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參數(shù)資料
| 型號： | LM90 |
| 廠商： | National Semiconductor Corporation |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | 3 Degree Celcious Accurate, Remote Diode and Local Digital Temperature Sensor with Two-Wire Interface |
| 中文描述： | 3度Celcious精確，遠(yuǎn)程二極管和本地?cái)?shù)字溫度傳感器兩線接口 |
| 文件頁數(shù)： | 10/20頁 |
| 文件大?。?/td> | 384K |
| 代理商： | LM90 |
1.0 Functional Description
(Continued)
Temperature
Digital Output
Binary
0001 1001 0000 0000
0000 0001 0000 0000
0000 0000 0010 0000
0000 0000 0000 0000
1111 1111 1110 0000
1111 1111 0000 0000
1110 0111 0000 0000
1100 1001 0000 0000
Hex
1900h
0100h
0020h
0000h
FFE0h
FF00h
E700h
C900h
+25C
+1C
+0.125C
0C
0.125C
1C
25C
55C
Local Temperature data is represented by an 8-bit, two’s
complement byte with an LSB (Least Significant Bit) equal to
1C:
Temperature
Digital Output
Binary
0111 1101
0001 1001
0000 0001
0000 0000
1111 1111
1110 0111
1100 1001
Hex
7Dh
19h
01h
00h
FFh
E7h
C9h
+125C
+25C
+1C
0C
1C
25C
55C
1.7 OPEN-DRAIN OUTPUTS
The
SMBData,
open-drain outputs and do not have internal pull-ups. A
“high” level will not be observed on these pins until pull-up
current is provided by some external source, typically a
pull-up resistor. Choice of resistor value depends on many
system factors but, in general, the pull-up resistor should be
as large as possible. This will minimize any internal tempera-
ture reading errors due to internal heating of the LM90. The
maximum resistance of the pull-up to provide a 2.1V high
level, based on LM90 specification for High Level Output
Current with the supply voltage at 3.0V, is 82k
(5%) or
88.7k
(1%).
ALERT
and
T_CRIT_A
outputs
are
1.8 DIODE FAULT DETECTION
The LM90 is equipped with operational circuitry designed to
detect fault conditions concerning the remote diode. In the
event that the D+ pin is detected as shorted to V
DD
or
floating, the Remote Temperature High Byte (RTHB) register
is loaded with +127C, the Remote Temperature Low Byte
(RTLB) register is loaded with 0, and the OPEN bit (D2) in
the status register is set. As a result, if the Remote T_CRIT
setpoint register (RCS) is set to a value less than +127C the
ALERT output pin will be pulled low, if the Alert Mask is
disabled. If the Remote HIGH Setpoint High Byte Register
(RHSHB) is set to a value less than +127C then ALERT will
be pulled low, if the Alert Mask is disabled. The OPEN bit
itself will not activate the ALERT or T_CRIT_A outputs. If the
remote temperature reading is greater than its T_CRIT level
when the OPEN bit is set the T_CRIT_A will remain inactive.
In the event that the D+ pin is shorted to ground or D, the
Remote Temperature High Byte (RTHB) register is loaded
with 128C (1000 0000) and the OPEN bit (D2) in the status
register will not be set. Since operating the LM90 at 128C
is beyond it’s operational limits, this temperature reading
represents this shorted fault condition. If the value in the
Remote Low Setpoint High Byte Register (RLSHB) is more
than 128C and the Alert Mask is disabled, ALERT will be
pulled low.
Remote diode temperature sensors that have been previ-
ously released and are competitive with the LM90 output a
code of 0C if the external diode is short-circuited. This
change is an improvement that allows a reading of 0C to be
truly interpreted as a genuine 0C reading and not a fault
condition.
1.9 COMMUNICATING with the LM90
The data registers in the LM90 are selected by the Com-
mand Register. At power-up the Command Register is set to
“00”, the location for the Read Local Temperature Register.
The Command Register latches the last location it was set
to. Each data register in the LM90 falls into one of four types
of user accessibility:
1.
Read only
2.
Write only
3.
Read/Write same address
4.
Read/Write different address
A
Write
to the LM90 will always include the address byte and
the command byte. A write to any register requires one data
byte.
Reading
the LM90 can take place either of two ways:
1.
If the location latched in the Command Register is cor-
rect (most of the time it is expected that the Command
Register will point to one of the Read Temperature Reg-
isters because that will be the data most frequently read
from the LM90), then the read can simply consist of an
address byte, followed by retrieving the data byte.
2.
If the Command Register needs to be set, then an
address byte, command byte, repeat start, and another
address byte will accomplish a read.
The data byte has the most significant bit first. At the end of
a read, the LM90 can accept either acknowledge or No
Acknowledge from the Master (No Acknowledge is typically
used as a signal for the slave that the Master has read its
last byte). It takes the LM90 31.25ms to measure the tem-
perature of the remote diode and internal diode. When re-
trieving all 10 bits from a previous remote diode temperature
measurement, the master must insure that all 10 bits are
from the same temperature conversion. This may be
achieved by using one-shot mode or by setting the conver-
sion rate and monitoring the busy bit such that no conversion
occurs in between reading the MSB and LSB of the last
temperature conversion.
L
www.national.com
10
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