Rainbow Electronics MAX6656 - Manual (Page 8)

MAX6655/MAX6656
The temperature data is stored in internal registers
RRTE, RRT2, and RLTS as 7 bits + sign in two’s com-
plement form with each LSB representing 1°C.
Additionally, the 3MSBs of the Extended Temperature
register contain fractional temperature data with
+0.125°C resolution (Tables 2 and 3). The voltage data
is stored in RV0, RV1, RV2, and RV3 as 8 bits in binary
form (Table 4).
OVERT
Output
OVERT output is an unlatched open-drain output that
behaves as a thermostat for fan control or system shut-
down (Figure 4). This output responds to the current
temperature. If the current temperature is above T
HIGH
,
OVERT activates and does not go inactive until the tem-
perature drops below T
HIGH .
Diode Fault Alarm
A continuity fault detector at DXP detects whether the
remote diode has an open-circuit condition, short-cir-
cuit to GND, or short-circuit DXP-to-DXN condition. At
the beginning of each conversion, the diode fault is
checked, and the Status Byte is updated. This fault
detector is a simple voltage detector; if DXP rises
above V
CC - 1V (typ) or below V DXN + 50mV (typ), a
fault is detected. Note that the diode fault isn’t checked
until a conversion is initiated, so immediately after POR,
the status byte indicates no fault is present, even if the
diode path is broken.
If the remote channel is shorted (DXP to DXN or DXP to
GND), the ADC reads 1111 1111 so as not to trip either
the T HIGH or T LOW alarms at their POR settings.
Similarly, if DXP_ is short circuited to V
CC
, the ADC
reads -1°C for both remote channels, and the ALERT
outputs are activated.
Alert
Interrupts
Normally, the ALERT interrupt output signal is latched
and can be cleared either by responding to the Alert
Response Address or by reading the Status register.
Interrupts are generated in response to T
HIGH and
T LOW , V HIGH and V LOW comparisons, and when the
remote diode is faulted. The interrupt does not halt auto-
matic conversions; new temperature data continues to
be available over the SMBus interface after
ALERT is
asserted. The interrupt output pin is open-drain so multi-
ple devices can share a common interrupt line.
The interface responds to the SMBus Alert Response
address, an interrupt pointer return-address feature
(see the Alert Response Address section). Before tak-
ing corrective action, always check to ensure that an
interrupt is valid by reading the current temperature.
The alert activates only once per crossing of a given
temperature threshold to prevent any reentrant inter-
rupts. To enable a new interrupt, rewrite the value of the
violated temperature threshold.
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 mas-
ter can broadcast a Receive Byte transmission to the
Alert Response slave address (0001100). Any slave
device that generated an interrupt then attempts to identi-
fy itself by putting its own address on the bus (Table 5).
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
acknowledgment and continues to hold the
ALERT line
low until serviced (implies that the host interrupt input is
level sensitive). The alert is cleared after the slave
address is returned to the host.
Command Byte Functions
The 8-bit Command Byte register (Table 6) is the mas-
ter index that points to the other registers within the
MAX6655/MAX6656. 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 internal temperature data.
Dual Remote/Local Temperature Sensors and
Four-Channel Voltage Monitors
8 _______________________________________________________________________________________
V CC
OVERT
MAX6655
MAX6656
SMBDATA
SMBCLK
ADD0
ADD1
GND
DXP2
DXN2
2200pF 2N3906
TO SYSTEM
SHUTDOWN
SMBus
SERIAL
INTERFACE
(TO HOST)
+3V TO +5.5V
ALERT
Figure 4. System Shutdown Application