Preliminary Technical Data
ADM1025/ADM1025A
Rev. P5 | Page 17 of 21| www.onsemi.com
USING THE ADM1025/ADM1025A
POWER-ON RESET
When power is first applied, the ADM1025/ADM1025A
performs a “power- on reset” on several of its registers.
Registers whose power-on values are not shown have power-on
conditions that are indeterminate. Value and limit registers are
reset to 00h on power-up. The ADC is inactive. In most
applications, usually the first action after power-on would be to
write limits into the Limit Registers.
Power-on reset clears or initializes the following registers (the
initialized values are shown in Table 9):
– Configuration Register
– Status Registers #1 and #2
– VID0-3 Register
– VID4 Register
– Test Register
INITIALIZATION
Configuration Register Initialization performs a similar, but not
identical, function to power-on reset.
Configuration Register Initialization is accomplished by setting
Bit 7 of the Configuration Register high. This bit automatically
clears after being set.
USING THE CONFIGURATION REGISTER
Control of the ADM1025/ADM1025A is provided through the
configuration register. The Configuration Register is used to
start and stop the ADM1025/ADM1025A, program the
operating modes of Pins 11 and 16, and provide the
initialization function described above.
Bit 0 of the Configuration Register controls the monitoring loop
of the ADM1025/ADM1025A. Setting Bit 0 low stops the
monitoring loop and puts the ADM1025/ADM1025A into a
low power mode thereby reducing power consumption. Serial
bus communication is still possible with any register in the
ADM1025/ADM1025A while in low power mode. Setting Bit 0
high starts the monitoring loop.
Bit 4 of the Configuration Register causes a low going 20 ms
(typ) pulse at the
RST
pin (Pin 16) when set. This bit is self-
clearing.
Bit 5 of the Configuration Register selects the operating mode
of Pin 11 between the default of 12 V analog input (Bit 5 = 0)
and VID4 (Bit 5 = 1).
Bit 7 of the Configuration Register is used to start a
Configuration Register Initialization when it is set to 1.
USING THE OFFSET REGISTER
This register contains a twos complement value that is added
(or subtracted if the number is negative) to either the internal
or external temperature reading. Note that the default value in
the offset register is zero, so zero is always added to the
temperature reading. The offset register is configured for the
external temperature channel by default. It may be switched to
the internal channel by setting Bit 0 of the Test Register to 1,
setting Bit 6 of the VID Register to 1, and clearing Bit 7 of the
VID Register.
STARTING CONVERSION
The monitoring function of the ADM1025/ADM1025A is
started by writing to the Configuration Register and setting
Start (Bit 0) high. Limit values should be written into the Limit
Registers before starting the ADC to avoid spurious out-of-limit
conditions. The time taken to complete the analog
measurements depends on how they are configured, as
described elsewhere. Once the measurements have been
completed, the results can be read from the Value Registers at
any time.
REDUCED POWER AND SHUTDOWN MODE
The ADM1025/ADM1025A can be placed in a low power mode
by setting Bit 0 of the Configuration Register to 0. This disables
the internal ADC. Full shutdown mode may then be achieved
by setting Bit 7 of the VID Register to 1 and Bit 0 of the Test
Register to 1. This turns off power to all analog circuits and
stops the monitoring cycle, if running, but it does not affect the
condition of any of the registers. The device will return to its
previous state when these bits are reset to zero.
5 V OPERATION
The ADM1025/ADM1025A may be operated with V
CC
connected to any supply voltage between 3.0 V and 5.5 V, but it
should be noted that the device has been optimized for 3.3 V
operation. In particular, the internal voltage divider used to
measure the supply voltage is optimized for 3.3 V. Powering the
device from 5 V will cause the V
CC
Reading Register (Register
25h) to overrange. In this case, the 5 V measurement should be
read from the 5 V Reading Register (Register 23h), instead of
the V
CC
Reading Register. Note also that when the 12 V
IN
/VID4
pin is programmed to read VID4, due to its internal voltage
divider, it will only read V
IH
= 2.1 V on the 12 V
IN
/VID4 pin as
logic high if the device is being powered from the 3.3 V supply.
