Datasheet

AD5421 Data Sheet
Rev. G | Page 28 of 36
CONTROL REGISTER
The control register is a read/write register and is addressed as described in Table 12. The data programmed to the control register
determines the mode of operation of the AD5421.
Table 17. Control Register Bit Map
MSB LSB
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
SPI watchdog timeout SPI
watchdog
timer
Auto fault
readback
Alarm on
SPI fault
Set min
loop
current
Select
ADC
input
On-chip
ADC
Power down
internal
reference
V
LOOP
fault
alert
Reserved
T0 T1 T2
Table 18. Control Register Bit Descriptions
Control Bits
Description
SPI watchdog
timeout
The T0, T1, and T2 bits allow the user to program the watchdog timeout period. The watchdog timer is reset when a valid
write to any AD5421 register occurs or when a NOP command is written.
T0 T1 T2 Timeout Period
0 0 0 50 ms
0 0 1 100 ms
0 1 0 500 ms
0
1
1
1 sec (default)
1 0 0 2 sec
1 0 1 3 sec
1 1 0 4 sec
1 1 1 5 sec
SPI watchdog
timer
0 = SPI watchdog timer is enabled (default).
1 = SPI watchdog timer is disabled.
Auto fault
readback
This bit specifies whether the fault register contents are automatically clocked out on the SDO pin on each write operation.
(The fault register can always be addressed for readback.)
0 = fault register contents are clocked out on the SDO pin (default).
1 = fault register contents are not clocked out on the SDO pin.
Alarm on SPI
fault
This bit specifies whether the loop current is forced to the alarm value when an SPI fault is detected (that is, the watchdog
timer times out). When an SPI fault is detected, the SPI fault bit of the fault register and the FAULT pin are always set.
0 = loop current is forced to the alarm value when an SPI fault is detected (default).
1 = loop current is not forced to the alarm value when an SPI fault is detected.
Set min loop
current
0 = normal operation (default).
1 = loop current is set to its minimum value so that the total current flowing in the loop consists only of the operating
current of the AD5421 and its associated circuitry.
Select ADC
input
0 = on-chip ADC measures the voltage between the V
LOOP
and COM pins (default).
1 = on-chip ADC measures the temperature of the AD5421 die.
On-chip ADC 0 = on-chip ADC is disabled (default).
1 = on-chip ADC is enabled.
Power down
internal
reference
0 = internal voltage reference is powered up (default).
1 = internal voltage reference is powered down and an external voltage reference source is required.
V
LOOP
fault
alert
This bit specifies whether the FAULT pin is set when the voltage between the V
LOOP
and COM pins falls to approximately 0.3 V.
(The V
LOOP
6V bit of the fault register is always set.)
0 = FAULT pin is not set when the V
LOOP
− COM voltage falls to approximately 0.3 V.
1 = FAULT pin is set when the V
LOOP
− COM voltage falls to approximately 0.3 V.