Datasheet

MAX31865
RTD-to-Digital Converter
12Maxim Integrated
Internal Registers
Communication is through eight 8-bit registers that
contain conversion, status, and configuration data. All
programming is done by selecting the appropriate
address of the desired register location. Table 1 illustrates
the addresses for the registers.
The registers are accessed using the 0Xh addresses
for reads and the 8Xh addresses for writes. Data is read
from or written to the registers MSB first.
Configuration Register (00h)
The configuration register selects the conversion mode
(automatic or triggered by the 1-shot command), enables
and disables BIAS pin output voltage V
BIAS
, initiates
1-shot conversions, selects the RTD connection (either
3-wire or 2-wire/4-wire), initiates a full fault detection
cycle, clears the Fault Status register, and selects the
filter notch frequencies. The effects of the configuration
bits are described below.
BIAS (D7)
When no conversions are being performed, V
BIAS
may
be disabled to reduce power dissipation. Write 1 to this
bit to enable V
BIAS
before beginning a single (1-Shot)
conversion. When automatic (continuous) conversion
mode is selected, V
BIAS
remains on continuously.
Conversion Mode (D6)
Write 1 to this bit to select automatic conversion mode, in
which conversions occur continuously at a 50/60Hz rate.
Write 0 to this bit to exit automatic conversion mode and
enter the “Normally Off” mode. 1-shot conversions may
be initiated from this mode.
1-Shot (D5)
When the conversion mode is set to “Normally Off”, write
1 to this bit to start a conversion. This causes a single
resistance conversion to take place. The conversion
is triggered when CS goes high after writing a 1 to
this bit. Note that if a multibyte write is performed, the
conversion is triggered when CS goes high at the end
of the transaction. If V
BIAS
is on (as selected by the
Configuration Register), the RTD voltage is sampled
when CS goes high and the conversion begins. Note
that if V
BIAS
is off (to reduce supply current between
conversions), any filter capacitors at the RTDIN inputs
need to charge before an accurate conversion can be
performed. Therefore, enable V
BIAS
and wait at least
10.5 time constants of the input RC network plus an
additional 1ms before initiating the conversion. Note that
a single conversion requires approximately 52ms in 60Hz
filter mode or 62.5ms in 50Hz filter mode to complete.
1-Shot is a self-clearing bit.
Table 1. Register Addresses and POR State
Table 2. Configuration Register Definition
REGISTER NAME READ ADDRESS (HEX) WRITE ADDRESS (HEX) POR STATE READ/WRITE
Configuration 00h 80h 00h R/W
RTD MSBs 01h — 00h R
RTD LSBs 02h — 00h R
High Fault Threshold MSB 03h 83h FFh R/W
High Fault Threshold LSB 04h 84h FFh R/W
Low Fault Threshold MSB 05h 85h 00h R/W
Low Fault Threshold LSB 06h 86h 00h R/W
Fault Status 07h — 00h R
D7 D6 D5 D4 D3 D2 D1 D0
V
BIAS
1 = ON
0 = OFF
Conversion
mode
1 = Auto
0 = Normally off
1-shot
1 = 1-shot
(auto-clear)
3-wire
1 = 3-wire RTD
0 = 2-wire or
4-wire
Fault Detection
Cycle Control
(see Table 3)
Fault Status
Clear
1 = Clear
(auto-clear)
50/60Hz filter
select
1 = 50Hz
0 = 60Hz