LM78 National Semiconductor, LM78 Datasheet - Page 16

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

www.national.com
Functional Description
3.0 USING THE LM78
3.1 Power On
When power is first applied, the LM78 performs a “power on
reset” on several of its registers. The power on condition of
registers in shown in Table I. Registers whose power on
values are not shown have power on conditions that are
indeterminate (this includes the value RAM and WATCH-
DOG limits). The ADC is inactive. In most applications, usu-
ally the first action after power on would be to write WATCH-
DOG limits into the Value RAM.
3.2 Resets
Configuration Register INITIALIZATION accomplishes the
same function as power on reset on most registers. The
POST RAM, Value RAM conversion results, and Value RAM
WATCHDOG limits are not Reset and will be indeterminate
immediately after power on. If the Value RAM contains valid
conversion results and/or Value RAM WATCHDOG limits
have been previously set, they will not be affected by a
Configuration Register INITIALIZATION. Power on reset, or
Configuration Register INITIALIZATION, clear or initialize
the following registers (the initialized values are shown on
Table I):
Configuration Register INITIALIZATION is accomplished by
setting Bit 7 of the Configuration Register high. This bit
automatically clears after being set.
The LM78-J allows the user to perform an unconditional
complete Power-on reset by writing a one to Bit 5 of the Chip
Reset/ID Register. The LM78-J can be differentiated from
the LM78 without the J suffix by reading Chip Reset/ID
Register Bit 6. A high would indicate that the LM78-J is being
used. The LM78-J allows an unconditional complete
Power-on reset to be initiated by taking the IOWR and IORD
signal lines low simultaneously, for at least 50 ns, while CS is
high. The delay between consecutive IORD and IOWR
pulses should be greater than 50 ns to ensure that an
Power-on reset does not occur unintentionally.
In systems where the serial bus is only being used it may be
advantageous to take both IOWR and IORD to the system
reset pulse. In this way whenever the system is reset the
LM78-J will also be reset to a known state.
3.3 Using the Configuration Register
The Configuration Register provides all control over the
LM78. At power on, the ADC is stopped and INT__Clear is
asserted, clearing the SMI and NMI/IRQ hardwire outputs.
The Configuration Register starts and stops the LM78, en-
ables and disables interrupt outputs and modes, and pro-
vides the Reset function described in Section 3.2.
Bit 0 of the Configuration Register controls the monitoring
loop of the LM78. Setting Bit 0 low stops the LM78 monitor-
• Configuration Register
• Interrupt Status Register 1
• Interrupt Status Register 2
• SMI Mask Register 1
• SMI Mask Register 2
• NMI Mask Register 1
• NMI Mask Register 2
• VID/Fan Divisor Register
• Serial Bus Address Register (Power on reset only, not
reset by Configuration Register INITIALIZATION)
(Continued)
16
ing loop and puts the LM78 in shutdown mode, reducing
power consumption. ISA and Serial Bus communication is
possible with any register in the LM78 although activity on
these lines will increase shutdown current, up to as much as
maximum rated supply current, while the activity takes place.
Taking Bit 0 high starts the monitoring loop, described in
more detail subsequently.
Bit 1 of the Configuration Register enables the SMI Interrupt
hardwire output when this bit is taken high. Similarly, Bit 2 of
the Configuration Register enables the NMI/IRQ Interrupt
hardwire output when taken high. The NMI/IRQ mode is
determined by Bit 5 in the Configuration Register. When Bit
5 is low the output is an active low IRQ output. Taking Bit 5
high inverts this output to provide an active high NMI output.
The Power Switch Bypass provides an active low at the
Power Switch Bypass output when set high. This is intended
for use in software power control by activating an external
power control MOSFET.
3.4 Starting Conversion
The monitoring function (Analog inputs, temperature, and
fan speeds) in the LM78 is started by writing to the Configu-
ration Register and setting INT__Clear (Bit 3), low, and Start
(bit 0), high. The LM78 then performs a “round-robin” moni-
toring of all analog inputs, temperature, and fan speed inputs
approximately once a second. The sequence of items being
monitored corresponds to locations in the Value RAM and is:
1. Temperature
2. IN0
3. IN1
4. IN2
5. IN3
6. IN4
7. -IN5
8. -IN6
9. Fan 1
10. Fan 2
11. Fan 3
3.5 Reading Conversion Results
The conversion results are available in the Value RAM.
Conversions can be read at any time and will provide the
result of the last conversion. Because the ADC stops, and
starts a new conversion whenever it is read, reads of any
single value should not be done more often then once every
120 ms. When reading all values, allow at least 1.5 seconds
between reading groups of values. Reading more frequently
than once every 1.5 seconds can also prevent complete
updates of Interrupt Status Registers and Interrupt Output’s.
A typical sequence of events upon power on of the LM78
would consist of:
1. Set WATCHDOG Limits
2. Set Interrupt Masks
3. Start the LM78 monitoring process
4.0 ANALOG INPUTS
The 8-bit ADC has a 16 mV LSB, yielding a 0V to 4.08V
(4.096–1LSB) input range. This is true for all analog inputs.
In PC monitoring applications these inputs would most often
be connected to power supplies. The 2.5V and 3.3V supplies
can be directly connected to the inputs. The 5V and 12V
inputs should be attenuated with external resistors to any
desired value within the input range.