Datasheet

MAX31865
RTD-to-Digital Converter
15Maxim Integrated
Table 6. Fault Threshold Registers Definition
Table 7. Fault Status Register Definition
Fault Threshold Registers (03h–06h)
The High Fault Threshold and Low Fault Threshold
registers select the trip thresholds for RTD fault detection.
The results of RTD conversions are compared with the
values in these registers to generate the “Fault” (D[7:6])
bits in the Fault Status register. The RTD Data Registers,
High Fault Threshold Registers, and Low Fault Threshold
Registers all have the same format.
The RTD High bit in the Fault Status Register is set if the
RTD resistance register value is greater than or equal to
the value in the High Fault Threshold register. The POR
value of the High Fault Threshold register is FFFFh.
The RTD Low bit in the Fault Status Register is set if the
RTD resistance value is less than or equal to the value in
the Low Fault Threshold register. The POR value of the
Low Fault Threshold register is 0000h.
Fault Status Register (07h)
The Fault Status register latches any detected fault bits;
writing a 1 to the Fault Status Clear bit in the Configuration
Register returns all fault status bits to 0.
Serial Interface
The MAX31865 supports SPI modes 1 and 3. Four pins
are used for SPI-compatible communications: SDO
(serial-data out), SDI (serial-data in), CS (chip select),
and SCLK (serial clock). SDI and SDO are the serial-
data input and output pins for the devices, respectively.
The CS input initiates and terminates a data transfer.
SCLK synchronizes data movement between the master
(microcontroller) and the slave (MAX31865).
The serial clock (SCLK), which is generated by the
microcontroller, is active only when CS is low and dur-
ing address and data transfer to any device on the
X = Don’t care
X = Don’t care
REGISTER
HIGH FAULT THRESHOLD MSB (03h) REGISTER HIGH FAULT THRESHOLD LSB (04h) REGISTER
LOW FAULT THRESHOLD MSB (05h) REGISTER LOW FAULT THRESHOLD MSB (06h) REGISTER
Bit
D7 D6 D5 D4 D3 D2 D1 D0 D7 D6 D5 D4 D3 D2 D1 D0
RTD
Resistance
Data
MSB — — — — — — — — — — — — — LSB X
Bit
Weighting
2
14
2
13
2
12
2
11
2
10
2
9
2
8
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
—
Decimal
Value
16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1 —
D7 D6 D5 D4 D3 D2 D1 D0
RTD High
Threshold
RTD Low
Threshold
REFIN- >
0.85 x V
BIAS
REFIN- < 0.85 x V
BIAS
(FORCE- open)
RTDIN- < 0.85 x
V
BIAS
(FORCE- open)
Overvoltage/
undervoltage fault
x x