ND Series

Industrial Middle Pressure Sensors

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10

10.4.3 I2C Clock Stretching

The figure to the right illustrates the I2C

clock stretching by the sensor. At times, the

sensor requires additional time to respond

to the host and utilizes the clock stretching

feature of the I2C protocol. This is

accomplished by holding the SCL low after

the ACK cycle of a data transfer. Refer to

Section 7.4 for the clock stretching timing.

Note, the maximum clock stretch time will

generally only occur once during the three

ACK cycles of a two byte transfer. That is,

the balance of ACK's during a multi-byte

transfer will generally include the typical clock stretching time.

10.4.4 I2C Bus Compatibility

The I2C specification allows any recessive voltage between 3.0 and 5.0 V. Different devices on the bus may operate at different

voltage levels. However, the maximum voltage on any port pin must conform to the electrical characteristics specifications (See

section 1). The bi-directional SCL (serial clock) and SDA (serial data) lines must be connected to a positive power supply voltage

through a pull-up resistor or similar circuit. Every device connected to the bus must have an open-drain or open-collector output for

both the SCL and SDA lines, so that both are pulled high (recessive state) when the bus is free.

10.5 Extended Data Acquisition

10.5.1 Available Extended Data

For either the SPI or I2C interface, additional data is available beyond the pressure. The means to access this extended data is to

continue reading data (either SPI or I2C) beyond the first 16 bits of pressure information. The following table defines the order of the

available data and respective format.

Data

Bytes

Format

Interpretation

Example

Pressure

1-2

2 byte, Signed Int

See Section 10.7

See Section 10.7

Temperature

3-4

2 byte, Signed Int

Fixed Decimal [8.8 bits], Upper 8

bits integer, lower 8 bits

fractional. Temperature in °C

1880H (18.80H) = 24.5°C

Model

5-12

8 byte, ASCII, null

terminated

Right reading ASCII with null

termination

4EH,44H,30H,30H,35H,44H,00H,xxH = ND005D

Serial Number

13-16

4 byte, Hex

Unique 4 byte serial for each part

2FD627A4H

Build Number

17-22

6 byte, ASCII, null

terminated

Right reading ASCII with null

termination

30H,31H,34H,37H,41H,00H = 0147A

10.5.2 SPI Extended Data Read

Reading the extended data while using the SPI interface is the same as shown in Section 10.3 with exception that the master

continues to read during the same nSS sequence to read all 22 bytes of the extended data. Any portion of the 22 bytes can be read

during the transfer. That is, for example, 4 bytes could be read to acquire only the pressure and temperature information. When

reading the extended data, only the first two bytes sent to the sensor (User Mode and User Rate) are used to set the internal

registers. The subsequent bytes (bytes 5 through 22) are ignored. Data read following the first 22 bytes is undefined.

10.5.3 I2C Extended Data Read

Reading the extended data via the I2C interface is similar to using the SPI interface where the master can simply continue to reading

the sensor during the pressure reading transfer. The master continues Ack'ing until the number of desired bytes are read.