MLX90308CAB Datasheet, PDF (10/26 Pages) List of Unclassifed Manufacturers – Programmable Sensor Interface

MLX90308CAB
Programmable Sensor Interface
Digital Features
Microprocessor, LX11 Core, Interrupt
Controller, Memories
The LX11 microcontroller core is described in its own
datasheet. As an overview, this implementation of the
LX11 RISC core has following resources:
Two accumulators, one index and two interrupt
accumulators.
15 - 8 bit I/O ports to internal resources.
64 byte RAM.
4 kbytes ROM : 3 kbytes is available for the
customer's application firmware. 1k is
reserved for test.
48 x 8 bit EEPROM.
Four interrupt sources, two UART interrupts
and two timers.
UART
The serial link is a potentially full-duplex UART. It is
receive-buffered, in that it can receive a second byte
before a previously received byte has been read from
the receiving register. However, if the first byte is not
read by the time the reception of the second byte is
completed, the first byte will be lost. The UART's
baud rate depends on the RC-oscillator's frequency
and the "TURBO"-bit (see output port). Transmitted
and received data has the following structure: start bit
= 0, 8 bits of data, stop bit = 1.
Sending Data
Writing a byte to port 1 automatically starts a
transmission sequence. The TX Interrupt is set when
the STOP-bit of the byte is latched on the serial line.
Receiving Data
Reception is initialized by a 1 to 0 transition on the
serial line (i.e., a START-bit). The baud rate period (i.
e., the duration of one bit) is divided into 16 phases.
The first six and last seven phases of a bit are not
used. The decision on the bit-value is then the result
of a majority vote of phase 7, 8 and 9 (i.e., the center
of the bit).
Spike synchronization is avoided by de-bouncing on
the incoming data and a verification of the START-bit
value. The RX Interrupt is set when the stop bit is
latched in the UART.
Timer
The clock of the timers TMI and TPI is taken directly
from the main oscillator. The timers are never
reloaded, so the next interrupt will take place 2x
oscillator pulses after the first interrupt.
Watch Dog
An internal watch dog will reset the whole circuit in
case of a software crash. If the watch dog counter is
not reset at least once every 26 milliseconds (@ 2.46
MHz main clock), the microcontroller and all the
peripherals will be reset.
Firmware
The MLX90308 firmware performs the signal
conditioning by either of two means: analog or
digital. The analog signal conditioning allows
separate offset and gain temperature coefficients for
up to four temperature ranges. Digital mode allows
for all of the analog capabilities plus up to five
different gain values based on the input signal level.
Also available in both modes is the capability of
range limiting and level steering.
Temperature Processing
In both analog and digital modes, the temperature
reading controls the temperature compensation. This
temperature reading is filtered as designated by the
user. The filter adjusts the temperature reading by
factoring in a portion of the previous value. This
helps to minimize the effect of noise when using an
external temperature sensor. The filter equation is:
If measured_temp > Temp_f(n) then
Temp_f(n+1) = Temp_f(n) + [measured_temp -
Temp_f(n)] / [2 ^n_factor].
If measured_temp < Temp_f(n), then
Temp_f(n+1) = Temp_f(n) - [measured_temp -
Temp_f(n)] [2 ^n_factor].
Temp_f(n+1) = new filtered temperature value.
Temp_f(n) = previous filtered temperature
value.
Measured_temp = Value from temperature A to
D.
N_factor = Filter value set by the user (four
LSB’s of byte 25 of EEPROM), range 0-15.
The filtered temperature value, Temp_f, is stored in
RAM bytes 58 and 59. The data is a 10 bit value, left
justified in a 16 bit field.
MLX90308CAB Programmable Sensor Interface
Page 10
Rev 3.0 21/May/01