Table 1. Debounce Settings
Reading Serial Data
The filtered outputs of the input comparators are
latched into a shift register at the falling-edge of CS.
Clocking the CLK pin, while CS is held low, shifts the
latched data out of SOUT 1 bit at a time.
The internal data serializer comprises a 16-bit shift
register, containing 8 bits of data corresponding to
the eight field inputs, as well as an 8-bit status byte
containing supplementary status and CRC information.
The status byte contains 1 bit representing the status
of the field-supply voltage (UV), 1 bit representing the
status of the internal temperature monitor (OT), a 5-bit
CRC code internally calculated and generated, and a
trailing 1 as a STOP bit.
The undervoltage (UV) bit is normally 0. If the supply
voltage falls below VOFFUVLO, the UV becomes a 1.
The UV bit returns to 0 once the supply voltage has
returned above VONUVLO.
The overtemperature (OT) bit is also normally 0. If the
junction temperature increases to above TALRM, the
OT bit becomes a 1. The bit returns to 0 once the junc-
tion temperature has returned below TALRM.
The CRC code can be used to check data integrity during
transfer from the device to an external microcontroller. In
applications where the integrity of data transferred is not
of concern, the CRC bits can be ignored. The CRC uses
the following polynomial:
P(x) = x5 + x4 + x2 + x0
The number of bits in the internal serializer can be
selected between 8 bits or 16 bits. The MODESEL
pin is used to configure the serializer as an 8-bit
(disabling the status byte) or 16-bit shift register.
In 8-bit mode, only the eight field input states are
transferred through the SPI port and the status byte is
ignored. Therefore, in multiple IC applications (input
channels greater than 8), if desired, only a single
status byte can be generated and transmitted for any
number of input channels.
The shift register contents are read only (no write
capability exists) through the SPI-compatible
interface.
For higher input counts than 8, multiple devices can
be cascaded. In this case, the SOUT pin of one
device should be connected to the SIN pin of the next
device, effectively cascading the internal shift regis-
ters. The CLK and CS pins of all the devices should be
connected together in this configuration. See the
Serial-Port Operation section for more detailed
information on operating the SPI interface.
Temperature Monitoring
The internal junction temperature of the device is con-
stantly monitored. An alarm is raised, by asserting the
FAULT pin, if the temperature rises above TALRM. In addi-
tion to asserting FAULT, the device sets the OT bit to a 1.
Supply Voltage Monitoring
A supply voltage monitor circuit constantly monitors
the field-supply voltage. If this voltage falls below a
threshold (VOFFUVLO), an alarm is raised by asserting
the FAULT pin, indicating that the part is experiencing
a fault condition and the data in the serializer is not
to be trusted. In addition, the device sets the UV bit
to a 1. Once the field-supply voltage has recovered
and goes above VONUVLO, the FAULT pin is released,
indicating normal operation of the part.
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MAX31911 Industrial, Octal, Digital Input Translator/Serializer
DB1 DB0 BINARY VALUE DEBOUNCE TIME
0000
01125Fs
1 0 2 0.75ms
1 1 3 3ms