Charles H. Small, Contributing Technical Editor, EDN
Analog Devices has teamed with Intel to develop a new serial bus that efficiently communicates computer-system heat dissipation and voltage-management and control information to safeguard the performance and reliability of highperformance desktop computers, workstations, and servers. The two companies have co-developed and launched the SST (simple serial transport) bus, which enables faster and more precise communication of system temperature and voltage levels.
"Thermal management is becoming increasingly important in today's electronics applications, which are growing more complex while shrinking in size. Accurately monitoring temperature in computing products and complex cores is key to ensuring protection against malfunction or failure due to excessive heat," says Susie Inouye, research director and principal analyst with marketresearch company DataBeans (www.databeans.com). "Thermalmanagement technology allows designers to successfully monitor these complex cores that are designed on submicron geometries and continue to push the limits of heat dissipation."
By more quickly and accurately relaying this environmental information to a computer's corelogic chip set, the SST bus can dramatically reduce thermalmanagement errors that can lead to a drop in computing performance. "Next-generation platforms will communicate critical system information, such as temperatures, in more and varied locations within the PC platform for enhanced thermal management and reliability. Highly accurate temperature sensing enables PCs that provide a better user experience," notes Eric Ingersoll, product-marketing engineer for Intel.
SST BUS By communicating data in a robust, noise-immune, and scalable way, the single-wire SST bus improves on the two-wire, 100kbps SMBus (system management bus) in highperformance- computing applications by offering increased bandwidth and higher noise immunity. Reducing fan noise and improving platform performance in desktop PCs, servers, and workstations, the SST bus relays key environmental information, such as temperature and voltage, directly to the system’s south-bridge core logic or dedicated ASIC fan-speed controllers at a rate of 1Mbps. Table 1 compares the SST bus to the legacy SMBus and I2C bus.
Figure 1 shows the architecture of an SST-bus PC. The processor is an Intel Core Duo. Comparable AMD processors still use the SMBus. The SST architecture includes the MCH (memory-channel-controller) northbridge IC, the ICH8 (I/O-channel-controller) southbridge IC, and external thermal sensors. The ICH8 is the SST-bus host controller, and the thermal sensors are the slave or client devices. On the SST bus, a 25% duty cycle is a logic zero, and a 75% duty cycle is a logic one.
The SST bus enhances system reliability and performance by significantly reducing communication errors. In particular, when you test the buses in the same environment on new PC motherboards, the SMBus measures about one error every 10kbits, compared with the SST bus’s one error for every 1Gbit processed. As a result, the PC user may see improved boot time and less chance of delays when the bus does not properly relay a thermal event to the core logic. In addition, the SST bus allows PC and workstation designers to use the new features, such as the recently announced Intel QST (quiet-system technology), in some nextgeneration Intel chip sets. With the integration of fan-speed control in the core logic, QST reduces the number of discrete fan-control components in the system, which can lower BOM (bill-of-materials) costs and allow system developers to use more programming options.
"A bus was required to enable industrywide compatibility with system-management devices, such as temperature sensors and voltage monitors in computing applications," says Steve Peterson, Intel’s director of chip-set and software marketing. "Working with Analog Devices, we developed a common, robust interface that all licensed vendors can easily incorporate, allowing them to add custom capabilities, such as the Intel QST, for new environmental features in PCs, servers, and workstations."
SMBUS AND I2C BUS Intel's earlier systemmanagement bus, the SMBus, is a two-wire bus similar to the I2C bus. In the early 1980s, Philips Semiconductors developed the I2C (Inter-IC), a simple, bidirectional, two-wire bus for efficient inter-IC control in TV sets. Philips' IC range currently includes more than 150 CMOS and bipolar I2C-buscompatible types for performing communication functions between intelligent control devices, such as microcontrollers; general-purpose circuits, such as LCD drivers, remote I/O ports, and memories; and application-oriented circuits, such as digital-tuning and signalprocessing circuits for radio and video systems.
All I2C-bus-compatible devices incorporate an on-chip interface, which allows them to communicate directly with each other through the I2C bus. This design concept solves the many interfacing problems you encounter when designing digital-control circuits. I2C has become a worldwide de facto standard that is now implemented in more than 1,000 ICs and is licensed to more than 50 companies.
MEASURING TEMPERATURE A simple method for measuring the temperature of highperformance processors and other circuitry is to exploit the negativetemperature coefficient of a diodeconnected transistor by measuring the base-emitter voltage, VBE, of a transistor operated at constant current. Unfortunately, this technique requires calibration to null the effect of the absolute value of VBE, which varies from device to device.
Analog Devices' chips use a technique that measures the change in VBE when you operate the device at three different currents. In this scheme, beta compensation, the beta of a transistor varies with current. National Semiconductor pioneered beta-compensation technology and was the first to bring it to market, with the introduction of TruTherm technology in 2005. Figure 2 shows the inputsignal conditioning that the SST-bus temperature sensor uses to measure the output of a remote temperature sensor. This figure shows the remote sensor as a substrate transistor, which Core Duo microprocessors include for temperature monitoring, but it could also be a discrete transistor near the processor. If you use a discrete transistor, do not ground the collector and link it to the base. If the sensor is operating in an extremely noisy environment, you can add C1 as a noise filter. Its value should not exceed 1,000pF. To prevent ground noise from interfering with the measurement, you do not reference the more negative terminal of the sensor to ground, but you bias it above ground by an internal diode at the D1– input (Figure 3).
DISCRETE TRANSISTORS If you use a discrete transistor, you do not ground the collector but instead link it to the base. If you use a PNP transistor, the base connects to the D1- input, and the emitter connects to the D1+ input. If you use an NPN transistor, the emitter connects to the D1– input, and the base connects to the D1+ input.
Figure 3 shows how to connect the ADT7484/ADT7486 to an NPN or PNP transistor for temperature measurement. To prevent ground noise from interfering with the measurement, do not reference the more negative terminal of the sensor to ground but bias it above ground by an internal diode at the D1– input.
PRECAUTIONS Digital boards can be electrically noisy environments. Analog Devices advises you to take the following precautions to protect the analog inputs from noise, particularly when measuring the small voltages from a remote diode sensor: - Place the device as close as possible to the remote-sensing diode. Provided that you avoid the worst noise sources, such as clock generators, data/ address buses, and CRTs, this distance can be 4 to 8in. - Route the D1+ and D1– tracks close together in parallel with grounded guard tracks on each side. Provide a ground plane under the tracks if possible. - Use wide tracks to minimize inductance and reduce noise pickup. Analog Devices recommends a minimum track width and spacing of 5mils. - Minimize the number of copper/solder joints, which can cause thermocouple effects. If you use copper/ solder joints, make sure that they are in both the D1+ and the D1–paths and are at the same temperature. - Place a 0.1μF bypass capacitor close to the device. - If the distance to the remote sensor is more than 8in., use a twisted-pair cable. This approach works for distances of approximately six to 12 feet. - For distances as long as 100 feet, use shielded twisted-pair cables, such as Belden #8451 microphone cables. Connect the twisted-pair cable to D1+ and D1– and the shield to ground, close to the device. Leave the remote end of the shield unconnected to avoid ground loops.
Thermocouple effects should not be major problems because the specified 1C resolution corresponds to about 240μV, and thermocouple voltages are about 3μV/°C of the temperature difference. Unless there are two thermocouples with a big temperature differential between them, thermocouple voltages should be much less than 200mV.
According to the spec sheet, you select the client address for Analog Devices’ ADT7484 and ADT7486 using the address pins. The address pins connect to a floatdetection circuit, which allows the devices to distinguish between three input states: high, low (ground), and floating. The address range for fixed-address, discoverable devices is 048 to 050, permitting as many as nine slave devices per SST bus (Table 2).
The SST bus, like Ethernet, is a networked, message-passing bus. For a summary of the commands that the ADT7484 and ADT7486 devices support when directed at the target address that the fixedaddress pins select, check out Table 3. The table contains the command name, command code, write-data length, read-data length, and a brief description.
AVAILABLE DEVICES Analog Devices’ ADT748x family comes in eight- or 10-pin MSOPs for easy placement in areas in which space is at a premium. The ADT7484A and ADT7486A are simple, ±1C-accurate, digitaltemperature sensors that monitor their own temperature, as well as one (ADT7484A) or two (ADT7486A) remote-sensor diodes. The ADT7485A digital-temperature sensor and voltage monitor can sense its own temperature, as well as that of a remote-sensor diode. The ADT7485A can also monitor four external voltage channels and its own supply voltage using its onboard 10-bit ADC.
The Andigilog aSC7521 SST-bus remote digital-temperature sensor, accurate to ±1°C and an operating range of –40 to +125°C, measures its own temperature and that of a remote diode on the CPU or on other critical system components in which heat build-up may suddenly occur.
The Andigilog aSC7531 SST-bus remote digital-temperature sensor and voltage monitor also measures two temperatures and adds accurate monitoring of critical system voltages. The combination of these two parts in a system plays a critical role in reporting system health with great precision to supervisory programs in Intel’s high-performance chip sets. The aSC7521 sells for $1.25, and the aSC7531 sells for $1.50 (1,000).
SMSC offers the EMC1102 and EMC1152. Both devices employ beta compensation to accurately measure temperature from a 65nm processor. Each device is a dualtemperature sensor, but the EMC1152 also measures five supply voltages in desktop PCs, allowing it to monitor supply-voltage rails on the motherboard. Production samples of both the EMC1102 and the EMC1152 are available now. Prices for the parts are 90 cents and $1.20 (10,000), respectively. The EMC1102 is available in an eightpin, "green," lead-free MSOP, and the EMC1152 is available in a 10-pin, green, lead-free MSOP.
AUTHOR INFORMATION Charles H. Small is a contributing technical editor for EDN. You can reach him at charles.small@att.world.net.
