New bus buffers from NXP Semiconductors overcome the previous system capacitance limitation of 400pF. These new bus buffers allow designers more freedom in their design to expand the use of the I2C or SMBus:
These devices can be used in a wide variety of applications:
More I2C or SMBus Devices in the Same System
All bus buffer devices isolate the system capacitance and, at least, double the number of devices allowed in the same I2C or SMBus system. Typically 20 devices are the limit for the 400pF maximum capacitance specified by the I2C protocol. By using these bus buffers, the designer can extend the use of the I2C or SMBus in systems with many more devices.
Long Distance I2C or SMBus
The P82B715, P82B96, and PCA9600 are specifically designed to extend the reach of the I2C or SMBus over long distances, in the range of kilometers, depending on the maximum bus speed. This allows the designer to use I2C to interconnect equipment cabinets or for large area systems such as warehouse pick and place systems.
The P82B96 and improved PCA9600 use a static level offset on the slave side to isolate noise and loadings on either side of the device, allowing 4000pF on the high drive side. The nominal offset is 0.8V. That VOL level may create worst-case design tolerance problems with slave devices that do not use I2C switching levels, for example TTL levels. It also precludes operation with other bus buffers using special non-compliant I2C levels. A lower VOL level (<0.4V), combined with the ability to operate with any master, slave, or bus buffer, is the primary advantage of using the P82B715 for long-distance buses, but it doesn't isolate bus capacitance the way the P82B96 and the improved PCA9600 do, so the total system loading must be less than 3000pF.
Isolation of the I2C or SMBus
Most devices have an enable/disable feature that can be used to electrically isolate an I2C or SMBus segment. In addition, all devices except the P82B715 release their I/O pins should their supply voltage fail.
Examples where this enable/disable feature could be useful include:
PCA954X devices (Application Note AN262 PCA954X Family of I2C/SMBus Multiplexers and Switches) provide a variety of lower cost 2-, 4-, and 8-channel I2C controlled multiplexers for applications that don't require bus buffering and can also be used for voltage level translation. PCA954X devices operate at 400kHz with voltage supply range of 2.3V to 5.5V.
CBTXXXX devices offer the lowest cost manually controlled multiplexers and voltage level translators at 5V operation in non bus buffering application.
Voltage Level Translation of the I2C or SMBus
The bus buffers isolate the system capacitance and also allow translation of the I2C or SMBus voltage levels if required.
The PCA9515/16/18 operate at 3.3V VCC but, since they have 5V-tolerant I/Os, they support different I2C or SMBus segments at 3.3V and 5V. This extends the life of older 5V circuit blocks while allowing use of newer 3.3V devices for improved system performance. The PCA9515A/16A/18A operate at 2.5V and 3.3V VCC with 5V-tolerance.
The PCA9508 and PCA9517(A) operate between 2.7V and 3.6V VCC with 5V-tolerant I/O. The second VCC adjusts between 0.9V and 5.5V allowing voltage level translation with optimum noise margin between 0.9V to 5V levels. The low voltage SDA/SCL pins also pull to ground providing excellent VIL margins even at lower voltages and allowing connections to other static or dynamic level shifting bus buffers. The PCA9508 provides stop and idle detection for hot-swap applications.
The PCA9507 is similar to the PCA9517A but has a rise-time accelerator on the low-voltage pin side allowing up to 1500pF loading. It was specifically designed for the DDC bus in HDMI cables but could be used in any I2C bus application.
The PCA9509 is similar to the PCA9517A but is reversed with the offset on the low-voltage side and the normal bus on the SMBus side. The low-voltage side has integrated current source. The device was specifically designed to interface a low-voltage/weak-drive processor I2C port to normal SMBus slave and is offered in the very small 1.6 x 1.6 x 0.5 mm XQFN-8 "GM" package for space-constrained applications.
The PCA9512(A) has two VCCs for optimum noise margin in 3.3V-to-5V level translation applications.
The P82B96 and PCA9600 operate at 3.3V or at 5V logic levels on the Sx side that is independent of the Vcc. The logic levels on the higher drive Rx/Tx side are half-rail CMOS switching levels related to the Vcc that can range from 2V to 15V. The P82B715 does NOT support logic level translations.
GTL20XX devices (Application Note AN10145 Bi-Directional Low Voltage Translators GTL2000, GTL2002, GTL2010) and the new PCA9306 offer lower-cost voltage level translations at any voltage between the range of 1V and 5V for applications that don't also require bus buffering.
Multi-Point Distribution of the I2C or SMBus
The I2C bus is very effective at communicating amongst equipment in a multi-point arrangement. The I2C bus is increasingly being used to interconnect multiple backplane cards such as those in Intel IPMI (Intelligent Platform Management Interface), VME, PICMG (PCI Industrial Computer Manufacturers Group), CompactPCI©, and AdvancedTCA™ architectures using the PCA9510/11/12/13/14. We have more information on IPMI, VME, and PICMG architectures.
The higher drives on the special side of the P82B96, PCA9600, and P82B715 provide reliable, very-long distance, multi-point communications at higher voltages for better noise margin.
The normal VOL side of the PCA9507/08/09 and PCA9517A can be connected to each other or any other master/slave/bus buffer in a multi-point or star arrangement.
The PCA9510(A)/11(A)/12(A)/13(A)/14(A) have been specifically designed for multi-point system implementations and include features like SDA/SCL precharge, idle detection, ready hardware pin, and rise-time accelerators to provide optimum protection of active I2C bus data in hot swap backplane applications.
Opto-Electrical Isolation of the I2C or SMBus
The I2C bus may need to be electrically isolated due to safety reasons such as in heath care or where high voltages are involved or in cases where the ground plane is at different potentials such as in power supplies. The P82B96 and PCA9600, with split input and output pins on their high-voltage, high-drive sides, are optimized for interconnection with optical isolators and, depending on the opto-isolator used, speeds up to 1MHz are achievable.
Usage Legend: = Good, = Better, = Best
All I2C Repeaters/Hubs/Extenders
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New I2C Repeaters/Hubs/Extenders
AN255 covers useage in a generic, multiple-master environment. Discusses pull-up resistor sizing in detail. Information on the permitted interfaces and bus propagation delays between P82B715/96 Buffers and the PCA9515/16/18 is also included.