Low-Power Peripherals in AXI and Their Implementation

ARM AXI Low-Power Interface for Peripheral Clock Management

The ARM AXI (Advanced eXtensible Interface) protocol includes an optional low-power interface extension designed to manage clock gating for peripherals in a system-on-chip (SoC). This interface is particularly useful for optimizing power consumption in designs where certain peripherals may remain idle for extended periods. The low-power interface allows peripherals to signal when they require an active clock (ACLK) and when they can operate without it, thereby reducing dynamic power consumption. This mechanism is critical in modern SoC designs, where power efficiency is a key design constraint.

The low-power interface targets two classes of peripherals: those that can have their clocks stopped immediately without any preparation and those that require a specific power-down or power-up sequence before the clock can be stopped or restarted. The interface is optional, meaning it only needs to be implemented if the design requires dynamic clock gating for power optimization. If not implemented, the ACLK input to the peripheral remains permanently active.

The primary motivation for using the low-power interface in AXI is to reduce power consumption in scenarios where peripherals are not actively in use. For example, in a mobile device, peripherals such as Bluetooth or GPS modules may not be needed for extended periods. By gating the clock to these peripherals, the system can significantly reduce dynamic power consumption, extending battery life. The low-power interface provides a standardized way to manage this clock gating, ensuring compatibility across different peripherals and simplifying the design of the clock controller.

Peripheral Clock Gating Requirements and Power-Down Sequences

The implementation of the low-power interface in AXI depends on the specific requirements of the peripherals in the system. Peripherals can be broadly categorized into two types based on their clock gating requirements: simple peripherals and complex peripherals.

Simple peripherals are those that can have their clocks stopped immediately without any additional preparation. These peripherals typically do not have any internal state that needs to be preserved when the clock is stopped, or they can automatically save and restore their state when the clock is restarted. For example, a simple GPIO peripheral may fall into this category, as it does not require any special handling when the clock is gated.

Complex peripherals, on the other hand, require a specific power-down or power-up sequence before the clock can be stopped or restarted. These peripherals often have internal state that needs to be preserved during clock gating, or they may need to perform certain operations to ensure proper functionality when the clock is restarted. For example, a USB controller may need to complete any ongoing transactions and save its internal state before the clock can be stopped. Similarly, when the clock is restarted, the USB controller may need to reinitialize its internal state and resume any pending transactions.

The low-power interface in AXI provides signals to support these requirements. The peripheral can use the CACTIVE signal to indicate whether it requires an active clock. When CACTIVE is high, the clock controller must provide an active clock to the peripheral. When CACTIVE is low, the clock controller can gate the clock to the peripheral, provided that the peripheral has completed any necessary power-down sequence. The peripheral can also use the CSYSREQ and CSYSACK signals to coordinate the power-down and power-up sequences with the clock controller.

Implementing Low-Power AXI Peripherals and Clock Controller Integration

To implement the low-power interface in an AXI-based SoC, the designer must first identify which peripherals require clock gating and determine whether they are simple or complex peripherals. For simple peripherals, the implementation is straightforward: the CACTIVE signal can be tied high if the peripheral always requires an active clock, or it can be controlled by the peripheral’s internal logic if the peripheral can operate without a clock in certain states.

For complex peripherals, the implementation is more involved. The peripheral must include logic to manage the power-down and power-up sequences, and it must coordinate these sequences with the clock controller using the CSYSREQ and CSYSACK signals. The clock controller, in turn, must be designed to respond to these signals by gating or ungating the clock as required.

The following steps outline the process for implementing the low-power interface in an AXI-based SoC:

1. Identify Peripherals Requiring Clock Gating: Review the system design to identify which peripherals can benefit from clock gating. Consider the power consumption of each peripheral and the expected duration of idle periods.

2. Classify Peripherals as Simple or Complex: Determine whether each peripheral is simple or complex based on its clock gating requirements. Simple peripherals can have their clocks stopped immediately, while complex peripherals require a power-down or power-up sequence.

3. Design Peripheral Logic for Clock Gating: For simple peripherals, implement logic to control the CACTIVE signal based on the peripheral’s operational state. For complex peripherals, implement logic to manage the power-down and power-up sequences and to coordinate with the clock controller using the CSYSREQ and CSYSACK signals.

4. Integrate Clock Controller: Design the clock controller to respond to the CACTIVE, CSYSREQ, and CSYSACK signals by gating or ungating the clock to the peripheral as required. Ensure that the clock controller can handle multiple peripherals with different clock gating requirements.

5. Verify Low-Power Operation: Use simulation and formal verification techniques to verify that the low-power interface operates correctly under all expected conditions. Test the system with different combinations of active and idle peripherals to ensure that power consumption is minimized without compromising functionality.

6. Optimize Power Consumption: Analyze the power consumption of the system with the low-power interface enabled and identify opportunities for further optimization. Consider adjusting the clock gating strategy based on the observed power consumption and system performance.

By following these steps, designers can effectively implement the low-power interface in AXI-based SoCs, reducing power consumption and improving the overall efficiency of the system. The low-power interface is a powerful tool for optimizing power consumption in modern SoC designs, and its proper implementation can lead to significant improvements in battery life and thermal performance.