significantly lower than bus pull-up resistances. Additionally, the accelerators permit the use of larger bus pull-ups, which reduce power consumption and improve logic low noise margin. Another common timing issue is when the I2C bus sticks low – communication issues, data full of zeroes, and strange GUI behaviour are all symptoms of a bus stuck low, where SCL or SDA can be stuck at ground usually due to faulty or confused I2C devices. If not corrected, a stuck device prevents other devices on the bus from communicating until the stuck device releases high.
When a bus sticks low, the host must manually
troubleshoot the bus, which usually results in a system reset that is disruptive, costly and time consuming.
Stuck busses are easily resolved by using Linear
Technology bus buffers with selectable stuck bus protection, like the LTC4315, where the SDA and
completed, a stop bit is generated to clear the bus for further communications and the input-to- output I2C connections are reconnected only when the bus is idle and not in the middle of an I2C transaction.
Figure 3
SCL outputs are continuously monitored for a low state of more than 30ms. If this occurs, the input- to-output connections for both I2C signals are broken, which prevents further communication with the stuck device, and the bus buffer generates up to 16 clock pulses on the SCLOUT pin in an attempt to free the card-side stuck bus. If the bus unsticks or if the 16 clock pulses are
Signaling simplified The 111 physical addresses allotted by standard mode I2C may be sufficient for most systems, but larger systems may need to provide communication for more than 111 devices. Perhaps a more common issue is that system designers are forced to use slave devices that provide only a few unique addresses, necessitating the reuse of the same address (nested addressing). In these cases, Linear Technology I2C multiplexers should be used to permit the reuse of I2C physical addresses. Due to their bi-directional nature, these multiplexers can also be used to drive I2C data from the output buses to the input bus. The LTC4314 1:4 pin-selectable mux, where four I2C devices are all assigned the same address and individual enable pins allow users to choose which downstream busses the upstream bus will is be connected to. An LTC4306 1:4 software- selectable mux is also available and works in a similar fashion, but uses I2C commands instead of pins to choose which downstream busses the upstream bus will be connected to. These I2C multiplexers also integrate bus buffers, so all of the unique features Linear Technology bus buffers bring are also provided, including high noise margin, rise time acceleration and stuck bus protection.
Conclusion
At over 30yrs old, the I2C bus is still being widely used, where larger more complex systems continue to challenge its specifications and limitations. Designers can address a wide variety of common electrical, timing and signalling issues
Linear Technology |
www.linear.com
Chris Gobok is Product Marketing Engineer, Mixed Signal Products, Linear Technology
encountered during design, but a better practice would be to take preventative measures against these issues because you do not always know what types of I2C devices will ultimately connect to your system – a non-compliant I2C device or
stuck bus may just be around the corner. By designing in the right combination of I2C bus buffers, rise time accelerators, and muxes from start, you can easily meet I2C specifications, while expanding your system without impractical restrictions and issues to slow you down.
Obsolete Chip?
E-tec offers a range of competitively priced package adaptors to solve obsolete package problems. These high quality adaptors are typically produced in 1.6mm thickness material or where the upper footprint interferes with the lower footprint then 2.4mm material is used. Here the pins are fitted into blind holes from below and the combination of a press fit and a solder fillet around the pin shoulder provides a reliable and robust connection.
Daughter boards can be created to customer requirements and raised pins are available to raise the complete assemble off the pcb to the required level.
E-tec welcomes special adaptor enquiries, send the chip package details for the top and bottom parts and we will respond promptly with a quotation.
Example pricing Top Part
BGA24 DIP624 LCC52
PLCC28 PLCC44 PLCC44 PLCC52 SOIC14 SOIC14 SOIC16
Bottom Price 10 £ea DIP-624 DIP324 PGA52 DIP324 DIP640 PGA44 PGA52 DIP314 TO10
DIP316
SOIC18 wide DIP318 SOIC20 wide DIP320 SOIC8N SSOP28
DIP308 DIP328
8.20 2.25 4.20 4.50 2.40 8.40 5.80 1.70 2.20 2.28 1.90 2.20 1.45 4.20
t: 01892 530260 f: 01892 515560 e:
info@e-tec.co.uk
www.e-tec.co.uk CIE electronica 2014 11
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