tmp03fruz-reel7 Analog Devices, Inc., tmp03fruz-reel7 Datasheet - Page 11

• Manufacturer Part Number: tmp03fruz-reel7
• Description: Serial Digital Output Thermometers
• Manufacturer: Analog Devices, Inc.

The TMP03 and TMP04 are superior to analog-output trans-
ducers for measuring temperature at remote locations, because
the digital output provides better noise immunity than an analog
signal. When measuring temperature at a remote location, the
ratio of the output pulses must be maintained. To maintain the
integrity of the pulsewidth, an external buffer can be added. For
example, adding a differential line driver such as the ADM485
permits precise temperature measurements at distances up to
4000 ft. (Figure 9). The ADM485 driver and receiver skew is
only 5 ns maximum, so the TMP04 duty cycle is not degraded.
Up to 32 ADM485s can be multiplexed onto one line by pro-
viding additional decoding.
As previously mentioned, the digital output of the TMP03
provides excellent noise immunity in remote measurement appli-
cations. The user should be aware, however, that heat from an
external cable can be conducted back to the TMP03. This heat
conduction through the connecting wires can influence the
temperature of the TMP03. If large temperature differences
exist within the sensor environment, an opto-isolator, level
shifter or other thermal barrier can be used to minimize measure-
ment errors.
TMP03
TMP03
TMP03
GND
GND
GND
5V
V+
V+
5V
5V
V+
D
D
D
OUT
OUT
OUT
10k
680
620
4.3k
2N2907
270
H11L1
OPTO-COUPLER
OPTO-COUPLER
5V
V
V
LOGIC
LOGIC
4.7k
4.7k
430
Microcomputer Interfaces
The TMP03 output is easily decoded with a microcomputer.
The microcomputer simply measures the T1 and T2 periods in
software or hardware, and then calculates the temperature using
the equation in the Output Encoding section of this data sheet.
Since the TMP03’s output is ratiometric, precise control of the
counting frequency is not required. The only timing require-
ments are that the clock frequency be high enough to provide
the required measurement resolution (see the Output Encoding
section for details) and that the clock source be stable. The
ratiometric output of the TMP03 is an advantage because the
microcomputer’s crystal clock frequency is often dictated by the
serial baud rate or other timing considerations.
Pulsewidth timing is usually done with the microcomputer’s
on-chip timer. A typical example, using the 80C51, is shown in
Figure 10. This circuit requires only one input pin on the micro-
computer, which highlights the efficiency of the TMP04’s
pulsewidth output format. Traditional serial input protocols,
with data line, clock and chip select, usually require three or
more I/O pins.
The 80C51 has two 16-bit timers. The clock source for the timers
is the crystal oscillator frequency divided by 12. Thus, a crystal
frequency of 12 MHz or greater will provide resolution of 1 µs
or less.
The 80C51 timers are controlled by two dedicated registers. The
TMOD register controls the timer mode of operation, while
TCON controls the start and stop times. Both the TMOD and
TCON registers must be set to start the timer.
TMP04
GND
V+
5V
D
OUT
TMP04
GND
5V
V+
D
OUT
INPUT
PORT 1.0
MICROCOMPUTER
(16-BITS)
(16-BITS)
TIMER 0
TIMER 1
80C51
NC
5V
OSC
DI
DE
TMP03/TMP04
12
V
ADM485
CC
TIMER 0 TIMER 1
TIMER 0 TIMER 1
TMOD REGISTER
TCON REGISTER
B
A