PC8374L datasheet(125/239 Pages) NSC

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7.0 Glue Functions (Continued)

Revision 1.1

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7.2.7

Hard-Disk LED Indicator Control

This function generates the HD_LED signal, which controls the Hard Drive, red LED indicator.

The Hard Drive LED is turned on by an active low HD_LED signal, generated when at least one of the PRIMARY_HD or

SECONDARY_HD or SCSI pins is active (low).

Figure 27 shows a simplified diagram of the circuit that generates the HD_LED signal.

Figure 27. HD_LED Generation (Simpliﬁed Diagram)

7.2.8

SMBus Voltage Translation

This function performs “passive” level translation between the VDD3-powered (or 2.5V-powered) CC_DDCSCL and

CC_DDCSDA signals and the VDD5-powered 5V_DDCSCL and 5V_DDCSDA signals, respectively, for interfacing the Data

Display Channel.

The signals connected to the CC_DDCSCL, CC_DDCSDA, 5V_DDCSCL and 5V_DDCSDA pins are compatible with Intel's

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

The CC_DDCSCL and 5V_DDCSCL pins are connected through a switch. This switch is controlled by a Level Translation

Control circuit, according to the voltage levels at the CC_DDCSCL and 5V_DDCSCL 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 (CC_DDCSCL, and 5V_DDCSCL) 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 VDD3 or VDD25 (2.5V Main Power) (for the CC_DDCSCL pin) and to VDD5 (for the 5V_DDCSCL pin).

In addition, when the VDD3 power is off, the switch is open, regardless of the voltage levels at the CC_DDCSCL and

5V_DDCSCL pins.

The CC_DDCSDA and 5V_DDCSDA pins are connected through a similar level translation circuit and behave like

CC_DDCSCL and 5V_DDCSCL.

Figure 28 shows simplified a diagram of the circuit that performs SMBus voltage translation.

Figure 28. SMBus Voltage Translation for VDD3 Tolerant Chipset Cluster (Simpliﬁed Diagram)

220

Ω

HD_LED

VDD3

SCSI

PRIMARY_HD

LED

SECONDARY_HD

2.2 K

Ω

5V_DDCSnn

VDD5

4.7 K

Ω

CC_DDCSnn

VDD3 or VDD25

Snn = SCL or SDA

Level

Control

Translation

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PC8374L Datasheet(PDF) 125 Page - National Semiconductor (TI)

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