PARALLEL TO SERIAL TRANSITION REDUCES WIRING
Circuitry and ultrasmall packaging enable use in ultra-portable applications
With portable products getting smaller and smaller, engineers look for every opportunity to save space. One approach that has been used in computers for many years is changing parallel data lines into serial data path to eliminate wires that have to be routed between various components, such as the display and the processor in a portable PC. An integrated circuit (IC) designed specifically for ultra-portable products makes this approach feasible for applications such as a hinged display in a flip phone.
High-speed data communication for higher resolution and increased number of colors can mean increased electromagnetic interference (EMI). In addition, ultra-portable applications require very low power, ultrasmall packaging, and very low cost. All of these issues have been addressed in recently introduced serial/deserialization (SerDes) ICs known as µSerDes products. Using differential signaling technologies, including complementary transistor logic (CTL) and enhanced low voltage differential signaling (LpLVDS), engineers have reduced EMI by as much 30 to 40 dBm over previous approaches. LpLVDS is LVDS technology for shorter distances and lower power with a voltage swing of 250 mV and maximum power dissipation of 5 mW per channel. CTL is the first differential signaling technology to sense current direction at the receiver. Its voltage swing is about 50 mV, power dissipation per channel is less than 1 mW at 1.8V cc, and EMI is -90 dB lower than levels in LpLVDS at fundamental frequencies. Both of these technologies have been used to develop 12-, 22-, and 24-bit bidirectional serializer/deserializer ICs that provide up to 56 MHz parallel interface operation and draw as little as 100 nA in standby. Packaging that ranges from an 8 × 8 mm 40-pin molded leadless package (MLP) to a 42-pin ball grid array (BGA) that is only 3.5 × 4.5 mm addresses both size and cost constraints in ultra-portable products.
A typical unidirectional communication application between a processor and a color LCD display can reduce the number of wires in a flex cable from 25 to only four. If the application requires bidirectional communication, the cable reduction can be as much as 50:7 and with bussing this increases to as much as a 96:7 reduction.
|Less Wires: Unidirectional communication with a pair of serial/deserializer ICs between a processor and a color LCD display can reduce the number of wires from 25 for horizontal and vertical sync (H/VSYNC) to only four.
CONTACT: Mike Fowler, Fairchild Semiconductor Tel: 207- 671-1403; e-mail: Michael.firstname.lastname@example.org://rbi.ims.ca/4393-505
EMERGING SCHEMES SOLVE VEHICLE SENSOR NOISE PROBLEMS
Both digital and current loop could be future options
To communicate high-resolution sensor data in vehicle applications such as power train, the Society of Automotive Engineers (SAE) is developing the Single Edge Nibble Transmission (SENT) encoding scheme. The Vehicle Architecture for Data Communications Standards committee document is Single Edge Nibble Transmission for Automotive Applications, SAE J2716. SENT can replace lower-resolution methods using 10 bit A-D converters and PWM techniques and provides a simpler, low-cost alternative to digital buses such as CAN or LIN.
Time Trigger: With SENT, the receiver calibrates the system on the fly by measuring the calibration pulse from falling edge to falling edge. A 4-bit status and communications nibble follows terminated by the cyclical redundancy check (CRC) nibble. Each successive nibble contains 4 bits of data, which can be used to represent two 12-bit words. For example, this could be two throttle body sensor values, but the data can be grouped in a different manner for alternative applications. The status byte transmits serial data for sensor identification or slow sensor data as temperature.
With 8-bit data and the typical 0.5 to 4.5V full-scale output in a 5V power bus, 1-bit of resolution is 16 mV. Today, 10-bit resolution is common for the more sophisticated control systems to meet emission standards and to achieve improved fuel economy and driveability. In the 10-bit system, a single bit is 4 mV, which presents a severe problem in automotive applications due to noise. Vehicle electrical systems typically show 25 to 50 mV of noise under most conditions and under extremely poor conditions this increases to 150 mV of noise. The noise level does not change in spite of the increase from 8-bit to 10-bit resolution and the need for 12-bit data is not far away.
In the digital implementation of SENT, the sensor signal is transmitted as a series of pulses with data measured as falling edge to falling edge times. The serial data protocol operates with an update rate higher than 1 kHz using a 168 µsec (±20 percent) nominal framing pulse for the start of transmission pulse, the standardization pulse. The existing noise voltage pulses do not matter when time is measured in the SENT approach, since it is a time critical system. However, in the digital three-wire system, spark type noise can be inductively coupled and produce edges in the wrong place. A two-wire 10 to 17 mA current loop greatly reduces the potential noise problem. These values are being considered instead of the 4 to 20 mA used in analog industrial applications based on the current draw of many legacy sensors. Power dissipation in the sensor and power dissipation at the receiving end in the module are among the factors being considered. In addition to reducing the noise problem beyond the digital three-wire implementation, wiring costs can be reduced as well. The current interface is being added to the standard, which should go to ballot in the summer of 2005.
CONTACT:Nick Richards, General Motors Powertrain Tel: 248-857-0163; e-mail: email@example.com
The status of SAE J2716 Standard is shown at: http://rbi.ims.ca/4393-506