Datasheet

Data Sheet AD5421
Rev. G | Page 29 of 36
FAULT REGISTER
The read-only fault register is addressed as described in Table 12. The bits in the fault register indicate a range of possible fault conditions.
Table 19. Fault Register Bit Map
MSB LSB
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
SPI PEC I
LOOP
Over
I
LOOP
Under
Temp
140°C
Temp
100°C
V
LOOP
6V
V
LOOP
12V
V
LOOP
/temperature value
Table 20. Fault Register Bit Descriptions
Fault Alert
FAULT
Pin Set
Description
SPI Yes
This bit is set high to indicate the loss of the SPI interface signaling. This fault occurs if there is no valid
communication to the AD5421 over the SPI interface for more than the user-defined timeout period. The
occurrence of this fault also forces the loop current to the alarm value if Bit D10 of the control register is at
Logic 0. The alarm current direction is determined by the state of the ALARM_CURRENT_DIRECTION pin.
PEC (packet
error check)
Yes
This bit is set high when an error in the SPI communication is detected using cyclic redundancy check (CRC)
error detection. See the Packet Error Checking section for more information.
I
LOOP
Over Yes This bit is set high when the actual loop current is greater than the programmed loop current.
I
LOOP
Under Yes This bit is set high when the actual loop current is less than the programmed loop current.
Temp 140°C
Yes
This bit is set high to indicate an overtemperature fault. This bit is set if the die temperature of the AD5421
exceeds approximately 140°C. This bit is cleared when the temperature returns below approximately 125°C.
Temp 100°C No
This bit is set high to indicate an increasing temperature of the AD5421. This bit is set if the die temperature of the
AD5421 exceeds approximately 100°C. This bit is cleared when the temperature returns below approximately 85°C.
V
LOOP
6V Yes
This bit is set high when the voltage between the V
LOOP
and COM pins falls below approximately 0.3 V (representing
a 6 V loop supply voltage with 20:1 resistor divider connected at V
LOOP
). This bit is cleared when the voltage returns
above approximately 0.4 V.
V
LOOP
12V No
This bit is set high when the voltage between the V
LOOP
and COM pins falls below approximately 0.6 V (representing
a 12 V loop supply voltage with 20:1 resistor divider connected at V
LOOP
). This bit is cleared when the voltage returns
above approximately 0.7 V.
V
LOOP
/temper-
ature value
N/A
These eight bits represent either the voltage between the V
LOOP
and COM pins or the AD5421 die temperature,
depending on the setting of Bit D8 of the control register (see the On-Chip ADC Transfer Function Equations
section).
8-Bit Value V
LOOP
− COM Voltage (V) Die Temperature (°C)
00000000 0 +312
… … …
11111111 2.49 −86
On-Chip ADC Transfer Function Equations
The transfer function equation for the measurement of the
voltage between the V
LOOP
and COM pins is as follows:
V
LOOP
− COM = (2.5/256) × D
where D is the 8-bit digital code returned by the on-chip ADC.
The transfer function equation for the die temperature is
as follows:
Die Temperature = (−1.559 × D) + 312
where D is the 8-bit digital code returned by the on-chip ADC.