Rainbow Electronics MAX6690 User Manual

Page 12

MAX6690

2°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface

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Alert

Interrupts

The

ALERT interrupt output signal is latched and can

only be cleared by either reading the status register or
by receiving an Alert Response address, if the fault
condition has ceased. Interrupts are generated in
response to T

HIGH

and T

LOW

comparisons and when

the remote diode is disconnected (for continuity fault
detection). The interrupt does not halt automatic con-
versions; new temperature data continues to be avail-
able over the SMBus interface after

ALERT is asserted.

The interrupt output pin is open drain so that multiple
devices can share a common interrupt line. The inter-
rupt rate can never exceed the conversion rate.

The MAX6690 responds to the SMBus Alert Response
address, an interrupt pointer return-address feature
(see Alert Response Address section). Prior to taking
corrective action, always check to ensure that an inter-
rupt is valid by reading the current temperature.

Alert Response Address

The SMBus Alert Response interrupt pointer provides
quick fault identification for simple slave devices that
lack the complex, expensive logic needed to be a bus
master. Upon receiving an

ALERT interrupt signal, the

host master can broadcast a Receive Byte transmission
to the Alert Response slave address (0001 100). Then
any slave device that generated an interrupt attempts
to identify itself by putting its own address on the bus
(Table 4).

The Alert Response can activate several different slave
devices simultaneously, similar to the I

C General Call.

If more than one slave attempts to respond, bus arbitra-
tion rules apply, and the device with the lower address
code wins. The losing device does not generate an
acknowledge and continues to hold the

ALERT line low

until cleared. (The conditions for clearing an

ALERT

vary depending on the type of slave device. Successful
reading of the alert response address clears the inter-
rupt latch provided that the condition that caused the

ALERT has already ceased. The ALERT is cleared after
the slave address has been returned to the host.)

Command Byte Functions

The 8-bit command byte register (Table 5) is the master
index that points to the various other registers within the
MAX6690. The register’s POR state is 0000 0000, so
that a Receive Byte transmission (a protocol that lacks
the command byte) that occurs immediately after POR
returns the current local temperature data. The one-
shot command immediately forces a new conversion
cycle to begin. If the one-shot command is received
when the MAX6690 is in software standby mode
(RUN/STOP bit = high), a new conversion is begun,
after which the device returns to standby mode. If a
conversion is in progress when a one-shot command is
received, the command is ignored. If a one-shot com-
mand is received in autoconvert mode (RUN/STOP bit
= low) between conversions, a new conversion begins,
the conversion rate timer is reset, and the next automat-
ic conversion takes place after a full delay elapses.

Configuration Byte Functions

The configuration byte register (Table 6) is a read-write
register with several functions. Bit 7 is used to mask
(disable) interrupts. Bit 6 puts the MAX6690 into soft-
ware standby mode (STOP) or autoconvert (RUN) mode.
Bit 5 selects the extended temperature range mode,
which allows temperature data to be read down to
-65°C. Bit 4 puts the MAX6690 into parasitic resistance
cancellation mode (PRCM), which can reduce tempera-
ture measurement errors due to resistance in series with
the sensing junction. Bit 3 should always be set to zero
(default value). Bits 2, 1, and 0 are internally set to zero.

Parasitic Resistance Cancellation

Mode

Resistance in series with the remote-sensing junction
causes conversion errors on the order of 0.5°C per
ohm. The MAX6690 can cancel the effect of parasitic
series resistance by using the PRCM. If bit 4 of the con-
figuration byte is set HIGH, then the PRCM is invoked,
provided the conversion rate is set

≤ 1Hz. If the conver-

sion rate is faster than this, then the setting of bit 4 in
the configuration register is ignored. In the PRCM, the
conversion time is doubled (to typically 500ms to read
both local and remote diodes) but external resistances
as high as 100

Ω can be compensated.

Table 4. Read Format for Alert Response
Address (0001100)

NAME

BIT

Logic 1

(LSB)

(MSB)

ADD7

ADD1

ADD2

ADD3

ADD4

ADD5

FUNCTION

Provide the current MAX6690
slave address that was latched at
POR (Table 9)

ADD6