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FEATURES
=Temperature measurements require no
external components
Measures temperatures from –55°C to
+125°C. Fahrenheit equivalent is –67°F to
+257°F.
Thermometer accuracy is ±2.0°C.
Thermometer resolution is configurable from
nine (default) to 12 bits (0.5°C to 0.0625°C
resolution)
9–bit readout mode features a max
conversion time of 150 ms.
Thermostatic settings are user definable.
Data is read from/written via a 2–wire serial
interface. (open drain I/O lines). 3–bit
addressability
Wide power supply range (2.7V – 5.5V).
Applications include personal computers,
cellular telephones, office equipment, or any
thermally sensitive system.
Pin/software compatible to LM75CIM–x
Thermal Watchdog in 9–bit (default) mode.
8–pin 150 mil SOIC package.
PIN ASSIGNMENT
PIN DESCRIPTION
SDA – 2–Wire Serial Data Input/Output
SCL – 2–Wire Serial Clock
GND – Ground
O.S. – Thermostat Output Signal
A0 – Chip Address Input
A1 – Chip Address Input
A2 – Chip Address Input
V
DD
– Power Supply Voltage
DESCRIPTION
The DS75 2–wire thermal watchdog provides 9–bit temperature readings which indicate the temperature
of the device. Thermostat settings and temperature readings are all communicated to/from the DS75 over
a simple 2–wire serial interface. No additional components are required; the device is truly a
“temperature–to–digital” converter.
The DS75 has three address bits that allow a user to multidrop up to eight sensors along the 2–wire bus,
greatly simplifying the bussing of distributed temperature sensing networks.
The open–drain thermal alarm output, O.S., becomes active when the temperature of the d evice exceeds a
user–defined temperature T
OS
. The number of consecutive faults required to set O.S. active is
configurable by the user. The device can also be configured in the interrupt or comparator mode, to
customize the method which clears the fault condition.
For applications that require greater temperature resolution, the user can adjust the readout resolution
from 9 to 12 bits. This is particularly useful in applications where thermal runaway conditions must be
detected quickly.
DS75
2–Wire Thermal Watchdog
www.dalsemi.com
6
3
1
2
4
8
7
5
SD
A
O.S.
GND
V
DD
A
0
A
1
A
2
SCL
DS75S
8-PIN SOIC (150 MIL)
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DS75
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Applications for the DS75 include personal computers/servers, cellular telephones, of fice equipment, or
any microprocessor–based thermally–sensitive system.
DETAILED PIN DESCRIPTION Table 1
PIN SYMBOL DESCRIPTION
1SDAData input/output pin for 2–wire serial communication port.
2SCLClock input/output pin for 2–wire serial communication port.
3 O.S. Thermostat output Becomes active when temperature exceeds TOS. Device
configuration defines means to clear over–temperature state.
4 GND
Ground pin.
5A
2
Address input pin.
6A
1
Address input pin.
7A
0
Address input pin.
8VDDSupply Voltage 2.7V – 5.5V input power pin.
OVERVIEW
A block diagram of the DS75 is shown in Figure 1. The DS75 consists of five major components:
1. Precision temperature sensor
2. Analog–to–digital converter
3. 2–wire interface electronics
4. Data registers
5. Thermostat comparator
The factory–calibrated temperature sensor requires no external components. Upon power–up, the DS75
begins temperature conversions with the default resolution of 9 bits (0.5°C resolution). The host can
periodically read the value in the temperature register, which contains the last completed conversion. As
conversions are performed in the background, reading the temperature register does not affect the
conversion in progress.
In power–sensitive applications, the user can put the DS75 into a shutdown mode, under which the sensor
will complete and store the conversion in progress and revert to a low–power standby state. In
applications where small incremental temperature changes are critical, the user can change the conversion
resolution from 9–bits to 10, 11, or 12. Each additional bit of resolution approximately doubles the
conversion time. This is accomplished by programming the configuration register. The configuration
register defines the conversion state, thermometer resolution/conversion time, active state of the
thermostat output, number of consecutive faults to trigger an alarm condition, and the method to
terminate an alarm condition.
The user can also program over–temperature (TOS) and under–temperature (T
HYST
) setpoints for
thermostatic operation. The power–up state of TOS is 80°C and that for T
HYST
is 75°C. The result of each
temperature conversion is compared with the TOS and T
HYST
setpoints. The DS75 offers two modes for
temperature control, the comparator mode and the interrupt mode. This allows the user the flexibility to
customize the condition that would generate and clear a fault condition. Regardless of the mode chosen,
the O.S. output will become active only after the measured temperature ex ceeds the respe ctive trippoint a
consecutive number of times; the number of consecutive conversions beyond the limit to generate an O.S.
is programmable. The power–up state of the DS75 is in the comparator mode with a single fault
generating an active O.S. Digital data is written to/read from the DS75 via a 2–wire interface, and all
communication is MSb first. Multipoint sensing is possible with the DS75 by uniquely setting the 3–bit
address of up to 8 parts on the 2–wire bus.
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DS75
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DS75 FUNCTIONAL BLOCK DIAGRAM Figure 1
OPERATION–Measuring Temperature
The core of DS75 functionality is its direct–to–digital temperature sensor. The DS75 measures
temperature through the use of an on–chip temperature measurement technique with an operating range
from –55°C to +125°C. Temperature conversions are initiated upon power–up, and the most recent result
is stored in the thermometer register. Conversions are performed continuousl y unless the user interv enes
by altering the configuration register to put the DS75 into a shutdown mode. Regardless of the mode
used, the digital temperature can be retrieved from the temperature register by setting the pointer to that
location (00h, power–up default). The DS75 power–up default has the sensor automatically performing
9–bit conversions continuously. Details on how to change the settings after pow-er up are contained in the
“OPERATION– Programming” section.
The resolution of the temperature conversion is configurable (9, 10, 11, or 12 bits), with 9–bit readings
the default state. This equates to a temperature resolution of 0.5°C, 0.25°C, 0.125°C, or 0.0625°C.
Following each conversion, thermal data is stored in the thermometer register in two’s complement
format; the information can be retrieved over the 2–wire interface with the device pointer set to the
temperature register. Table 2 describes the exact relationship of output data to measured tempe rature. T he
table assumes the DS75 is configured for 12–bit resolution; if the device is configured in a lower
resolution mode, those bits will contain zeros. The data is transmitted serially over the 2–wire serial
interface, MSb first. The MSb of the temperature register contains the “sign” (S) bit, denoting whether the
temperature is positive or negative. For Fahrenheit usage, a lookup table or conversion routine must be
used.
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Temperature/Data Relationships Table 2
S262
5
2
4
2
3
2
2
2
1
2
0
MSB
MSb (UNIT = °C) LSb
2
–1
2
–2
2
–3
2
–4
0000LSB
TEMPERATURE/DATA RELATIONSHIPS
Table 2 cont’d
TEMP
DIGITAL OUTPUT
(Binary)
DIGITAL
OUTPUT
(Hex)
+125°C
0111 1101 0000 0000 7D00h
+25.0625°C
0001 1001 0001 0000 1910h
+10.125°C
0000 1010 0010 0000 0A20h
+0.5°C
0000 0000 1000 0000 0080h
+0°C
0000 0000 0000 0000 0000h
–0.5°C
1111 1111 1000 0000 FF80h
–10.125°C
1111 0101 1110 0000 E5E0h
–25.0625°C
1110 0110 1111 0000 E6F0h
–55°C
1100 1001 0000 0000 C900h
OPERATION–Thermostat Control
In its comparator operating mode, the DS75 functions as a thermostat with programmable hysteresis, as
shown in Figure 2. When the DS75’s temperatur e meets or exceeds t he value stored in the hi gh temper ature trip register (T
OS
) a consecutive number of times defined by the configuration register, the output
becomes active, and will stay active until the temperature falls below the temperature stored in the low
temperature trigger register (T
HYST
) the first time. In this way, any amount of hysteresis may be obtained.
The DS75 powers up in the comparator mode with T
OS
=80°C and T
HYST
=75°C, and the device can be
used as a standalone thermostat (no 2–wire interface required) with those setpoints.
In the interrupt mode, the O.S. output will first become active following the programmed number of
consecutive conversions above T
OS
. The fault can only be cleared by either setting the DS75 in a
shutdown mode or by reading any register (temperature, configuration, TOS, or T
HYST
) on the device.
Following a clear, a subsequent fault can only occur if consecutive conversions fall below T
HYST
. This
interrupt/clear process is thus cyclical (TOS, clear, T
HYST
, clear, TOS, clear, T
HYST
, clear, . . .). Only the
first of multiple consecutive T
OS
violations will activate O.S., even if each fault is separated by a clearing
function. The same situation applies to multiple consecutive T
HYST
events.
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O.S. OUTPUT TRANSFER FUNCTION Figure 2
Regardless of the mode chosen, the O.S. output is open–drain and the active state is set in the configuration register. The power–up default is active low. Refer to the “OPERATION–Programming” section for
instructions in adjusting the thermostat setpoints, thermostat mode, and O.S. active state.
OPERATION–Programming
There are three areas of interest in programming the DS75: Configuration register, T
OS
register, and the
T
HYST
register. All pro gramming is done via the 2–wire interface by setting the pointer to the appropriate
location. Table 3 illustrates the pointer settings for the four registers of the DS75.
Pointer Register Structure Table 3
POINTER ACTIVE REGISTER
00h Temperature (default)
01h Configuration
02h T
HYST
03h T
OS
The DS75 will power up with the temperature register selected. If the host wishes to change the data
pointer, it simply addresses the DS75 in the write mode (R/W=0), receives an acknowledge, and writes
the 8 bits that correspond to the new desired location. The last pointer location is always maintained so
that consecutive reads from the same register do not require the host to always provide a pointer addr ess.
The only exception is at power–up, in which case the pointer will always be set to 00h, the temperature
register. The pointer address must always proceed data in writing to a register, regardless of which
address is currently selected. Please r efer to the “2–Wire Serial Data Bus” section for det ails of the 2–
wire bus protocol.
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DS75
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Configuration Register Programming
The configuration register is accessed if the DS75 pointer is currentl y set to the 01h location. W riting to
or reading from the register is determined by the R/W bit of the 2–wire control byte (See “2–wire Serial
Data Bus” section). Data is read from or written to the configuration register MSb first. The format of the
register is illustrated below in Figure 3. The effect each bit has on DS75 functionality is described below
along with the power–up state of the bit. The user has read/write access to all bits in the configuration
register. The entire register is volatile, and thus it will power–up in the default state.
CONFIGURATION/STATUS REGISTER
Figure 3
0R1R0F1F0POLTMSD
MSb LSb
SD = Shutdown bit. If SD is “0”, the DS75 will continuously perform temperature conversions and store
the last completed result in the thermometer register. If SD is changed to “1”, the conversion in pro gress
will be completed and stored; then the device will revert to a low–power standby mode. The O.S. output
will be cleared if the device is in the interrupt mode and remain unchanged in the comparator mode. The
2–wire port remains active. The power–up default state is “0” (continuous conversion mode).
TM = Thermostat mode. If TM=“0”, the DS75 is in the comparator mode. TM=“1” sets the device to the
interrupt mode. See “OPERATION–Thermostat Control” section for a description of the difference
between the two modes. The power–up default state of the TM bit is “0” (comparator mode).
POL = O.S. Polarity Bit. If POL =“1”, the active state of the O.S. output will be high. A “0” stored in
this location sets the thermostat output to an active low state. The user has read/write access to the POL
bit, and the power–up default state is “0” (active low).
F0, F1 = O.S. Fault Tolerance bits. The fault tolerance defines the number of consecutive conversions
returning a temperature beyond limits is required to set the O.S. output in an active state. This may be
necessary to add margin in noisy environments. Table 4 below defines the four settings. The DS75 will
power up with F0=F1=“0”, such that a single occurrence will trigger a fault.
Fault Tolerance Configuration Table 4
F1 F0 Consecutive conversions beyond limits
to generate fault
00 1
01 2
10 4
11 6
R0, R1 = Thermometer resolution bits. Table 5 below defines the resolution of the digital thermometer,
based on the settings of these two bits. There is a direct trade-off between resolution and conversion time,
as depicted in the AC Electrical Characteristics. The default state is R0=0 and R1=0 (9–bit conversions).
“0” = Reserved Location. The master can write to this bit, but it will always read out as a “0”. The
power–on default state is “0”.
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Thermometer Resolution Configuration
Table 5
R1 R0 Thermometer
Resolution
Max Conversion
Time
0 0 9–bit 0.15s
0 1 10–bit 0.3s
1 0 11–bit 0.6s
1 1 12–bit 1.2s
Thermostat Setpoints Programming
The thermostat registers (T
OS
and T
HYST
) can be programmed or read via the 2–wire interface. TOS is
accessed by setting the DS75 data pointer to the 03h location, and that for the T
HYST
setting is 02h.
The format of the T
OS
and T
HYST
registers is identical to that of the Thermometer register; that is, 12–bit
2’s complement representation of the temperature in °C. The user can program the number of bits (9, 10,
11, or 12) for each T
OS
and T
HYST
that correspond to the thermometer resolution mode chosen. If the 9–bit
mode is chosen, for example, the 3 least significant bits of T
OS
and T
HYST
will be ignored by the
thermostat comparator. The format for both T
OS
and T
HYST
is shown below in Table 6. The power–up
default of T
OS
is 80°C and that for T
HYST
is 75°C.
Thermostat Setpoint (TOS/THYST) Format
Table 6
S262
5
2
4
2
3
2
2
2
1
2
0
MSB
MSb
(UNIT = °C)
LSb
2
–1
2
–2
2
–3
2
–4
0000LSB
TEMPERATURE/DATA RELATIONSHIPS
TEMP
DIGITAL OUTPUT
(Binary)
DIGITAL
OUTPUT
(Hex)
+80°C
0101 0000 0000 0000 5000h
+75°C
0100 1011 0000 0000 4B00h
+10.125°C
0000 1010 0010 0000 0A20h
+0.5°C
00000000 1000 0000 0080h
+0°C
00000000 0000 0000 0000h
–0.5°C
1111 1111 1000 0000 FF80h
–10.125°C
1111 0101 1110 0000 E5E0h
–25.0625°C
1110 0110 1111 0000 E6F0h
–55°C
1100 1001 0000 0000 C900h
If the user does not wish to take advantage of the thermostat capabilities of the DS75, the 24 bits can be
used for general storage of system data that need not be maintained following a power loss. The O.S.
output should be left floating if this is done.
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2–WIRE SERIAL DATA BUS
The DS75 supports a bi–directional two–wire bus and data transmission protocol. A device that sends
data onto the bus is defined as a transmitter, and a device receiving data as a receiver. The device that
controls the message is called a “master”. Th e devices that ar e controlled b y the master are “sla ves”. The
bus must be controlled by a master device which generates the serial clock (SCL), controls the bus access,
and generates the START and STOP conditions. The DS75 operates as a slave on the two–wire bus.
Connections to the bus are made via the open–drain I/O lines SDA and SCL.
The following bus protocol has been defined (See Figure 4):
• Data transfer may be initiated only when the bus is not busy.
• During data transfer, the data line must remain stable whenever the clock line is HIGH. Changes in
the data line while the clock line is high will be interpreted as control signals.
Accordingly, the following bus conditions have been defined:
Bus not busy: Both data and clock lines remain HIGH.
Start data transfer: A change in the state of the data line, from HIGH to LOW, while the clock is
HIGH, defines a START condition.
Stop data transfer: A change in the state of the data line, from LOW to HIGH, while the clock line is
HIGH, defines the STOP condition.
Data valid: The state of the data line represents valid data when, after a START condition, the data line
is stable for the duration of the HIGH period of the clock signal. The data on the line must be changed
during the LOW period of the clock signal. There is one clock pulse per bit of data.
Each data transfer is initiated with a START condition and terminated with a STOP condition. The
number of data bytes transferred between START and STOP conditions is not limited, and is determined
by the master device. The information is transferred byte–wise and each receiver acknowledges with a
ninth bit.
Within the bus specifications a regular mode (100 kHz clock rate) and a fast mode (400 kHz clock rate)
are defined. The DS75 works in both modes.
Acknowledge: Each receiving device, when addressed, is obliged to generate an acknowledge after
the reception of each byte. The master device must generate an extra clock pulse which is associated with
this acknowledge bit.
A device that acknowledges must pull down the SDA line during the acknowledge clock pulse in such a
way that the SDA line is stable LOW during the HIGH period of the acknowledge related clock pulse. Of
course, setup and hold times must be taken into account. A master must signal an end of d ata to the slave
by not generating an acknowledge bit on the last byte that has been clocked out of the slave. In this case,
the slave must leave the data line HIGH to enable the master to generate the STOP condition.
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DATA TRANSFER ON 2–WIRE SERIAL BUS Figure 4
Figure 5 details how data transfer is accomplished on the two–wire bus. Depending upon the state of the
R/W bit, two types of data transfer are possible:
1. Data transfer from a master transmitter to a slave receiver. The first byte transmitted by the
master is the slave address. Next follows a number of data bytes. The slave returns an acknowledge
bit after each received byte.
2. Data transfer from a slave transmitter to a master receiver. The first byte (the slave address) is
transmitted by the master. The slave then returns an acknowledge bit. Next follows a number of data
bytes transmitted by the slave to the master. The master returns an acknowled ge bit after all received
bytes other than the last byte. At the end of the last received byte, a ’not acknowledge’ is returned.
The master device generates all of the serial clock pulses and the START and STOP conditions. A
transfer is ended with a STOP condition or with a repeated START condition. Since a repeated
START condition is also the beginning of the next serial transfer, the bus will not be released.
The DS75 may operate in the following two modes:
1. Slave receiver mode: Serial data and clock are received through SDA and SC L. After each byte is
received, an acknowledge bit is transmitted. START and STOP conditions are recognized as the
beginning and end of a serial transfer. Address recognition is performed by hardware after reception
of the slave address and direction bit.
2. Slave transmitter mode: The first b yte is received and handled as in the slave receiver mode. How-
ever, in this mode, the direction bit will indicate that the transfer direction is reversed. Serial data is
transmitted on SDA by the DS75 while the serial clock is input on SCL. START and STOP
conditions are recognized as the beginning and end of a serial transfer.
SLAVE ADDRESS
A control byte is the first byte received following the START condition from the master device. The
control byte consists of a four bit control code; for the DS75, this is set as 1001 binary for read and write
operations. The next three bits of the control byte are the device select bits (A2, A1, A0). The y are used
by the master device to select which of eight devices are to be accessed. The set bits are in effect t he three
least significant bits of the slave address. The last bit of the control byte (R/W) defines the operation to be
performed. When set to a one a read op eration is selected, and when set to a zero a write operation i s
selected. Following the START condition, the DS75 monitors the SDA bus checking the device type
identifier being transmitted. Upon receiving the 1001 code and appropriate device select bits, the slave
device outputs an acknowledge signal on the SDA line.
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2–WIRE SERIAL COMMUNICATION WITH DS75 Figure 5
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ABSOLUTE MAXIMUM RATINGS*
Voltage on VDD, Relative to Ground –0.3V to +7.0V
Voltage on any other pin, Relative to Ground –0.3V to (V DD + 0.3V)
Operating Temperature –55°C to +125°C
Storage Temperature –55°C to +125°C
Soldering Temperature 260°C for 10 seconds
* This is a stress rating only and functional operation of the device at these or an y other conditions above
those indicated in the operation sections of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods of time may affect reliability.
The Dallas Semiconductor DS75 is built to the highest quality standards and manufactured for long term
reliability. All Dallas Semiconductor devices are made using the same quality materials and
manufacturing methods. However, the DS75 is not exposed to environmental stresses, such as burn–in,
that some industrial applications require. For specific reliability information on this product, please
contact the factory in Dallas at (972) 371–4448.
RECOMMENDED DC OPERATING CONDITIONS
(-55°C to +125°C; 2.7V ≤ V
DD
≤ 5.5V)
PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES
Supply Voltage V
DD
2.7 5.5 V
DC ELECTRICAL CHARACTERISTICS (-55°C to +125°C; 2.7V ≤ VDD ≤ 5.5V)
PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES
Input Logic High V
IH
0.7V
DD
VDD+0.5 V 1
Input Logic Low V
IL
-0.5 0.3V
DD
V1
V
OL1
3 mA sink
current
0 0.4
SDA Output Logic Low
Voltage
V
OL2
6 mA sink
current
0 0.6
V1
O.S. Saturation Voltage V
OL
4 mA sink
current
0.8 V 1, 9
Input current each I/O pin
0.4 < V
I/O
<
0.9 V
DD
-10 +10
µA
2
I/O Capacitance C
I/O
10 pF
Standby Current I
DD1
1 µA 3, 4
Active Temp.
Conversions
1000
Active Current I
DD
Communica-
tion only
100
µA 3, 4
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DS75
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ELECTRICAL CHARACTERISTICS:
DIGITAL THERMOMETER (-55°C to +125°C; 2.7V ≤ V
DD
≤ 5.5V)
PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES
Thermometer Error T
ERR
-25 to +100
-55 to +125
± 2.0
± 3.0
°C
9
Resolution 9 12 bits
9-bit
conversions
125 150
10-bit
conversions
250 300
11-bit
conversions
500 600
Conversion Time t
CONVT
12-bit
conversions
1000 1200
ms
AC ELEC TRICAL CHARACTERI STICS (-55°C to +125°C; 2.7V ≤ VDD ≤ 5.5V)
PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES
SCL clock frequency f
SCL
Fast Mode
Standard Mode
400
100
KHz
Bus free time between
a STOP and START condition
t
BUF
Fast Mode
Standard Mode
1.3
4.7
µs
Hold time (repeated)
START condition
t
HD:STA
Fast Mode
Standard Mode
0.6
4.0
µs
5
LOW period of SCL t
LOW
Fast Mode
Standard Mode
1.3
4.7
µs
HIGH period of SCL t
HIGH
Fast Mode
Standard Mode
0.6
4.0
µs
Set-up time for a
repeated START
t
SU:STA
Fast Mode
Standard Mode
0.6
4.7
µs
Data hold time t
HD:DAT
Fast Mode
Standard Mode
0
0
0.9
µs
6
Data set-up time t
SU:DAT
Fast Mode
Standard Mode
100
250
ns 7
Rise time of both SDA
and SCL signals
t
R
Fast Mode
Standard Mode
20
+
0.1C
B
300
1000
ns 8
Fall time of both SDA
and SCL signals
t
F
Fast Mode
Standard Mode
20
+
0.1C
B
300 ns 8
Set-up time for a STOP t
SU:STO
Fast Mode
Standard Mode
0.6
4.0
µs
Capacitive load for
each bus line
C
b
400 pF 8
Input Capacitance C
I
5pF
Page 13
DS75
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NOTES:
1. All voltages are referenced to ground.
2. I/O pins of fast mode devices must not obstruct the SDA and SCL lines if VDD is switched off.
3. I
DD
specified with O.S. pin open.
4. I
DD
specified with VDD at 5.0V and SDA, SCL = 5.0V, 0°C to 70°C.
5. After this period, the first clock pulse is generated.
6. The maximum t
HD:DAT
has only to be met if the device does not stretch the LOW period (t
LOW
) of the
SCL signal.
7. A fast mode device can be used in a standard mode system, but the requirement t
SU:DAT ≤
250 ns must
then be met. This will automatically be the case if the device does not stretch the LOW period of the
SCL signal. If such a device does stretch the LOW period of the SCL signal, it must output the next
data bit to the SDA line t
RMAX
+t
SU:DAT
= 1000+250 = 1250 ns before the SCL line is released.
8. Cb – total capacitance of one bus line in pF.
9. Internal heating caused by O.S. loading will cause the DS75 to read approximately 0.5°C higher if
O.S. is sinking the max rated current.
TIMING DIAGRAMS Figure 6
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