Rainbow Electronics MAX6649 - Manual (Page 9)

MAX6646/MAX6647/MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensors with Overtemperature Alarms
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The alert response can activate several different slave
devices simultaneously, similar to the I
2 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 com-
pletion of the read alert response protocol clears the
interrupt latch, provided the condition that caused the
alert no longer exists.
OVERT
Overtemperature Alarm/Warning
Outputs
OVERT asserts when the temperature rises to a value
stored in one of the OVERT limit registers (19h, 20h). It
deasserts when the temperature drops below the stored
limit, minus hysteresis. OVERT can be used to activate a
cooling fan, send a warning, invoke clock throttling, or trig-
ger a system shutdown to prevent component damage.
Command Byte Functions
The 8-bit command byte register (Table 5) is the master
index that points to the various other registers within the
MAX6646/MAX6647/MAX6649. The register’s POR state
is 0000 0000, so a receive byte transmission (a protocol
that lacks the command byte) that occurs immediately
after POR, returns the current local temperature data.
The MAX6646/MAX6647/MAX6649 incorporate collision
avoidance so that completely asynchronous operation is
allowed between SMBus operations and temperature
conversions.
One-Shot
The one-shot command immediately forces a new con-
version cycle to begin. If the one-shot command is
received while the MAX6646/MAX6647/MAX6649 are in
standby mode (RUN bit = 1), a new conversion begins,
after which the device returns to standby mode. If a one-
shot conversion is in progress when a one-shot com-
mand is received, the command is ignored. If a one-shot
command is received in autonomous mode (RUN bit = 0)
between conversions, a new conversion begins, the con-
version rate timer is reset, and the next automatic conver-
sion 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 (dis-
able) interrupts. Bit 6 puts the MAX6646/MAX6647/
MAX6649 into standby mode (STOP) or autonomous
(RUN) mode.
Status Byte Functions
The status byte register (Table 7) indicates which (if
any) temperature thresholds have been exceeded. This
byte also indicates whether the ADC is converting and
whether there is an open-circuit fault detected in the
external sense junction. After POR, the normal state of
all flag bits is zero, assuming no alarm conditions are
present. The status byte is cleared by any successful
read of the status byte, after conversion is complete
and if the fault condition no longer exists. Note that the
ALERT interrupt latch is not automatically cleared when
the status flag bit indicating the ALERT is cleared. The
fault condition must be eliminated before the ALERT
output can be cleared.
When autoconverting, if the T
HIGH
and T
LOW
limits are
close together, it is possible for both high-temp and low-
temp status bits to be set, depending on the amount of
time between status read operations (especially when
converting at the fastest rate). In these circumstances, it
is best not to rely on the status bits to indicate reversals in
long-term temperature changes. Instead use a current
temperature reading to establish the trend direction.
BIT NAME
POR
STATE
FUNCTION
7 RFU 1
Reserved. Always write 1 to
this bit.
6 to 3 RFU 0
Reserved. Always write
zero to this bit.
2 FQ1 1
Fault queue-length control
bit (see Table 4).
1 FQ0 1
Fault queue-length control
bit (see Table 4).
0 RFU 0
Reserved. Always write
zero to this bit.
Table 3. Fault Queue Register Bit Definition
(22h)
FQ1 FQ0 FAULT QUEUE LENGTH (SAMPLES)
0 0 1
0 1 2
1 1 3
1 0 1
Table 4. Fault Queue Length Bit Definition
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under the Philips I 2 C Patent Rights to use these components in an I 2 C system, provided that the system conforms to the I 2 C Standard
Specification as defined by Philips.