Rainbow Electronics MAX6660 - Manual (Page 10)

MAX6660
Remote-Junction Temperature-Controlled
Fan-Speed Regulator with SMBus Interface
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The SMBus interface includes a Timeout, which resets
the interface any time the data or clock line is held low
for more than 35ms, ensuring that the MAX6660 can
never “lock” the bus.
Remote Temperature Data Register
Two registers, at addresses 00h and 01h, store the
measured temperature data from the remote diode. The
data format for the remote-diode temperature is 10 bit
+ sign, with each bit corresponding to 0.125°C, in two’s
complement format (Table 2). Register 01h contains the
sign bit and the first 7 bits. Bits 7, 6, 5 of Register 00h
are the 3LSBs. If the two registers are not read at the
same time, their contents may be the result of two dif-
ferent temperature measurements leading to erroneous
temperature data. For this reason, a parity bit has been
added to the 00h register. Bit 4 of this is zero if the data
in 00h and 01h are from the same temperature conver-
sion and are 1 if they are not. The remaining bits are
“don’t cares.” When reading temperature data, register
01h must be read first.
Alarm Threshold Registers
The MAX6660 provides four alarm threshold registers
that can be programmed with a two’s complement tem-
perature value with each bit corresponding to 1°C. The
registers are T
HIGH , T LOW , T MAX , and T HYST . If the
measured temperature equals or exceeds T HIGH , or is
less than T LOW , an ALERT interrupt is asserted. If the
measured temperature equals or exceeds T MAX , the
OVERT output is asserted (see Over-Temperature
Output (
OVERT) section). If ALERT and OVERT are acti-
vated by the temperature exceeding T MAX , they can
only be deasserted by the temperature dropping below
T
HYST . The POR state for T HIGH is +127°C, for T LOW is -
55°C, for T MAX is +100°C, and for T HYST is +95°C.
Over-Temperature Output (
OVERT
)
The MAX6660 has an over-temperature output ( OVERT)
that is set when the remote-diode temperature crosses
the limits set in the T
MAX register. It is always active if
the remote-diode temperature exceeds T MAX . The
OVERT line clears when the temperature drops below
T
HYST . Bit 1 of the Configuration register can be used
to mask the
OVERT output. Typically, the OVERT output
is connected to a power-supply shutdown line to turn
system power off. At power-up,
OVERT defaults to
active-low but the polarity can be reversed by setting
bit 5 of the Configuration register.
The
OVERT line can be taken active, either by the
MAX6660 or driven by an external source. An external
source can be masked by bit 2 of the Configuration
register. When
OVERT is active, the fan loop forces the
fan to full speed and bit 1 of the Status register is set.
Diode Fault Alarm
A continuity fault detector at DXP detects an open cir-
cuit between DXP and DXN. If an open or short circuit
exists, register 01h is loaded with 000 0000.
Additionally, if the fault is an open circuit, bit 2 of the
status byte is set to 1 and the ALERT condition is acti-
vated at the end of the conversion. Immediately after
POR, the Status register indicates that no fault is pre-
sent until the end of the first conversion.
ALERT Interrupts
The ALERT interrupt output signal is activated (unless it
is masked by bit 7 in the Configuration register) when-
ever the remote-diode’s temperature is below T
LOW or
exceeds T HIGH . A disconnected remote diode (for con-
tinuity detection), a shorted diode, or an active
OVERT
also activates the ALERT signal. The activation of the
ALERT signal sets the corresponding bits in the Status
register. There are two ways to clear the
ALERT: send-
ing the ALERT Response Address or reading the Status
register.
The interrupt does not halt automatic conversions. New
temperature data continues to be available over the
SMBus interface after
ALERT is asserted. ALERT is an
active-low open-drain output so that devices can share
a common interrupt line. The interrupt is updated at the
end of each temperature conversion so, after being
cleared, reappears after the next temperature conver-
sion, if the cause of the fault has not been removed.
By setting bit 0 in the Configuration register to 1, the
Status register can only be cleared by sending the
SMBus Alert Response Address (see Alert Response
Address section). Prior to taking corrective action,
always check to ensure that an interrupt is valid by
reading the current temperature. To prevent recurring
interrupts, the MAX6660 asserts
ALERT only once per
crossing of a given temperature threshold. To enable a
new interrupt, the value in the limit register that trig-
gered the interrupt must be rewritten. Other interrupt
conditions can be caused by crossing the opposite
temperature threshold, or a diode fault can still cause
an interrupt.
Example: The remote temperature reading crosses
T
HIGH , activating ALERT. The host responds to the
interrupt and reads the Alert Response Address, clear-
ing the interrupt. The system may also read the status
byte at this time. If the condition persists, the interrupt
reappears. Finally, the host writes a new value to
T
HIGH . This enables the device to generate a new
T HIGH interrupt if the alert condition still exists.