Power and cooling of multichip SHARC DSP boards: is it all smoke and mirrors?
By Kim R. Fowler
Something interesting has been happening in the DSP market recently. The power consumption of multiprocessor DSP boards has been ignored, misunderstood, or hidden. The same may be said for cooling of these boards. It seems that either a basic law of physics has been forgotten or some vendors are working a sleight of hand.
The basics
As designers increase the numbers of processors on their board designs, they also increase power consumption and heat production. Power (P) is defined as voltage (V) multiplied by current (I): P = V x I. Since most power supplies maintain a fixed voltage, additional processors will draw incrementally more current. Consequently, power consumption and heat production will increase linearly.
Consider a simple example of multiple processors. The SHARC DSP processor typically consumes between two and three watts of power when it runs on a 5-volt supply and is reasonably loaded with continuous computations and link port communications.
A SHARC might consume only 2 watts when running less than full computational load, making few accesses across the external bus, and supporting light communications on the link parts. Alternately, a SHARC might consume 3 watts when performing continuous computations, making many accesses across the external bus, and supporting full bandwidth on the link ports. Keep in mind that the specified maximum for the SHARC is 4.25 watts. Even though two to three watts per SHARC is an observed reality for a typical DSP application, it can be higher.
The VME specification for power consumption per slot in a VMEbus chassis is 30 watts. The capability of most commercial chassis is nearly twice that, or 60 watts per slot. Table 1 reflects the power demand by the SHARC processors only, and does not include the power demand of other components and circuits, such as memory, interface circuitry, and I/O. Obviously, the figures for power will increase in Table 1 when other circuitry operates on the board.
Ixthos has measured power consumption of 27 watts by its 5-volt IXZ8 (eight-SHARC) board under typical maximum conditions. Using Table 1 as a guide, the additional power required by the board support circuitry is between three watts and 11 watts.
Table 2 conservatively estimates power consumption for a single VME board with multiple 5volt processors, operating at 2.5 watts per processor, and support circuitry drawing a constant seven watts.
Buyer beware
What does this mean to the customer? Well, a board populated with 20 or more SHARC processors, each running at 5 volts, will exceed the VME specification by a factor of two for a single slot. How does a vendor supply that much current to a board without melting the power pins? You could use two slots in the VME chassis and a cabling kit to siphon current from two sets of P-l connectors onto a single board. But this would defeat fitting a board into a single slot.
Even if you can supply the current to the board, will the chassis be able to cool it? As board and system temperatures rise, temperature-sensitive components exhibit anomalous behavior, and mean time between failure rates decrease significantly, which result in compromises to system reliability.
Some vendors have been fond of using mezzanine cards to attach more processors to a single board. These cards leave only miniscule gaps between the mezzanine components and the components on the baseboard, which reduce airflow and cooling. Cooling options include opening up the slots on either side of the DSP board, using a chassis with fans which move high volumes of cooling air, or even moving to alternative cooling technologies. Regardless, some significant changes or precautions are necessary to address the increased risk to the system.
A cool solution
Ixthos designers have developed the first 16-SHARC processor board with their IXZ16. The board uses Quad-SHARC multichip modules attached directly to the baseboard, with each SHARC running at 3.3 volts to reduce power consumption and heat production over the 5-volt SHARC counterparts. The power consumption of the board, including support circuitry, is less than 31 watts under typical maximum conditions.
The IXZ16 adds eight more processors to a single VME slot over the IXZ8, but only slightly increases power consumption. The low power consumption for the IXZ16 also means that heat production is low. The temperature rise does not exceed 12 degrees Celsius above ambient for the hottest components on the 1XZ16. Consequently, the IXZ16 is ideal for large and small multi-processor systems since it provides massive computing with low power consumption and heat production.
Ixthos, in partnership with Analog Devices, developed the Quad-SHARC multichip module in the ceramic ball-grid-array package for use on the IXZ16. Had Ixthos used 5-volt SHARC processors on the IXZ16, the power consumption and the heat production would have nearly doubled. So how do other SHARC board manufacturers run 16, 20, or 24 SHARCs in a single VMEbus slot at 5 volts? It could be mirrors; most likely it will be smoke.
Kim R. Fowler is manager of applications engineering at Ixthos Inc., a designer of single-board digital signal processors (DSPs) in Leesburg, Va. Ixthos, a subsidiary of DY 4 Systems Inc. of Kanata, Ontario, specializes in boards based on the SHARC DSP from Analog Devices in Norwood, Mass.
