In order to meet the demands of increased functionality, performance, and reduced power, many modern circuit boards use highly integrated CPUs, ASSPs, ASICs, and memory devices to implement the circuit board’s main function (the payload function).
Boards of this complexity are particularly common in equipment designed for communications infrastructures, computer servers, and higher end industrial and medical systems. Because the ICs on the board are usually fabricated with fine transistor geometries, they require multiple power supply rails with tight tolerances to operate. Typically, seven to ten supplies are needed in a complex circuit board, with higher numbers not unusual.
The management of these supplies – along with other system management tasks – is increasing in complexity and cost. This is leading many board designers to ask: "How can I reduce the cost and complexity associated with implementing board management?"
Common board (platform) management functions
Board management (also referred to as platform management) consists of those portions of the board involved with the management of the power supplies as well as the portions responsible for providing the correct digital environment for the main ICs to operate. The blue (power management) and red (digital management) boxes in Figure 1 illustrate some common platform management functions.
Figure 1. Platform management functions in a complex circuit board
Traditional implementation of platform management
- Hot-swap Controller: Required if this card is required to be plugged into a backplane. This functionality is usually implemented using Hot-swap ICs.
- Supply OR’ing: Required if the circuit board is powered by two or more rails and the card is required to select one of the supplies from which to derive power. Typically this is done in systems that use redundant power supplies in order to improve reliability.
- Sequencing and Monitoring: Usually these payload ICs require that the supplies be turned on in a pre-determined sequence. This is controlled by the power supply sequencing ICs, and in some cases CPLDs are used. However, these supplies should also be monitored for faults. If a fault occurs, the CPU should be reset to prevent faults such as flash corruption. Supervisor ICs/ Reset ICs are used to implement voltage monitoring.
- Power Supply Trimming and Margining: In order to meet the dynamic power requirements of complex ASICs and CPUs, their core supply voltage is maintained within a tight range. This is called trimming. For quality control purposes, a circuit board’s operation headroom is measured by skewing the supply voltage to its maximum and minimum values. This is called margining. Margining and trimming require ADCs and DACs.
- Power Feed: Sometimes, circuit boards are required to feed power to a daughter card. A power feed circuit is implemented using hot-swap controller.
- Reset Distribution: During the power up process, the payload ICs should be held in reset and are brought out of their reset state in a controlled manner. This function is usually implemented in an FPGA or a CPLD.
- Power-on Configuration: After the reset for the payload ICs is released, they should be set into an operating mode depending on the hardware environment. For example, a CPU should know if the I/O interface bus is a 16- or 32-bit bus, or if its on-chip Ethernet controller is enabled or not, or the number of wait-states required to access a Flash ROM. These configuration words are loaded using either the data bus or any other multipurpose pins. This function is usually implemented using an FPGA, or in some cases discrete buffers.
- System Interface: Boards require the monitoring of multiple sensors, such as temperature or enclosure sensors and controlling LEDs or turn on fans, or any other peripherals. These functions require many memory mapped I/O signals that are usually implemented using CPLDs.
- Fault Logging: This captures a snapshot of the fault to help the repair centers fix the failed boards –similar in purpose to an airplane black box for the circuit board.
Traditional approaches to implementing board management utilize multiple discrete ICs to implement the major functions required on the circuit board. For instance, individual hot-swap controllers, reset generators, power supply supervisors, and CPLDs are commonly found on circuit boards to implement these functions as illustrated in Figure 2. This approach can be problematic in the areas of cost, reliability and risk.
Figure 2. A traditional platform management solution
Increased Bill of Materials (BOM) and Cost:
involves many single-function ICs and discretes.
Typically a designer is required to select a unique single function IC from a large list of part numbers to implement each of the board management functions. This results in an increased bill of materials (BOM) across the project. An Increased bill of materials limits the ability to reduce the price per component using a volume discount. In addition, the cost of inventory management also increases along with the increased BOM.
The cost of implementation is also high due of duplication of sub-functions in many of these single function ICs. For example, if a circuit board requires a hot-swap controller, sequencer, supervisor and reset generator ICs, then the building blocks of each of these ICs, such as voltage references, comparators and charge-pumps, are repeated multiple times.
Table 1 reflects the cost of traditional components used to implement reliable platform management in a high power blade.
Table 1. The component costs associated with
a typical platform management solution.
As can be seen, the cost of implementing functions that are just required just to manage the payload ICs can be quite high. To reduce cost, one of the common steps taken by the engineer is to sacrifice the performance of these ICs. For example, highly accurate supervisor and reset ICs are more expensive. So, engineers use less accurate devices for voltage monitoring. For example, if the accuracy of the supervisor IC is traded off from <1% to 2%, the cost savings can be more than half. But this trade off significantly increases the chances that the processor will be operating below its minimum operating voltage.
As a result, the probability of Flash corruption or other unpredictable behavior is high. Bottom line, the reliability of the board is reduced. Not only that, but the large number of components used on the board statistically further reduce reliability.
Inflexible, with Increased Risk of Board Re-spin:
Single function ICs are usually hardwired. That makes the traditional solution inflexible if changes need to be made after the circuit board is laid out. Many times, a circuit board re-spin is necessary.