LM78 National Semiconductor, LM78 Datasheet - Page 11

• Manufacturer Part Number: LM78
• Description: Microprocessor System Hardware Monitor
• Manufacturer: National Semiconductor

Test Circuit
Functional Description
1.0 GENERAL DESCRIPTION
The LM78 provides 7 analog inputs, a temperature sensor, a
Delta-Sigma ADC (Analog-to-Digital Converter), 3 fan speed
counters, WATCHDOG registers, and a variety of inputs and
outputs on a single chip. Interfaces are provided for both the
ISA parallel bus or Serial Bus. The LM78 performs power
supply, temperature, and fan monitoring for personal com-
puters.
The LM78 continuously converts analog inputs to 8-bit digital
words with a 16 mV LSB (Least Significant Bit) weighting,
yielding input ranges of from 0V to 4.096V. The two negative
analog inputs provide inverting op amps, with their
non-inverting input referred to ground. With additional exter-
nal feedback components, these inputs provide measure-
ments of negative voltages (such as -5V and -12V power
supplies). The analog inputs are useful for monitoring sev-
eral power supplies present in a typical computer. Tempera-
ture is converted to an 8-bit two’s-complement digital word
with a 1˚C LSB.
Fan inputs measure the period of tachometer pulses from
the fans, providing a higher count for lower fan speeds. The
fan inputs are digital inputs with an acceptable range of 0V to
5V and a transition level of approximately 1.4V. Full scale fan
counts are 255 (8-bit counter) and this represents a stopped
or very slow fan. Nominal speeds, based on a count of 153,
are programmable from 1100 to 8800 RPM on FAN1 and
FAN2, with FAN3 fixed at 4400 RPM. Signal conditioning
circuitry is included to accommodate slow rise and fall times.
The LM78 provides a number of internal registers, as de-
tailed in Figure 6 . These include:
• Configuration Register:
• Interrupt Status Registers:
• Interrupt Mask Registers:
• VID/Fan Divisor Registers: A register to read the sta-
figuration.
status of each WATCHDOG limit or Interrupt event.
vidual Interrupt sources, as well as separate masking for
each of both hardware Interrupt outputs.
tus of the VID input lines. The high bits of this register
contain the divisor bits for FAN1 and FAN2 inputs.
Provides control and con-
Allows masking of indi-
Two registers to provide
FIGURE 5. Digital Output Load Circuitry
11
When the LM78 is started, it cycles through each measure-
ment in sequence, and it continuously loops through the
sequence approximately once every second. Each mea-
sured value is compared to values stored in WATCHDOG, or
Limit registers. When the measured value violates the pro-
grammed limit the LM78 will set a corresponding Interrupt in
the Interrupt Status Registers. Two hardware Interrupt lines,
SMI and NMI/IRQ, are fully programmable with separate
masking of each Interrupt source, and masking of each
output. In addition, the Configuration Register has control
bits to enable or disable the hardware Interrupts.
Additional digital inputs are provided for chaining of SMI
(System Management Interrupt), outputs of multiple external
LM75 temperature sensors via the BTI (Board Temperature
Interrupt) input, and a Chassis Intrusion input. The Chassis
Intrusion input is designed to accept an active high signal
from an external circuit that latches when the case is re-
moved from the computer.
• Serial Bus Address Register: Contains the Serial Bus
• Chip Reset/ID Register: Allows reseting of all the reg-
• POST RAM:
• Value RAM:
address. At power on it assumes the default value of
0101101 binary, and can be altered via the ISA or Serial
Bus interface.
isters to the default power-on reset value. Provides a bit
for identification between the current version of this de-
vice and an older version which does not have this reset
capability.
POST codes. Overflow of the POST RAM will set an
Interrupt. The POST RAM, located at base address x0h
and x4h, allows for easy decoding to address 80h and
84h, the normal addresses for outputting of POST codes.
Interrupt will only be set when writing to port x0h or x4h.
The POST RAM can be read via ports 85h and 86h.
ages, fan counts, and WATCHDOG limits are all con-
tained in the Value RAM. The Value RAM consists of a
total of 64 bytes. The first 11 bytes are all of the results,
the next 19 bytes are the WATCHDOG limits, and are
located at 20h-3Fh, including two unused bytes in the
upper locations. The next 32 bytes, located at 60h-7Fh,
mirror the first 32 bytes with identical contents. The only
difference in the upper bytes are that they auto-increment
the LM78 Internal Address Register when read from or
written to via the ISA bus (auto-increment is not available
for Serial Bus communications).
DS012873-8
The monitoring results: temperature, volt-
FIFO RAM to store up to 32 bytes of 8-bit
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