Improving isolated I2C interface with hot swap

Article By : Manny Chavez, Texas Instruments

The article explains what hot swap is, the benefits of hot-swappable I2C, and the additional benefits of isolated I2C with hot swap.

Inter-integrated circuit (I²C) is a popular inter-chip communication standard for electronic systems in industrial, automotive, aerospace, and computing settings. The versatility of I²C has led to continuous improvements in its capabilities and the integration of additional features.

Because I²C is an open-drain or open-collector communication standard, each I²C device connects the drain of an output switch or field-effect transistor (FET) from I²C pins—SDA and SCL—to an I²C bus. When you add or replace a device in an I²C system without stopping, shutting down or rebooting the system, a transient occurs within the device. If this transient is fast or strong enough, it could couple into the internal switch, causing the FET to turn on momentarily, which in turn discharges the bus. This discharge could appear as a valid data signal to other devices on the I²C bus, leading to data corruption.

Hot-swap capabilities in I²C devices help maintain the data integrity of I²C buses during device and node plug-in, preventing data corruption on a bus caused by transients when I²C devices are plugged in. This article will explore what hot swap is, the benefits of hot-swappable I²C, and the additional benefits of isolated I²C with hot-swap compliance.

Defining hot swap

Hot swap refers to a system’s ability to continue normal operation while components are added or replaced without pausing, powering down or restarting. For an I²C bus, you can maintain continuous operation when additional devices do not load the bus or corrupt an ongoing bitstream.

You can add devices with hot-swap abilities to an I²C bus whether or not you supply power to the node and system. Because these devices help preserve the communication integrity of an active I²C bus, they are beneficial to systems that you can maintain or upgrade without shutting down. To preserve communication during hot-swap events in a system with multiple removable nodes or modules, you must be able to withdraw or replace each removable node without affecting the operation of any adjacent nodes, regardless of power-supply levels and bus activity.

Benefits of hot-swappable I²C

Today’s sensitive, high-speed serial communication devices are not all designed to support hot-swap capabilities. In isolated I²C buses where hot swap is necessary, you can typically implement the capability with a staggered-pin design at the point of connection, which ensures the reliable connection of ground and local power supplies before making signal connections. Figure 1 is an example of a staggered connector.

Figure 1 The above example shows a staggered connector used in hot-swap applications. Source: Texas Instruments

Some I²C devices are compatible with “power-on hot swap” using staggered connectors or hot-swap controllers, which means that I²C nodes using these devices might preserve communication on the bus only if the device’s bus-side power supply (V_CC) level is always above or equal to the bus voltage levels during connection.

When you connect partially hot-swappable devices to a loaded, idle bus, you can unintentionally reduce the bus voltage levels by more than 60%. The magnitude of this reduction in bus voltage varies for each system based on external factors such as the bus’s R and C values. In different systems, this dip could be low enough to cross several I²C devices’ low-level input voltage (V_IL) thresholds, potentially causing false low readings by devices being connected to the bus. This dip is less significant for hot-swappable devices that include pin pre-charging, as shown in Figure 2 and Figure 3.

Figure 2 Non-hot-swappable I²C device without pin pre-charge is loading the 3.3-V bus down to 1.2 V during plug-in. Source: Texas Instruments

Figure 3 Hot-swap device with pin pre-charge reduces bus loading to 2.3 V during a hot swap plug-in. Source: Texas Instruments

Along with cases when you connect unpowered power-on hot swap devices to I²C buses, communication errors can also occur unexpectedly when using regular, non-hot-swappable devices as a result of transients coupling to unintended sections of an I²C device’s internal circuitry during low to high transitions on an I²C bus if signal rise times are fast enough.

Benefits of isolated I²C

The I²C protocol helps controllers communicate with sensors, data converters and other nearby ICs while isolated I²C-compatible signal isolators facilitate the use of the protocol in systems that are physically distant, operating at different local voltage potentials, and requiring isolation protection for operation or safety. Digital signal isolation protects low-voltage, logic-level subsystems from mid- to high-voltage sensors, actuators and transient events, so I²C-compatable digital isolators and their hot-swap capability combine bus dependability when adding or removing I²C nodes with isolation protection from undesired or unexpected voltage shifts.

When used with isolated power supplies, digital isolators can protect sensitive systems or those in need of protection from large DC and AC currents, ground-potential differences, and high-voltage events. Although most digital isolation devices isolate signals in one direction for communication across an isolation barrier, bidirectional signal isolators can also isolate bidirectional signals for I²C.

Bidirectional communication is inherent to I²C, and using an isolator with integrated bidirectional lines provides the benefits of low discrete component count, small footprint, fewer system-wide variations, and built-in protection over discrete solutions. Depending on the additional features of a hot-swappable I²C device, including isolated I²C devices, you can protect the bus from data corruption with capabilities such as non-fail-safe electrostatic discharge (ESD) structures or pin pre-charge and protect I²C nodes from damage with capabilities such as isolation and built-in ESD protection cells.

Isolated I²C devices with hot-swap-compliant circuitry can help maintain the data integrity of I²C buses during device and node plug-in without the need for a staggered connector. They can prevent the corruption of data on a bus caused by transients from plugging in a device or high data rates, while the integration of hot-swap circuitry and isolation simplifies reliable system-level design and shortens the system’s bill of materials (BOM).

This article was originally published on EDN.

Manny Chavez is application engineer at Texas Instruments.

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