What is zero wait state memory in Arm Cortex-M series?

Zero wait state memory in Arm Cortex-M series microcontrollers refers to the ability to access external memory without any additional wait states or pipeline stalls. This enables the Cortex-M core to run at maximum efficiency by eliminating memory access delays.

Introduction to Wait States

In microcontroller systems, wait states are often necessary when accessing external memories or peripherals. This is because external devices are generally slower than the microcontroller’s core clock speed. So the microcontroller must insert idle clock cycles, called wait states, when accessing slower external devices to account for their longer response times.

For example, if the Cortex-M3 core is running at 100 MHz but the external SRAM has an access time of 10 ns, the core would have to insert a wait state for every external access. This is because at 100 MHz, each clock cycle takes only 10 ns. So with a 10 ns access time, the external SRAM will not be ready with the data within a single clock cycle. By inserting an extra wait state, the microcontroller waits for 20 ns instead of just 10 ns for the SRAM to respond.

Without wait states, the core would attempt to sample the external memory before its data is ready, leading to incorrect results. So wait states are essential to ensure synchronous and reliable operation between the fast processor core and slower external devices.

Problems with Wait States

While wait states are necessary, they also come with some disadvantages:

• Reduced performance – Each wait state inserted means an extra clock cycle of delay for every external access. This reduces the effective core clock speed.
• Higher power consumption – The extra cycles spent waiting translate into higher power consumption as more clocks are needed per instruction.
• Complex software – The number of required wait states may need complex software management since access times can vary between devices.
• Memory bottlenecks – External memory accesses may become a bottleneck since they take much longer than internal activities.

Eliminating wait states can help overcome these problems and improve the responsiveness and efficiency of the microcontroller.

Zero Wait State Memory

Zero wait state (ZWS) memory refers to external memories that can be accessed without the need for extra wait states. Two conditions must be met to implement ZWS memories:

1. The ZWS memory must have an access time equal to or less than the minimum clock period of the microcontroller core clock.
2. The ZWS memory interface must be simple enough such that the address and controls signals meet the setup and hold time requirements within a single clock cycle.

For example, to have ZWS operation with a 100 MHz Cortex-M3, the external memory must have an access time of 10 ns or less. This allows it to respond within one clock period. And the memory interface must have enough time to transmit the address and control signals within 10 ns as well.

ZWS memories allow the microcontroller to run at maximum speeds without being hampered by external memory accesses. This helps overcome the bottlenecks mentioned previously and provides significant performance advantages.

Types of Zero Wait State Memories

Some common memory technologies capable of ZWS operation with Cortex-M cores include:

• SRAM – Static RAM can be designed with fast access times meeting ZWS requirements.
• PSRAM – Pseudo SRAM provides DRAM density with SRAM interfaces conducive to ZWS operation.
• RLDRAM – Reduced latency DRAM with fast row access can attain ZWS speeds at lower costs.
• DDR – Double data rate synchronous DRAM modules with ZWS capabilities are available.
• SDRAM – Some synchronous DRAMs can be clocked fast enough to enable ZWS.

Of these, SRAM provides the simplest interface and fastest access times to enable zero wait states. PSRAM and RLDRAM tradeoffs make them viable, cost-effective alternatives in many applications requiring medium/high densities and sizes.

ZWS Memories in Arm Cortex-M MCUs

Many Arm Cortex-M microcontrollers include memory interfaces designed to support ZWS capabilities. Some examples include:

• Cortex-M7 – Supports SRAM/PSRAM/SDRAM with ZWS at up to 300 MHz.
• Cortex-M4 – Up to 150 MHz ZWS operation with SRAM or PSRAM.
• Cortex-M3 – External bus interface includes ZWS logic supporting SRAM at up to 100 MHz.
• Cortex-M0+ – Capable of 50 MHz ZWS access for SRAM.

Higher-end Cortex-M cores typically have faster interfaces and dedicated control signals to enable seamless ZWS memory integration. The external memory is treated as an extension of internal memory, capable of single cycle accesses.

In addition to the microarchitecture features, Arm MCU vendors implement physical layer optimizations such as minimizing trace lengths, ideal termination schemes, etc. to attain maximum ZWS memory performance. Many vendors provide memory recommendations and interface guidelines when using their Cortex-M devices for ZWS operation.

Benefits of ZWS Operation

The benefits of integrating ZWS memories with Cortex-M cores include:

• Faster processing – Eliminating wait states allows the core to operate at maximum rated speeds.
• Predictable latency – ZWS accesses take exactly one clock cycle, simplifying software scheduling.
• Lower power – Increased performance per clock cycle reduces overall energy usage.
• Real-time responsiveness – ZWS operation provides very consistent response times.
• Simplified software – No complex wait state management code required.

These benefits enable Cortex-M systems to tackle increasingly complex and demanding real-time processing requirements. Tasks such as high-resolution sensor fusion, servo control, wireless communication, and digital signal processing are made possible using ZWS memory interfaces.

Use Cases and Applications

Some common applications that leverage ZWS memory with Cortex-M microcontrollers include:

• IoT endpoints – ZWS SRAM improves responsiveness for time-critical sensor aggregation and wireless transmission tasks.
• Industrial automation – Real-time control and predictive maintenance algorithms demand ZWS data access.
• Automotive – Safety-critical applications like motor control and ADAS require ZWS memory interfaces.
• 5G infrastructure – Low latency and jitter necessitate ZWS memory in 5G gear such as small cells.
• Aerospace – Hard real-time control systems in UAVs, spacecraft etc. use ZWS memories.

In essence, any application with demanding latency, determinism, throughput, or real-time requirements can benefit greatly from harnessing ZWS memory configurations in Cortex-M implementations.

Design Considerations for ZWS Operation

Some key design aspects to consider when implementing ZWS external memory include:

• Memory technology selection – Assess tradeoffs like density, power, cost, and interface complexity.
• Memory access patterns – Whether mostly sequential or random accesses to optimize throughput.
• Memory controller options – Optimized Arm CoreLink controllers for managing ZWS memories.
• Board layout guidelines – Minimize trace lengths, optimal topology, termination schemes.
• Signal integrity analysis – Ensure clean signals and account for reflections, crosstalk.
• Power delivery design – Low noise, adequate decoupling for memory and SoC.

Vendors also provide memory interface design guidelines for their specific Cortex-M devices. Utilizing hardware and software performance analyzers is crucial to validate and optimize the ZWS memory subsystem design.

Conclusion

Zero wait state memory interfaces unlock the true performance potential of Arm Cortex-M cores. By eliminating external memory access delays, ZWS configurations enable Cortex-M-based microcontrollers to tackle increasingly demanding embedded applications. Arm MCU vendors provide system architects the flexibility to choose from a range of suitable memory technologies while providing the necessary design guidelines and optimizations required to implement an efficient ZWS memory system tuned for their application needs.