You cannot perform simultaneous read and write operations on a single NXP FlexSPI channel because the hardware controller uses a shared command engine and data buffer. The FlexSPI peripheral is designed for sequential instruction, address, and data phases, meaning any attempt to overlap these transactions will result in data corruption or bus conflicts rather than true parallelism.
Understanding FlexSPI Architecture
The NXP FlexSPI (Flexible Serial Peripheral Interface) controller is a sophisticated peripheral found in i.MX RT and i.MX 8 series processors. It is primarily designed to interface with external serial memory devices such as QSPI NOR Flash, Serial NAND, and SRAM. A common misconception among developers is that "FlexSPI" implies multiple independent SPI buses operating in parallel. In reality, the standard FlexSPI controller presents a single logical interface to the CPU, even if it supports multiple chip selects (CS) lines.
The term "spi mem" in your query likely refers to the Linux kernel driver spi-mem or the generic concept of SPI memory access. This is not a separate physical entity but rather a software abstraction layer used by the operating system to manage SPI flash operations. When you invoke a read or write operation through this subsystem, it translates high-level commands into specific sequences of clock cycles and data transfers over the physical pins.
The Shared Command Engine
At the hardware level, the FlexSPI controller contains a finite state machine that orchestrates the transfer. This state machine handles the command phase, address phase, and data phase sequentially. Because the internal DMA channels and LUT (Look-Up Table) engines are tied to a single transaction stream, initiating a new write command while a read is still pending will abort the current operation. There are no independent pipelines for reading and writing within a single FlexSPI instance that allow them to run concurrently on the same wire pair.
Why Simultaneous Operations Fail
Attempting to schedule a read and a write at the exact same time leads to contention on the bidirectional data lines (DQ0-DQ3). In full-duplex SPI modes, some devices can theoretically transmit and receive simultaneously, but this requires strict timing alignment and usually only applies to simple register reads or status queries, not complex memory map accesses.
For memory-mapped reads, where the CPU fetches instructions directly from the flash via the FlexSPI, the controller must maintain a continuous stream of address updates. If a write operation interrupts this stream, the address counter resets, causing the CPU to fetch incorrect code or data. Similarly, program or erase operations require exclusive access to the memory device's internal buffers, which prevents any other traffic from occurring during the critical timing windows.
Proper Sequencing Strategies
To achieve high throughput without violating hardware constraints, you must serialize your operations intelligently. The Linux kernel's spi-mem framework provides tools to optimize these sequences. Instead of trying to run reads and writes in parallel, you should batch operations.
- Batched Reads: Group multiple small read requests into larger contiguous blocks. This reduces the overhead of command and address phases, maximizing the effective bandwidth.
- Asynchronous Erase/Program: Some advanced QSPI flashes support non-volatile configuration registers that allow background programming. You can initiate a write command and then immediately switch to reading other parts of the memory map, provided the flash device supports this specific feature set.
- DMA Buffer Management: Use Direct Memory Access (DMA) to move data between RAM and the FlexSPI data buffer without CPU intervention. While this does not make reads and writes simultaneous, it allows the CPU to prepare the next transaction while the previous one completes.
Hardware Alternatives for Parallelism
If your application genuinely requires simultaneous read and write bandwidth, relying on a single FlexSPI controller is insufficient. You have two viable architectural options.
First, utilize multiple FlexSPI instances. Many NXP SoCs provide more than one FlexSPI controller (e.g., FlexSPI1 and FlexSPI2). You can attach different memory devices to each controller and manage them independently from the OS perspective. This effectively doubles your available SPI bandwidth by splitting the load across two separate hardware peripherals.
Second, consider using a dual-bank memory device. Some high-end QSPI flashes feature two independent banks that share the same physical pins but are accessed via different Chip Select (CS) signals. By asserting CS1 instead of CS0, you can talk to Bank B while Bank A is idle or performing its own internal operations. However, note that even with dual-bank memories, you cannot read from Bank A and write to Bank B simultaneously if they share the same DQ lines and the controller cannot toggle between them fast enough. True simultaneous I/O usually requires physically separate buses or controllers.
For more details on how memory controllers handle startup sequences and initial reads, you may find Understanding CPU Startup: The Role of ROM x86 and Memory Controller SPI Reads helpful for context on how the system initializes these interfaces.
Debugging Common Issues
When troubleshooting unexpected behavior, ensure that your LUT configurations match the specific timing requirements of your flash device. Mismatched wait states or incorrect dummy cycle counts can cause the controller to hang or return garbage data, which might be mistaken for a concurrency issue. Always verify the status register after every command to ensure the previous operation has fully completed before issuing the next one.
Frequently Asked Questions
Can I use DMA for both read and write at the same time?
No, the FlexSPI controller typically shares a single data path for DMA transfers. While the CPU can trigger a DMA request for a read and another for a write, the hardware will serialize these requests because they compete for the same physical pins and internal FIFO buffers.
What is the difference between spi-mem and FlexSPI?
FlexSPI is the physical hardware peripheral in the NXP processor, whereas spi-mem is a software subsystem in the Linux kernel that abstracts SPI flash operations. The kernel driver uses spi-mem to send commands to the FlexSPI hardware, but they are not separate physical entities.
Does dual-bank QSPI allow true simultaneous access?
Not exactly. Dual-bank QSPI allows you to access two different memory regions using different chip selects, but you generally cannot read from one bank and write to the other simultaneously on the same bus due to pin sharing. You would need two separate FlexSPI controllers for true parallel operation.
How do I prevent bus conflicts during memory mapped reads?
Ensure that your application does not interrupt active memory-mapped fetches with explicit write commands. Use the flash's status register polling mechanism to confirm that any previous write or erase operation is complete before allowing the CPU to resume normal instruction fetching from that region.
Are there NXP chips with multiple FlexSPI controllers?
Yes, many i.MX RT and i.MX 8 series processors include multiple FlexSPI instances (e.g., FlexSPI1 and FlexSPI2). You can connect different memory devices to each instance to achieve higher aggregate bandwidth and independent operation.
Why does my SPI read fail when I try to write immediately after?
This is likely due to timing violations. The flash device needs time to process the write command internally. If you issue a read immediately, the controller may intercept the command before the flash is ready, leading to a bus error. Always poll the busy bit before issuing subsequent commands.