The sixth chapter focuses on reusing peripheral driver code, specifically discussing SPI NOR Flash memory. This section uses the MX25L1606 from Wanghong Electronics in Taiwan as an example to illustrate how similar flash memory can be utilized within AMetal.
### 6.2 SPI NOR Flash Memory
SPI NOR Flash is a type of non-volatile memory that uses an SPI interface. The MX25L1606 has a total capacity of 16M bits (2M bytes). Each byte corresponds to a unique memory address, with the address range spanning from 0x000000 to 0x1FFFFF.
The memory structure of the MX25L1606 includes blocks, sectors, and pages. A page consists of 256 bytes, a sector contains 16 pages (4K bytes), and a block contains 16 sectors (64K bytes). Table 6.5 provides a detailed breakdown of this structure.
The communication interface of the MX25L1606 is a standard 4-wire SPI interface, supporting modes 0 and 3. The pins involved are CS, MOSI, MISO, and CLK. Additional pins like WP (write protection) and HOLD (data transmission suspension) are typically not used and can be pulled high via a resistor. The MicroPort-NorFlash module connects to the AM824-Core through the MicroPort interface.
### 6.2.1 Basic Functions
The MX25L1606 features a structured memory layout with specific sizes for blocks, sectors, and pages. Understanding these units is essential for efficient data management and access.
### 6.2.2 Initialization
AMetal offers driver functions for popular SPI Flash devices like the MX25L8006 and MX25L1606. Before using any other functionalities, the device must be initialized. The initialization function `am_mx25xx_init()` returns the instance handle of the MX25L1606.
To initialize the device, users define an instance of `am_mx25xx_dev_t` and provide specific information about the device, such as the SPI chip select pin, mode, speed, and model. The SPI handle is obtained through the SPI instance initialization function, which is then passed to the device's initialization function.
### 6.2.3 Interface Function
SPI Flash requires erasing before writing. The erase function targets sectors, which are the smallest unit of erasure. Writing to an address must ensure it has been erased first, as SPI Flash cannot overwrite bits from 1 to 0 directly.
The read and write functions allow data to be accessed and stored efficiently. For example, writing 128 bytes of data starting at address 0x001000 requires erasing one sector (4096 bytes) beforehand.
### 6.2.4 MTD Universal Interface Function
To abstract the device-specific details, the MX25L1606 is represented as an MTD (Memory Technology Device). This allows cross-platform usage, making it easier to interact with different flash memory devices without rewriting code.
The MTD interface includes functions for initialization, erasing, reading, and writing. These functions work independently of the underlying hardware, providing a consistent way to manage flash memory across different platforms.
### 6.2.5 FTL Universal Interface Function
The FTL (Flash Translation Layer) interface further simplifies flash memory management by abstracting the complexities of erasing and wear leveling. It allows data to be written to logical blocks, which are mapped to physical storage locations, thus extending the lifespan of the flash memory.
FTL introduces additional overhead in terms of RAM usage and storage space, but it significantly improves reliability and performance, especially in applications where frequent writes are required.
### 6.2.6 Micro Database
AMetal also provides a micro-database interface based on non-volatile memory, allowing persistent storage of key-value pairs. This feature is useful for storing critical data that must remain intact even after power loss or program termination.
By utilizing the hash table interface, users can store and retrieve data efficiently, ensuring that information remains available across system restarts. This makes the micro-database ideal for applications requiring long-term data retention.
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