AN352 SILABS | Alldatasheet

Document overview

  • Manufacturer or author: Provided By ALLDATASHEET.COM(FREE DATASHEET DOWNLOAD SITE)
  • PDF pages: 4

Technical content

Rev. 1.0 8/08 Copyright © 2008 by Silicon Laboratories AN352 AN352 LOW -C OST , H IGH -S PEED I2C I SOLATION WITH D IGITAL I SOLATORS 1. Introduction Many articles have been published using opto-couplers for I2C isolation (See “5. References” on page 3). These circuits are somewhat complex, sensitive to bus cap acitance, and limited in speed. They are also not compatible with high-speed digital isolators having standard CMOS input levels. This application note shows how to convert a standard Si8 442 high-speed digital isolator to a bidirectional I2C isolator. In addition to being compatible with digital isolators, the circuit is simpler than previously published solutions, completely insens itive to bus capacitance, and can easily support the standard 400 kHz maximum I2C bus ra te. 2. Difficulty of Making a Bidirectional Circuit Standard I2C SDA and SCL signals are driven by open drain drivers. In all cases, SDA can be driven by any device on the bus so that the SDA bus wire communicates information from the I2C master to the slaves and from the slaves to the master. That is, the data transfer is bidirectional. In some cases, the SCL only has a driver for the I2 C master. However, in many cases, such as multiple I2 C masters or if the slave needs to stretch SCL by holding it low while it retrieves data, the SCL line must also be bidirectional. For the wires that need to be bidirectional, if digital isolators are inserted as in Figure 1, there are several problems: „ The isolators must be open-drain. „ There is a latch-up condition that can occur. If, for example, the side A driver pulls low, isolator A pulls low on side B. This causes isolator B to pull low on side A, and the circuit latches with both isolators pulling low. Side A Driver Side B Driver Isolator A Isolator B Figure 1. The open drain problem is easily solved by putting a Schottky diode in series with the isolator output. However, the latch up problem is much more difficult to solve. Previous attempts to solve this problem have used dio des inherent to the opto-coupler input and additional circuitry to avoid the latch-up condition (See “5. References” on page 3). Some of these approaches are sensitive to bus capacitance. They tend to be slow due to the slow response of the opto-couplers. Finally, when higher speed digital isolators with standard CMOS input levels are used, the circuit tricks using the diode input of the opto-couplers no longer apply. 3. Circuit Using Digital Isolators The full circuit using a Silicon Laboratories, Inc. Si8442 high-speed isolator is shown in Figure 2. The circuit assumes 1 kΩ pull- up resistors on the SCL and SDA lines. It can be easily adjusted for other bus pull-up resistors. This circuit has been tested with Silicon Laboratories, Inc. C8051Fxxx series MCUs at a bus speed of approximately 300 kHz including bus tr ansactions that require SCL clock stretching.

Figure 2. Full Circuit of the Si8442 to open drain. (side B). Transistors Q1 and Q2 act as the comparators. do turn on, and the low is propagated. n one side and can be used on either side A or side B. consideration for the side without the comparator circuit. drain driver must turn on the comparator. temperature compensation to match diodes D2A,B. eshold is well centered between these values.

4 Rev. 1.0 CONTACT INFORMATION Silicon Laboratories Inc.

400 West Cesar Chavez

Austin, TX 78701 Tel: 1+(512) 416-8500 Fax: 1+(512) 416-9669 Toll Free: 1+(877) 444-3032 Email: PowerProducts@silabs.com Internet: www.silabs.com The information in this document is believed to be accurate in all respects at the time of publication but is subject to change without notice. Silicon Laboratories assumes no responsibility for errors and omissions, and disclaims responsibility for any consequences resulting from the use of information included herein. Additionally, Silicon Laboratories assumes no responsibility for the functioning of undescribed features or parameters. Silicon Laboratories reserves the right to make changes without further notice. Silicon Laboratories makes no warranty, rep- resentation or guarantee regarding the suitability of its products for any particular purpose, nor does Silicon Laboratories assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation conse- quential or incidental damages. Silicon Laboratories products are not designed, intended, or authorized for use in applications intended to support or sustain life, or for any other application in which the failure of the Silicon Laboratories product could create a situation where per- sonal injury or death may occur. Should Buyer purchase or use Silicon Laboratories products for any such unintended or unauthorized ap- plication, Buyer shall indemnify and hold Silicon Laboratories harmless against all claims and damages. Silicon Laboratories, Silicon Labs, and ISOcap are trademarks of Silicon Laboratories Inc.