DS7505U+T&R【PDF 9/14 页】IC DGTL THERMOMETER 2WIRE 8-USOP

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The four DS7505 registers each have a unique 2-bit

pointer designation, which is defined in Table 5. When

reading from or writing to the DS7505, the user must

point the DS7505 to the register that is to be

accessed. When reading from the DS7505, once the

pointer is set, it remains pointed at the same register

until it is changed. For example, if the user desires to

perform consecutive reads from the temperature regis-

ter, then the pointer only has to be set to the tempera-

ture register one time, after which all reads are

automatically from the temperature register until the

pointer value is changed. When writing to the DS7505,

the pointer value must be refreshed each time a write is

performed, even if the same register is being written to

twice in a row.

At power-up, the pointer defaults to the temperature

immediately without resetting the pointer.

Changes to the pointer setting are accomplished as

described in the 2-Wire Serial Data Bus section.

2-Wire Serial Data Bus

The DS7505 communicates over a standard bidirection-

al 2-wire serial data bus that consists of a serial clock

(SCL) signal and serial data (SDA) signal. The DS7505

interfaces to the bus through the SCL input pin and

open-drain SDA I/O pin. All communication is MSB first.

The following terminology is used to describe 2-wire

communication:

Master Device: Microprocessor/microcontroller that

controls the slave devices on the bus. The master

device generates the SCL signal and START and STOP

conditions.

Slave: All devices on the bus other than the master.

The DS7505 always functions as a slave.

Bus Idle or Not Busy: Both SDA and SCL remain high.

SDA is held high by a pullup resistor when the bus is

idle, and SCL must either be forced high by the master

(if the SCL output is push-pull) or pulled high by a

pullup resistor (if the SCL output is open drain).

Transmitter: A device (master or slave) that is sending

data on the bus.

Receiver: A device (master or slave) that is receiving

data from the bus.

START Condition: Signal generated by the master to

indicate the beginning of a data transfer on the bus.

The master generates a START condition by pulling

SDA from high to low while SCL is high (see Figure 6).

A repeated START is sometimes used at the end of a

data transfer (instead of a STOP) to indicate that the

master performs another operation.

STOP Condition: Signal generated by the master to

indicate the end of a data transfer on the bus. The mas-

ter generates a STOP condition by transitioning SDA

from low to high while SCL is high (see Figure 6). After

the STOP is issued, the master releases the bus to its

idle state.

Acknowledge (ACK): When a device (either master or

slave) is acting as a receiver, it must generate an

acknowledge (ACK) on the SDA line after receiving

every byte of data. The receiving device performs an

ACK by pulling the SDA line low for an entire SCL peri-

od (see Figure 6). During the ACK clock cycle, the

transmitting device must release SDA. A variation on

the ACK signal is the not acknowledge (NACK). When

the master device is acting as a receiver, it uses a

NACK instead of an ACK after the last data byte to indi-

cate that it is finished receiving data. The master indi-

cates a NACK by leaving the SDA line high during the

ACK clock cycle.

Digital Thermometer and Thermostat

_______________________________________________________________________________________ 9

Table 3. Resolution Configuration

THERMOMETER

RESOLUTION (BITS)

MAX CONVERSION

TIME (ms)

100

200

Table 4. Fault Tolerance Configuration

CONSECUTIVE OUT-OF-LIMITS

CONVERSIONS TO TRIGGER O.S.

Table 5. Pointer Definition

Temperature

Configuration

HYST

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