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INA3221 Datasheet(PDF) 20 Page - Texas Instruments |
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INA3221 Datasheet(HTML) 20 Page - Texas Instruments |
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20 / 46 page  20 INA3221 SBOS576B – MAY 2012 – REVISED MARCH 2016 www.ti.com Product Folder Links: INA3221 Submit Documentation Feedback Copyright © 2012–2016, Texas Instruments Incorporated 8.5 Programming 8.5.1 Bus Overview The INA3221 offers compatibility with both I2C and SMBus interfaces. The I2C and SMBus protocols are essentially compatible with one another. The I2C interface is used throughout this data sheet as the primary example, with the SMBus protocol specified only when a difference between the two systems is discussed. Two I/O lines, the serial clock (SCL) and data signal line (SDA), connect the INA3221 to the bus. Both SCL and SDA are open-drain connections. The device that initiates a data transfer is called a master, and the devices controlled by the master are slaves. The bus must be controlled by the master device that generates the SCL, controls the bus access, and generates start and stop conditions. To address a specific device, the master initiates a start condition by pulling SDA from a high to a low logic level while SCL is high. All slaves on the bus shift in the slave address byte on the SCL rising edge, with the last bit indicating whether a read or write operation is intended. During the ninth clock pulse, the slave being addressed responds to the master by generating an acknowledge bit and pulling SDA low. Data transfer is then initiated and eight bits of data are sent, followed by an acknowledge bit. During data transfer, SDA must remain stable while SCL is high. Any change in SDA while SCL is high is interpreted as a start or stop condition. After all data are transferred, the master generates a stop condition by pulling SDA from low to high while SCL is high. The INA3221 includes a 28-ms timeout on the interface to prevent locking up the bus. 8.5.1.1 Serial Bus Address To communicate with the INA3221, the master must first address slave devices with a slave address byte. This byte consists of seven address bits and a direction bit to indicate whether the intended action is a read or write operation. The INA3221 has one address pin, A0. Table 1 describes the pin logic levels for each of the four possible addresses. The state of the A0 pin is sampled on every bus communication and must be set before any activity on the interface occurs. Table 1. Address Pins and Slave Addresses A0 SLAVE ADDRESS GND 1000000 VS 1000001 SDA 1000010 SCL 1000011 8.5.1.2 Serial Interface The INA3221 only operates as a slave device on the I2C bus and SMBus. Bus connections are made using the open-drain I/O lines, SDA and SCL. The SDA and SCL pins feature integrated spike-suppression filters and Schmitt triggers to minimize the effects of input spikes and bus noise. While there is spike suppression integrated into the digital I/O lines, use proper layout to minimize the amount of coupling into the communication lines. Noise introduction occurs from capacitively coupling signal edges between the two communication lines themselves, or from other switching noise sources present in the system. Routing traces in parallel with ground between layers on a printed circuit board (PCB) typically reduces the effects of coupling between the communication lines. Shield communication lines to reduce the possibility of unintended noise coupling into the digital I/O lines that could be incorrectly interpreted as start or stop commands. The INA3221 supports a transmission protocol for Fast (1 kHz to 400 kHz) and High-speed (1 kHz to 2.44 MHz) modes. All data bytes are transmitted MSB first. |
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