ADM1021A
Rev. 7 | Page 14 of 19 | www.onsemi.com
When reading data from a register there are two possibilities:
1.
If the ADM1021A’s address pointer register value is
unknown or not the desired value, it is first necessary to
set it to the correct value before data can be read from the
desired data register. This is done by performing a write to
the ADM1021A as before, but only the data byte contain-
ing the register read address is sent, because data is not to
be written to the register. This is shown in Figure 16.
A read operation is then performed consisting of the
serial bus address, R/
W
bit set to 1, followed by the data
byte read from the data register. This is shown in Figure 17.
2.
If the address pointer register is known to be already at
the desired address, data can be read from the
corresponding data register without first writing to the
address pointer register, so Figure 16 can be omitted.
NOTES
1.
Although it is possible to read a data byte from a data
register without first writing to the address pointer
register, if the address pointer register is already at the
correct value, it is not possible to write data to a register
without writing to the address pointer register; this is
because the first data byte of a write is always written to
the address pointer register.
2.
Remember that the ADM1021A registers have different
addresses for read and write operations. The write address
of a register must be written to the address pointer if data
is to be written to that register, but it is not possible to
read data from that address. The read address of a register
must be written to the address pointer before data can be
read from that register.
ALERT
OUTPUT
The
ALERT
output goes low whenever an out-of-limit
measurement is detected, or if the remote temperature sensor is
open-circuit. It is an open drain and requires a 10 kΩ pull-up to
V
DD
. Several
ALERT
outputs can be wire-ANDed together so
the common line goes low if one or more of the
ALERT
outputs
goes low.
The
ALERT
output can be used as an interrupt signal to a
processor, or it can be used as an
SMBALERT
. Slave devices on
the SMBus cannot normally signal to the master that they want
to talk, but the
SMBALERT
function allows them to do so.
One or more
ALERT
outputs are connected to a common
SMBALERT
line connected to the master. When the
SMBALERT
line is pulled low by one of the devices, the
following procedure occurs, as shown in Figure 18.
00056-018
MASTER
RECEIVES
SMBALERT
MASTER SENDS
ARA AND READ
COMMAND
NO
ACK
START ALERT RESPONSE ADDRESS
RD
ACK DEVICE ADDRESS
STOP
DEVICE SENDS
ITS ADDRESS
Figure 18. Use of SMBALERT
1.
SMBALERT
is pulled low.
2.
Master initiates a read operation and sends the alert
response address (ARA = 0001 100). This is a general call
address that must not be used as a specific device address.
3.
The device whose
ALERT
output is low responds to the
alert response address and the master reads its device
address. The address of the device is now known and it
can be interrogated in the usual way.
4.
If more than one device’s
ALERT
output is low, the one
with the lowest device address has priority, in accordance
with normal SMBus arbitration.
5. Once the ADM1021A has responded to the alert response
address, it resets its
ALERT
output, provided that the
error condition that caused the
ALERT
no longer exists. If
the
SMBALERT
line remains low, the master sends the
ARA again, and so on until all devices whose
ALERT
outputs were low have responded.
LOW POWER STANDBY MODES
The ADM1021A can be put into a low power standby mode
using hardware or software, that is, by taking the
STBY
input
low, or by setting Bit 6 of the configuration register. When
STBY
is high or Bit 6 is low, the ADM1021A operates normally.
When
STBY
is pulled low or Bit 6 is high, the ADC is inhibited,
so any conversion in progress is terminated without writing the
result to the corresponding value register.
The SMBus is still enabled. Power consumption in the standby
mode is reduced to less than 10 μA if there is no SMBus activity,
or 100 μA if there are clock and data signals on the bus.
These two modes are similar but not identical. When
STBY
is
low, conversions are completely inhibited. When Bit 6 is set but
STBY
is high, a one-shot conversion of both channels can be
initiated by writing 0xXX to the one-shot register (Address 0x0F).
