THMC50

REMOTE/LOCAL TEMPERATURE MONITOR AND

FAN CONTROLLER WITH SMBus INTERFACE

SLIS090 – JULY 1999

10

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

functional description

The THMC50 provides a remote thermal diode temperature sensor, an internal ambient temperature sensor,

an analog output, and two voltage supervisors/reset generators.

Temperatures read from the remote and internal sensors are converted into an 8-bit, 2s-complement digital

word with a 1

°C LSB (least significant bit). The analog output is a 0-V to 2.5-V output from an 8-bit DAC that

is used along with external circuitry to control the fan speed. The analog output is automatically set by the

THMC50 to full on (0

×FF), the FAN_OFF output floats (fan is on), and THERM is asserted low when either of

two automatic trip points is exceeded for more than three monitoring cycle times. The FAN_SPD analog output

will be reset to full off (0x00) whenever RST is asserted.

The THMC50 provides a number of internal registers, as detailed in Table 1. These include:

Register:

Function:

Configuration register

Provides control and configuration, as well as initialization

Interrupt status register

Provides status of each limit or interrupt event

Interrupt status register mirror

Mirror registers can be used by second agent needing to monitor

the THMC50 status.

Interrupt mask register

Allows masking of individual interrupt sources, as well as

separate masking for the hardware interrupt output

Value RAM

The monitoring results and limits for temperature are all

contained in the value RAM.

When the THMC50 is first started, it performs temperature conversions at a rate of 1Hz. Each measured

temperature value is compared to values stored in the limit registers. When the measured value exceeds the

programmed limit, the THMC50 sets a corresponding error bit in the interrupt status register. An open drain

hardware interrupt line, INT, is available to generate an interrupt. INT is fully programmable with masking of each

interrupt source, and with masking of the INT output.

The temperature monitoring section also has an open drain input/output, THERM. This line is asserted low

internally whenever a critical temperature limit is exceeded for at least three monitoring cycles. It can also be

asserted low externally. Whenever THERM is asserted low, either internally or externally, the analog output

automatically goes to full scale (0xFF) and the FAN_OFF output floats (fan is on) in order to command the fan

to full speed.

SMBus interface

When using the SMBus interface, a write always consists of the THMC50 SMBus interface address byte,

followed by the internal address register byte, then the data byte (see Figure 2). There are two cases for a read:

1.

If the internal address register is known to be at the desired address, simply read the THMC50 with the

SMBus interface address byte, followed by the data byte read from the THMC50 (see Figure 5).

2.

If the internal address register value is unknown, write to the THMC50 with the SMBus interface address

byte, followed by the internal address register byte (see Figure 4). Next, restart the serial communication

with a read consisting of the SMBus interface address byte, followed by the data byte read from the

THMC50.

The default power-on SMBus address for the THMC50 is 01011XX binary, where XX reflects the state defined

by the add terminal. This allows up to three THMC50 devices to be used on a single system. Table 1 shows how

the state of the add terminal is used to define the THMC50 SMBus slave address.