LPC47M172_07 SMSC [SMSC Corporation], LPC47M172_07 Datasheet - Page 138

• Manufacturer Part Number: LPC47M172_07
• Description: Advanced I/O Controller with Motherboard GLUE Logic
• Manufacturer: SMSC [SMSC Corporation]

Advanced I/O Controller with Motherboard GLUE Logic
Datasheet
7.35
SMSC/Non-SMSC Register Sets (Rev. 01-11-07)
NOTE: The switch is implemented as an n-channel switch that will not pass a full voltage swing.
It provides a current path to ground.
The board designer should treat each signal pair to the switch as a separate bus with a
resistance in the path.
The maximum resistance of the switch between any bus to any other bus is 25ohms (when the
switch is on). When the switch is off the impedance is Hi-Z and the current is zero.
The design requires pull-ups on each of the busses shown above.
It is recommended that the pullups be selected so that the total maximum current on both
busses does not exceed 2mA to limit the voltage drop across the switch.
MCH
SMBus Isolation Circuitry
The SMBus Isolation circuitry is used to isolate the main SMBus signals from the resume SMBus signals
during power down modes. The SMB data pins and the SMB clock pins function as inputs shorted together
through the isolation resistor when the switch is closed. The SMBus signals require external pull-up
resistors on LPC47M172. See Figure 7.11 for recommended schematic implementation. The switch is
controlled by the PWRGD_PS signal. The switch is closed as long as PWRGD_PS is ‘1’. The current flow
is controlled by the external signals on the SMB pins. See Table 7.35 and Table 7.36 for the current flow
across the switch based on the voltage levels on the pins. The switch provides a 25ohm resistance to
ground.
These pins are powered by VTR.
SMB_CLK_M
SMB_DAT_M
SMB_CLK_R
SMB_DAT_R
DDCSDA_3V
DDCSCL_3V
NAME
Figure 7.10 - VGA DDC Voltage Translation Circuit
4.7k
VCC
IO_SW
IO_SW
IO_SW
IO_SW
Table 7.34 - SMBus Isolation Pins
4.7k
BUFFER
DATASHEET
GPIO Alternate
25ohm
LPC47M172
25ohm
SEE NOTE
Max
Max
Select bit
Function
Page 138
EN
POWER
VTR
VTR
VTR
VTR
WELL
DDCSDA_5V
DDCSCL_5V
Main Well SMBus Clock
Main Well SMBus Data
Resume Well SMBus Clock
Resume Well SMBus Data
DESCRIPTION
6.2V
2.2k
VCC5V
2.2k
6.2V
Connector
VGA
SMSC LPC47M172