of each I/O expander signal and sets outputs to their default value. Outputs for ports that are
disabled are set to their negated value (e.g., the power indicator is turned off, the link is down, there is no
activity, etc.).
The default value of I/O expander outputs is shown in Table 5.5. Note that this default value may be
modified via serial EEPROM or SMBus configuration prior to SMBus initialization by changing the state of
the PCI Express Slot Control Register (PCIESCTL) or Hot-Plug Configuration Control (HPCFGCTL).
SMBus I/O
Expander
Section
Function
0
Lower
Port 2 hot-plug
Upper
Port 4 hot-plug
2
Lower
Unused
Upper
Power good inputs
4
Lower
Link status
Upper
Link activity
Table 5.4 I/O Expander Function Allocation
Summary of Contents for 89HPES24T3G2ZBAL
Page 8: ...IDT PES24T3G2 User Manual 6 February 22 2012 Notes...
Page 12: ...IDT Table of Contents PES24T3G2 User Manual iv February 22 2012 Notes...
Page 14: ...IDT List of Tables PES24T3G2 User Manual vi February 22 2012 Notes...
Page 16: ...IDT List of Figures PES24T3G2 User Manual viii February 22 2012 Notes...
Page 20: ...IDT Register List PES24T3G2 User Manual xii February 22 2012 Notes...
Page 32: ...IDT PES24T3G2 Device Overview PES24T3G2 User Manual 1 12 February 22 2012 Notes...
Page 72: ...IDT SMBus Interfaces PES24T3G2 User Manual 5 20 February 22 2012 Notes...
Page 76: ...IDT Power Management PES24T3G2 User Manual 6 4 February 22 2012 Notes...
Page 156: ...IDT Configuration Registers PES24T3G2 User Manual 8 74 February 22 2012 Notes...