How to reduce power consumption in CPLD designs with power supply cycling

The duty cycle approach to power consumption reduction relies on the fact that for much of the time equipment is operational there is often little processing required by the CPLD. For example, the CPLD may be scanning a key pad to see if a key press has occurred, or is waiting for some communication on a serial interface.

CPLDs, like all integrated circuits, consume some power associated with device bias and leakage, regardless of activity. This power is often referred to as static current. CPLDs also consume power as a result of signals switching inside the device. This power is often referred to as dynamic power.

The amount of energy consumed as dynamic power is proportional to the amount of processing undertaken. Thus, achieving an operation in 0.1 seconds at 100 MHz consumes approximately the same amount of energy (in terms of dynamic power) as achieving the same operation in ten seconds at 1 MHz.

These characteristics of dynamic and static power can be used to minimize the overall power consumption of a design by compressing required activity into a small time period and turning off the CPLD during periods of inactivity.

1. Overview of duty cycle approach.

For the duty cycle approach to work, the power up and down time of the CPLD clearly needs to be brief compared with the frequency at which the CPLD must be activated in order to achieve the desired processing and system response time. Fortunately, currently available CPLD devices utilize on-chip, non-volatile memory that yields power up and down times below 1 mS. This allows CPLDs to be activated at frequencies up to the low hundreds of Hz while still yielding useful power savings due to the duty cycling approach.

Implementing the switch
When implementing the duty cycle approach, it is necessary to engineer the design so that it is possible to turn the CPLD device on and off. There are several methods available to achieve this. However, the method chosen will be influenced by several factors. Key factors to consider include:

• The number of power supplies the CPLD has.
• What other devices, if any, share power supplies with the device.
• Whether devices sharing the same power supply rail can be duty cycled.
• The controls available within the power supply subsystem for disabling power supplies

There are three primary methods that can be considered when implementing duty cycling of the CPLD. The advantages and disadvantages of each method are described below.

Use Power Supply Disable on the Voltage Regulator Module (VRM): Many designs use VRMs to provide power to the chips within the design, and often these modules have a power enable/disable input signal that can be used to duty cycle the CPLD. The advantage of this approach is its relative simplicity. One disadvantage of this approach is its course grained nature, requiring all devices that are fed by the particular module to be cycled on and off at the same time. The second disadvantage is that the time required to power down and power up the VRM reduces the frequency at which the duty cycling can occur.

Use FETs on the Power Lines to the CPLD(s): In this approach FETs (Field Effect Transistors) are placed in the various power supply lines of the CPLD to be duty cycled. This approach has two key advantages. First, it allows specific device(s) to be duty cycled. Second, the power can be turned on and off very rapidly, which allows the device to be duty cycled at a higher frequency. The disadvantage of this approach is that additional devices are required on the circuit board, driving up cost and board area.

Use CPLD Sleep-Pin Functionality: An increasing number of PLDs, such as Lattice's MachXO family, incorporate a "sleep pin" that allows the device to be disabled and the static power consumption reduced to almost zero. This functionality can be used to implement duty cycling. With this approach, only the specific CPLD is duty cycled and the turn on and off times allow for a high frequency of duty cycling. This approach also uses minimal components, saving cost and board area.