Cache Memory Positioning in an MCU

Introduction

In a Microcontroller Units (MCUs), cache memory is strategically positioned to optimize performance by reducing data and instruction access times. This article delves into the intricacies of cache memory within an MCU, examining its architecture, the differences between data cache (D-cache) and instruction cache (I-cache), data flow, and the significance of cache memory.

MCU Architecture

To understand the positioning of cache memory, it is crucial to grasp the basic architecture of an MCU, which typically includes:

• CPU : The heart of the microcontroller, executing instructions and processing data. Composed of several functional units, including calculation units and registers.
• Flash memory : Non-volatile memory where program code is stored.
• RAM (Random Access Memory) : Volatile memory used to store temporary data and variables during program execution.
• D-cache (Data Cache) : Located between the processor's instruction control unit and the Flash memory. Stores frequently used instructions to reduce access time.
• I-cache (Instruction Cache): Located between the processor's data processing unit and RAM). Stores frequently accessed data to improve performance.

Data flow

Understanding the data flow within an MCU is essential for optimizing cache usage . There are two primary types of data flow: instruction access and data access :

Instructions Access :

1. Request: The processor requests an instruction

2. Cache Check: The instruction cache (I-cache) is checked first.

3. Cache Hit: If the instruction is present in the cache, it is provided to the processor quickly.

4. Cache Miss: If the instruction is not in the cache, it is read from flash memory and then stored in the cache for future accesses.

Data Access :

1. Request: The processor requests data.

2. Cache Check: The data cache (D-cache) is checked first.

3. Cache Hit: If the data is present in the cache, it is provided to the processor quickly.

4. Cache Miss: If the data is not in the cache, it is read from RAM and then stored in the cache for future accesses.

The importance of caches

Cache memory is vital in MCUs for several reasons:

• Performance improvement: Caches enable the processor to access instructions and data more quickly, reducing program execution times.
• Energy efficiency: By minimizing accesses to main memory, caches help reduce power consumption, which is essential for embedded devices.
• Latency Reduction: Caches reduce the latency associated with memory accesses, improving system responsiveness.

Comparison of Access Speeds

The speed at which the CPU can access different types of memory significantly impacts overall system performance. Here's a comparison of the access speeds for cache memory, RAM, and flash memory:

• Cache Memory: The fastest type of memory, located closest to the CPU. Access times are typically in the order of nanoseconds (ns).
• RAM (SRAM): Slower than cache memory but faster than flash memory. Access times are usually a bit longer than cache memory but still in the range of nanoseconds to low tens of nanoseconds.
• Flash Memory: The slowest among the three. Access times are in the range of microseconds (μs), which is significantly slower compared to cache and RAM

Conclusion

Instruction and data caches are fast memories located close to the processor in a microcontroller. They play a crucial role in storing frequently-used instructions and data, reducing access times and improving overall system performance.