
Operation 
 ALERT
  Output 
- 10 - 
 Andigilog, Inc. 2006 
www.andigilog.com 
August 2006 - 70A05003 
aSC7512 
The aSC7512 has an emergency alarm function, 
 ALERT
 that is optionally assigned to pin 6, the TACH / 
 ALERT
 pin. 
 ALERT
 is determined by both high and low 
limits and will also respond to a remote diode open circuit 
failure. These limits are settable separately for Zone 1 
and Zone 2 sensors. Any alarm condition is reported 
individually in the status register and may be read at any 
time on the SMBus. Alarm conditions are logically 
combined and used to drive an open-drain output, the 
 ALERT
 output, (pin 6). 
This output pin may be used as an interrupt signal the 
CPU or to turn on remote drivers for fans or indicators. 
The 
 ALERT
 pin will remain asserted until it has been 
reset by the host via the SMBus.  
 ALERT
 Limits 
Figure 7 shows use of the 
 ALERT
 high and low limits. 
The user sets up the alarm by writing the upper and lower 
limit temperatures into the limit registers over the SMBus. 
After each measurement, the comparator tests the 
measurement exceeds the high limit is or is equal to or 
less-than the low limit, it will assert the particular alarm 
bits in the status register and cause the
 ALERT
 pin to go 
low. 
Figure 7  
 ALERT
 Limits and Responses 
The status bits will remain high until the status register is 
read and then, if the condition is no longer present those 
bits will be reset, otherwise they will remain high until the 
conditions are no longer met and the register is read 
again. The same sequence applies to the local readings 
and limits. 
The
 ALERT
 pin will remain low until the status bits have 
been reset and an Alert Response has been issued by 
the master and responded by the aSC7512.  This flow is 
described below. 
The user may mask-out or disable the 
 ALERT
 signal pin 
should it be necessary to prevent a processor interrupt. 
This is controlled by setting bit 7 of the configuration 
register.  
SMBus Alert Output 
The 
ALERT
 pin may be used to signal an SMBus Alert to 
the host processor. This is a special mode of the SMBus 
interface that requires the SMBus host to send an Alert 
Response Address (ARA) to all slaves sharing the 
ALERT
 pin in order to isolate clear and service the 
alerting device. This sequence is described below and in 
Figure 6. 
The sequence of servicing this interrupt is as follows: 
1. 
ALERT
 is asserted by the aSC7512 driving pin 
6 low. 
2. The SMBus master begins a read operation with 
a start followed by the ARA response address, 
0001 100. This is an SMBus General Call 
Address to be used only for requesting an alert 
response. 
3. The device providing the
 ALERT
 signal responds 
to this by providing an ACK followed by its own 
bus address, an aSC7512 will provide, 101 
1000, with the LSB of the data byte set to 1. A 
NACK response is expected from all devices not 
giving an 
ALERT
 . 
4. If more than one device responds, the device 
with the lowest device address will have priority 
and will be serviced first by the master. 
5. The service routine must read the status register 
of the alerting device to determine the nature of 
the alert. If the alerting condition is still present, 
the status bit will remain set, continuing to 
activate the 
ALERT
  pin. If the condition is 
removed, the status bit will be cleared and an 
additional 
ARA 
will 
ALERT
 pin. 
now 
de-assert 
the 
ZN1 Low
 ALERT
 Limit 
Temperature 
Conversion 
ZN1 High
 ALERT
 Limit 
Status Bit-4, RHIGH 
Status Bit-3, RLOW 
Status Register Read 
ARA Response 
ALERT
  Pin 6