PC8374L datasheet(126/239 Pages) NSC

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7.0 Glue Functions (Continued)

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126

Revision1.1

7.2.9

SMBus Isolation

When the Main power is off, this function performs “passive” bus isolation between the SMB1_SCL, SMB1_SDA signals and

the SMB2_SCL, SMB2_SDA signals respectively. The bus isolation enables the operation of the serial bus section connect-

ed to Standby power, if the other serial bus is connected to the Main power and the Main power is off.

The signals connected to the SMB1_SCL, SMB1_SDA, SMB2_SCL and SMB2_SDA pins are compatible Intel's SMBus

(Speciﬁcation Rev 1.1, Dec. 11, 1998) two-wire synchronous serial interface speciﬁcations.

The SMB1_SCL and SMB2_SCL pins are connected through a switch. This switch is controlled by the voltage levels at the

SMB1_SCL and SMB2_SCL pins (for the DC characteristics, see Section 10.2.12 on page 216):

● For V

IN <VISC at either pin, the switch is closed; therefore, both pins (SMB1_SCL, and SMB2_SCL) are held at low

level.

● For V

IN >VISO at both pins, the switch is open; therefore, each pin is pulled up to high level by the external resistor

connected to VSUP1 (for the SMB1_SCL pin) and to VSUP2 (for the SMB2_SCL pin).

In addition, when the Main power supply voltage is not valid (PWRGD_PS = low), the switch is forced open and the unpow-

ered SMBx_SCL signal is disconnected from the powered SMBx_SCL signal (x is either 1 or 2). This isolation between the

two signals prevents the loading of the powered SMBx_SCL signal by the unpowered SMBx_SCL signal.

The SMB1_SDA and SMB2_SDA pins are connected through a similar level isolation circuit and behave like SMB1_SCL

and SMB2_SCL.

Figure 29 shows a simplified diagram of the circuit that performs SMBus isolation.

Figure 29. SMBus Isolation (Simpliﬁed Diagram)

For the AC characteristics, see “SMBus Voltage Translation and Isolation Timing” on page 235.

2.7 K

Ω

SMB2_Snn

VSUP2 (3.3V)

2.7 K

Ω

SMB1_Snn

VSUP1 (3.3V)

Snn = SCL or SDA

Control

Switch

PWRGD_PS

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PC8374L Datasheet(PDF) 126 Page - National Semiconductor (TI)