Adding external RAM to an Arm Cortex system can provide a significant performance boost by increasing the amount of available memory. However, properly interfacing external RAM requires careful planning and configuration to ensure reliable operation. This article provides best practices and step-by-step guidance for using external RAM with Arm Cortex CPUs.
Overview of External RAM Integration
The Arm Cortex CPUs provide interfaces for connecting external memories like SRAM, PSRAM, and SDRAM. The external memory can be used to supplement or completely replace the internal memory of the Cortex CPU. Using external RAM has several benefits:
- Increasing memory size – More RAM to run larger applications or store more data.
- Higher bandwidth – External RAM typically has higher bandwidth than internal RAM, improving performance.
- Flexibility – External RAM can be changed as per application requirements.
However, using external RAM also poses some challenges related to timing, power consumption, PCB routing, and configuration. This article provides recommendations to overcome these challenges.
Interfacing Guidelines
Here are some key guidelines for interfacing external RAM with Cortex CPUs:
Memory Interface Signals
The memory interface consists of address, data, and control signals. Some key signals are:
- Address bus – Connects to the address inputs of the RAM.
- Data bus – Bi-directional bus for reading/writing data.
- Clock – Used for synchronizing data transfers.
- Chip select – Activates the RAM device for access.
- Read/Write – Indicates read or write operation.
Matching Electrical Specifications
The interface signals must match the voltage levels and drive strength requirements of the external RAM device. Pay attention to factors like:
- Memory interface voltage – 1.8V or 3.3V.
- Input and output voltage levels.
- Current drive capabilities.
- Termination requirements.
Trace Routing
Use shortest possible traces for memory signals to minimize reflections and signal integrity issues. Some best practices are:
- Minimize vias and stubs.
- Route clock and data bus signals first.
- Match trace lengths for address/command/data.
- Add termination resistors if required.
Bypass Capacitors
Add multiple bypass capacitors close to the external memory device to provide a low impedance power supply. Use a mix of capacitors like tantalum, ceramic, etc. for filtering different frequency noise.
Configuration and Initialization
The Arm Cortex memory controller and related peripherals need to be configured correctly to interface with external memory. Here are some key steps for configuration:
1. Memory Interface Configuration
Configure the type of external RAM in the memory interface – static, pseudo-static or SDRAM. Set important parameters like RAM size, data bus width, timing values, and access latency in the configuration registers.
2. Address Decoding
Set up address decoding to generate the chip select signal for external RAM access. You can either use an address decoder or configure a Chip Select signal through General Purpose I/O (GPIO).
3. Clock Configuration
External RAM normally requires a higher frequency clock. Make sure to configure the appropriate clock divider and source in the PLL. SDRAM may need an inverted clock signal as well.
4. Pin Multiplexing
Configure pin multiplexing to connect the required RAM interface signals through GPIO/peripherals like FSMC, AHB to APB bridge etc. on your microcontroller board.
5. Timing Parameters
For SDRAM, load the correct timing parameters like refresh rate, CAS latency etc. into the SDRAM controller registers as per your SDRAM chip.
6. Bring-up and Initialization
Follow a proper bring-up sequence to apply power, provide clock, and initialize the SDRAM using predefined commands for precharge, load mode register etc.
Common Issues and Debugging
Some common issues faced during integration and their debugging techniques are:
Board Bring-up Problems
- No activity on address/data bus – Check soldering, shorts, opens on PCB.
- No CS activation – Verify address decoding logic.
- No clock – Check clock source, divider configuration.
Initialization Failures
- Improper bring-up sequence – Follow datasheet procedure.
- Incorrect timing parameters – Double check calculation against datasheet.
- Failing commands – Increase delays between commands.
Intermittent Crashes
- Add more bypass capacitors near RAM.
- Reduce clock frequency.
- Increase timing delays as per datasheet guidelines.
High Power Consumption
- Interface voltage level mismatch – Check voltage translators.
- Termination missing – Add resistors to terminate trace.
- Excessive switching – Reduce bus activity through firmware optimization.
Software Optimization Tips
Software optimizations can improve performance when using external RAM. Some techniques include:
1. Enabling Caching
Enabling instruction and data caching leverages faster cache instead of direct external RAM access. But pay attention to cache coherency with DMA and multicore systems.
2. Optimizing Data Alignment
Align data structures to optimize external memory accesses. For example, align buffers on cache line boundary.
3. Prefetching Data
Use prefetch instructions and DMA features to load data into caches before actual access.
4. Access Optimization
Optimize read/write patterns based on external RAM capabilities. For example, prioritize sequential accesses instead of random.
5. Avoiding Wait States
Minimize access delays and wait states by providing sufficient metadata instructions and optimizing code execution flow.
6. Using DMA Efficiently
Structure DMA transfers for bulk data movement to/from external RAM avoiding CPU overhead.
Example Implementation
Here is an example of interfacing 1MB external PSRAM with a Cortex-M7 CPU on an ARM development board. The PSRAM chip MT48LC1M16LF has 16-bit data bus and operates at 3.3V.
Hardware Configuration
- Use the FSMC peripheral to generate RAM interface signals.
- Connect FSMC A[23:1] pins to PSRAM address bus for 1MB range.
- Use FSMC D[15:0] for the 16-bit data bus.
- Connect chip select through GPIO port A, pin 5.
- Configure 3.3V power supply for PSRAM.
- Add 0.1uF and 10uF bypass capacitors on PSRAM power pins.
Software Initialization
- Enable FSMC clock and related GPIO clocks.
- Configure GPIOA pin 5 output for chip select.
- Configure FSMC for static memory in 16-bit mode.
- Set timing parameters based on PSRAM datasheet.
- Initialize PSRAM by issuing write command and verifying data.
Conclusion
Adding external RAM to Arm Cortex systems can significantly improve performance and memory space. Careful planning is required for signal integrity, configuration, timing, initialization and software optimization to reliably interface the external memory. Following the recommendations outlined in this article will help developers to successfully integrate external RAM and unlock the full potential of their Cortex application.
