Rainbow Electronics LM81 - Manual (Page 14)

Functional Description
(Continued)
3.0 USING THE LM81
3.1 Power On
When power is first applied, the LM81 performs a “power on
reset” on several of its registers. The power on condition of
the LM81’s registers in shown in
Table 1 Registers whose
power on values are not shown have power on conditions
that are indeterminate (this includes the value RAM ,exclu-
sive of the DAC data, and WATCHDOG limits). When power
is first applied the ADC is inactive. In most applications,
usually the first action after power on is usually to write
WATCHDOG limits into the Value RAM. Register values can
be returned to their default values after power is applied to
the LM81 by taking RESET low for at least 50 ns.
3.2 Resets
Configuration Register INITIALIZATION accomplishes the
same function as power on reset on most registers. The
Value RAM conversion results, and Value RAM WATCH-
DOG limits are not Reset and will be indeterminate immedi-
ately after power on. If the Value RAM contains valid con-
version results and/or Value RAM WATCHDOG limits have
been previously set, they will not be affected by a Configu-
ration Register INITIALIZATION. Power on reset, or Configu-
ration Register INITIALIZATION, clear or initialize the follow-
ing registers (the initialized values are shown on
Table 1:
• Configuration Register
• Interrupt Status Register 1
• Interrupt Status Register 2
• INT Mask Register 1
• INT Mask Register 2
• VID/Fan Divisor Register
• Serial Bus Address Register (Power on reset only, not
reset by Configuration Register INITIALIZATION)
• VID4 Register
• Temperature Configuration Register
• Extended Mode Register 1
• Extended Mode Register 2
Configuration Register INITIALIZATION is accomplished by
setting Bit 7 of the Configuration Register high. This bit
automatically clears after being set.
3.3 Using the Configuration Register
The Configuration Register controls the LM81 operation. At
power on, the ADC is stopped and INT_Clear is asserted,
clearing the INT hardwire output. The Configuration Register
starts and stops the LM81, enables and disables interrupt
output, and provides the Reset function described in
Section
3.2.
Bit 0 of the Configuration Register controls the monitoring
loop of the LM81. Setting Bit 0 low stops the LM81 monitor-
ing loop and puts the LM81 in shutdown mode, reducing
power consumption. Serial Bus communication can take
place with any register in the LM81 although activity on the
SMBData and SMBCLK 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 INT Interrupt
hardwire output when this bit is taken high.
Bit 3 of the Configuration Register clears the INT output
when set high, without affecting the contents of the Interrupt
Status Registers. The LM81 will stop monitoring. It will re-
sume upon clearing of this bit.
Bit 4 of the Configuration Register provides an active low 20
ms pulse at the RESET output when set high.
The CI_Clear provides an active low 20 ms pulse at the CI
output pin when set high. This is intended for resetting the
Chassis Intrusion circuitry.
The INITIALIZATION bit resets the internal registers of the
LM81 as described in
Section 3.2.
3.4 Starting Conversions
The monitoring function (Analog inputs, temperature, and
fan speeds) in the LM81 is started by writing to the Configu-
ration Register and setting INT_Clear (Bit 3), low, and Start
(bit 0), high. The LM81 then performs a “round-robin” moni-
toring of all analog inputs, temperature, and fan speed inputs
approximately once every 400 ms. The sequence of items
being monitored corresponds to locations in the Value RAM
and is:
1. Temperature
2. Vccp2
3. +12Vin
4. +5Vin
5. +3.3Vin
6. Vccp1
7. +2.5Vin
8. Fan 1
9. Fan 2
DACOut immediately changes after the DAC Data Register
in the Value RAM has been updated. For a zero to full scale
transition DACOut will typically settle within 100 µsec of the
stop by master in the write to the DAC Data Register Serial
Bus transaction. The DAC Data Register is not reset by the
INITIALIZATION bit found in the Configuration Register.
LM81
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