A Digital Thermometer Using the AT89C2051 Microcontroller(0597).pdf

(103 KB) Pobierz
A Digital Thermometer Using the
AT89C2051 Microcontroller
Introduction
The system presented in this application
note implements a simple digital ther-
mometer that includes a built-in LCD and
RS-485 communication port. It is
designed around Atmel’s AT89C2051
processor, a DS1620 digital thermome-
ter/thermostat from Dallas Semiconduc-
tor, a small 8 X 2 LED backlit LCD, and
an RS485 line interface. The system,
shown in Figure 1, can be used as the
basis for developing custom solutions for
networked and stand alone data collec-
tion and control equipment. It can be
centrally powered due to its low current
requirement and its small size allows it to
be placed almost anywhere.
and handles the newline character by
advancing the cursor to the beginning of
the next line.
PositionLcd simply sets the cursor
address to the value specified by the
caller. The ClearLcd function is used to
clear the entire LCD and home the cur-
sor.
The functions that follow concern them-
selves with actual physical communica-
tion to the LCD. Since there is not a
direct correspondence between the
LSI’s data RAM and the LCD’s physical
mapping, it is necessary to keep watch
for certain boundary conditions. When a
boundary is encountered, the cursor
must be repositioned in order to keep the
output contiguous. Since all displayable
data must pass through DataWr, it
makes sense to contain the corrective
actions here. To handle this problem,
keep track of the logical cursor position
and invoke a remedial maneuver when-
ever discontinuity may occur. There are
two ways you can accomplish this:
• Read the LCD’s status register (to get
the cursor address) or
• Keep a local copy just for your
reference.
Not wanting to waste a pin to control the
LCD’s read/write line (it runs in write-only
mode), the latter approach was adopted.
Here, the global register (IRAM actually)
variable Cursor is used for this purpose.
Cursor is consulted prior to any data
write operation. If a correction is neces-
sary, a new cursor address is generated
and dispatched to the LCD control regis-
ter via CommandWr.
Following this, DataWr splits the data
byte into nibbles (remember the LCD
operates using a 4-bit bus) and falls
AT89C2051
Flash
Microcontroller
Application
Note
Software
The LCD driver is written entirely in C
and compiles under Micro-C (from Dun-
field Development Systems) using the
tiny memory model. Although a canoni-
cal stack-based implementation, Micro-C
includes a number of special features
that make it quite suitable for generating
ROMable code for small systems. The
overhead incurred performing stack
manipulations is made up by the library
functions that are all hand coded in an
highly optimized assembler. As an
added benefit, Micro-C comes with fully
documented library source code so spe-
cial modifications can be made as cir-
cumstances arise.
The first few functions contained in the
LCD driver module are conventional C
library implementations. PutString dis-
plays a null terminated string by merely
passing characters off to PutChar until a
null byte is encountered. PutChar out-
puts a character at a time to the LCD
0597A-B–12/97
5-59
through to handle the actual physical transfer. Using Micro-
C’s extended preprocessor allows bit manipulation macros
that expand directly to 8051 SETB and CLR instructions.
Here, clearing DRS selects the LCD’s data register and
DEN is toggled to generate the data strobe. CommandWr
operates similarly but does not have to deal with any cursor
entanglements. It selects the command register as its desti-
nation by setting DRS high prior to clocking the nibbles
across the interface.
The initialization function InitLcd begins at a more rudimen-
tary nibble oriented level since no assumption can be made
as to the operational status of the LCD at this time. The first
three sequences ensure that the transfer mode is set to
operate over a 4-bit bus. Repeating the sequence three
times ensures that the command will be recognized regard-
less of the operational mode of the LSI. (It is wise to make
no assumptions when performing any low-level initializa-
tion.) Following this, the actual operating parameters are
transferred to the LCD using the standard CommandWr
function. Software for this application note may be down-
loaded from Atmel’s Web Site or BBS.
nated by pulling \RST low which forces DQ into a high
impedance state and concludes the transfer. Temperature
data is transmitted over the 3-wire bus in lsb first format. A
total of nine bits are transmitted where the most significant
bit is the sign bit. If all nine bits are not of interest, the trans-
fer can be terminated at any time by asserting \RST.
The DS1620 support routines are coded in assembly. The
DS1620 also has nonvolatile EEPROM configuration regis-
ters that hold thermostatic and operational control informa-
tion. TempConfig is hardcoded to set the mode for opera-
tion under CPU control and continuous temperature con-
version. Once in continuous conversion mode, the actual
conversion process is started by issuing the start conver-
sion command through TempConvert. Now the DS1620
can be read at any time and the last temperature conver-
sion that was performed will be returned. This is accom-
plished by calling TempRead. The result is returned in the
16 bit accumulator as defined by Micro-C consisting of the
B (msb) and ACC (lsb) registers.
Mainline Glue
Coordination of the support drivers is managed by the main
module. This module takes control after the Micro-C startup
routine finishes. On entry, the code initializes the serial
port, instructs the DS1620 to start performing temperature
conversions, initializes the LCD, displays a log on mes-
sage, and enters into an endless loop. This loop continu-
ously reads the DS1620, performs a Celsius to Fahrenheit
conversion, translates the resulting binary number to an
ASCII string, and displays the conversion result on the
LCD. Periodically the code falls through and toggles an
LED and transmits the temperature data serially.
The Celsius to Fahrenheit conversion is performed using
the familiar equation: F = C * 9/5 + 32. Since the DS1620
returns temperature in .5°C increments the value is first
divided by 2. Unlike the often impenetrable gyrations that
result when working with numbers in assembler, the C rep-
resentation of this calculation is perfectly clear in intent and
function. A short sequence of divisions, modulos, and logi-
cal OR operations results in decimal ASCII values that
make up the output string.
Digital Temperature
Temperature acquisition is handled using the DS1620 ther-
mometer/thermostat IC from Dallas Semiconductor. The
DS1620 contains all temperature measurement and signal
conditioning circuitry on-chip and presents the processor
with a 3-wire digital interface composed of a bi-directional
data line DQ, a reset input \RST, and a clock input CLK.
The temperature reading is provided in a 9 bit, two’s com-
plement format. The measurement range spans from -55°C
to +125°C in .5°C increments.
Data transfers into and out of the DS1620 are initiated by
driving \RST high. Once the DS1620’s reset is released, a
series of clock pulses is emitted by the processor to actu-
ally transfer the data. For transmission to the DS1620, data
must be valid during the rising edge of the clock pulse.
Data bits received by the processor are output on the fall-
ing edge of the clock and remain valid through the rising
edge. Taking the clock high results in DQ assuming a high
impedance state. The sequence can be immediately termi-
5-60
Microcontroller
Microcontroller
5-61

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika:

Zgłoś jeśli naruszono regulamin