Now that we’ve become used to USB you really appreciate the facility to connect or disconnect devices without having to turn the power off first. This was also true for RS232 (but not with LPT), but you never felt at ease with it. I2C or SMBus devices are unfortunately not hot-swappable. A component that solves half of the problem (the switching part) has been introduced recently by Linear Technology.
The LTC4300 is a 2-wire interface buffer that can isolate the signals between peripherals and the bus, making it possible to add another device to the bus at any time, without causing any interference. The next step is more difficult and you will have to find the solution for this yourself: you have to find a way to detect when there is no activity on the bus. At that time the interface chip can be enabled, causing the peripheral to be connected to the bus. The buffer contains active pull-ups, permitting the use of high-value (10 kΩ) pull up resistors. More information about the LTC4300 can be found at www.linear. com.
The LTC4300 series hot-swappable 2-wire bus buffers allow I/O card insertion into a live backplane without corruption of the data and clock busses. When the connection is made, the LTC4300-1/LTC4300-2 provides bidirectional buffering, keeping the backplane and card capacitances isolated. Rise time accelerator circuitry* allows the use of weaker DC pull-up currents while still meeting rise time requirements. During insertion, the SDA and SCL lines are precharged to 1V to minimize bus disturbances.
- Bidirectional Buffer for SDA and SCL Lines Increases Fanout
- Prevents SDA and SCL Corruption During Live
- Board Insertion and Removal From Backplane
- Isolates Input SDA and SCL Lines From Output
- Compatible with I2C, I2C Fast Mode and SMBus
- Standards (Up to 400kHz Operation)
- Low ICC Chip Disable: <1µA (LTC4300-1)
- READY Open-Drain Output (LTC4300-1)
- 1V Precharge on All SDA and SCL Lines
- Supports Clock Stretching, Arbitration, and Synchronization
- 5V to 3.3V Level Translation (LTC4300-2)
- High Impedance SDA, SCL Pins for VCC = 0V
- Small MSOP 8-Lead Package