ARM Cortex-M4 16-bit Memory Read Behavior on 32-bit Data Bus
When designing embedded systems with ARM Cortex-M4 processors, one common scenario involves interfacing a 16-bit memory chip with a 32-bit data bus. This configuration is often used to optimize cost and power consumption while maintaining sufficient performance for many applications. However, this setup can lead to unexpected behavior, particularly when the processor performs 16-bit memory reads on a 32-bit data bus. Understanding the behavior of the upper half of the data bus (D16-D31) during such operations is critical to ensuring reliable system performance and avoiding bus conflicts.
The Cortex-M4 processor, like other ARM Cortex-M series processors, is designed to handle various memory access widths, including 8-bit, 16-bit, and 32-bit. When a 16-bit memory chip is connected to the lower half of the 32-bit data bus (D0-D15), the behavior of the upper half of the data bus (D16-D31) during read operations becomes a key concern. Specifically, the question arises: What happens to the D16-D31 lines during a 16-bit read cycle? Are they left in a high-impedance state, or are they driven by the processor? This issue is particularly relevant when designing custom boards with dual 16-bit flash memory chips, where one chip is connected to the lower half of the data bus and the other to the upper half.
Memory Interface Configuration and Bus Conflict Risks
The behavior of the upper half of the data bus during 16-bit read operations depends on several factors, including the design of the external memory interface and the configuration of the microcontroller. At the processor level, the Cortex-M4 uses the Advanced High-performance Bus (AHB) for data transfers. The AHB handles data access based on the size of the access and the address value. For 16-bit read operations, the unused data bus lanes (D16-D31) are typically ignored by the processor. However, the external memory interface, which is part of the microcontroller and not the Cortex-M4 processor itself, may behave differently.
In some microcontroller designs, the external memory interface may drive the upper half of the data bus during 16-bit read operations, even though the processor does not use these lanes. This can lead to bus conflicts if a second 16-bit memory chip is connected to the upper half of the data bus. For example, if the processor drives the D16-D31 lines low while the second memory chip attempts to drive these lines high, a conflict occurs, resulting in intermediate voltage levels (e.g., 1V instead of the expected 3.3V). This behavior was observed in the case of the NXP LPC4088 microcontroller, where the processor drove the D16-D31 lines low during 16-bit read cycles, causing a bus conflict with the upper 16-bit flash memory chip.
To avoid such conflicts, it is essential to understand the configuration options of the external memory interface. Some microcontrollers provide programmable options to force the upper half of the data bus into a high-impedance state during 16-bit read operations. If this option is available, it should be enabled to prevent the processor from driving the D16-D31 lines. Additionally, the use of pull-up resistors on the data lines can help ensure that the upper half of the data bus remains in a known state when not actively driven by the memory interface. However, this approach may not be sufficient to resolve bus conflicts in all cases, particularly if the processor actively drives the D16-D31 lines.
Implementing Data Bus Management and Software Workarounds
When designing a system with dual 16-bit flash memory chips on a 32-bit data bus, careful consideration must be given to the memory interface configuration and software implementation. If the external memory interface does not support tristating the upper half of the data bus during 16-bit read operations, alternative approaches must be employed to avoid bus conflicts and ensure reliable operation.
One approach is to use software to manage 32-bit data reads from the 16-bit flash memory. Since the processor cannot perform a single 32-bit read from a 16-bit memory chip, two 16-bit reads must be performed and the results merged in software. For example, to read a 32-bit value from address N, the software would first read the lower 16 bits from address N and then read the upper 16 bits from address N+2. The two 16-bit values can then be combined to form the 32-bit result. While this approach introduces additional software overhead and reduces performance, it ensures compatibility with 16-bit memory chips and avoids bus conflicts.
Another consideration is the execution of code from 16-bit flash memory. If the external memory interface does not support automatic conversion of 32-bit accesses to two 16-bit accesses, executing code from 16-bit flash memory may not be feasible. In such cases, it is recommended to use a microcontroller with a programmable external memory interface that supports 16-bit memory configurations. This allows the memory interface to automatically handle 32-bit accesses by performing two 16-bit accesses, eliminating the need for software intervention and enabling code execution from 16-bit flash memory.
In summary, the behavior of the upper half of the 32-bit data bus during 16-bit read operations depends on the design of the external memory interface and the configuration of the microcontroller. To avoid bus conflicts and ensure reliable operation, it is essential to carefully review the microcontroller’s documentation and configure the memory interface appropriately. If the memory interface does not support tristating the upper half of the data bus, software workarounds may be necessary to manage 32-bit data reads and avoid conflicts. By understanding the underlying hardware and implementing appropriate solutions, designers can successfully integrate 16-bit memory chips into systems with 32-bit data buses.
