I am building a variant of the Arduino platform which will have an analogue output capability in the form of a dual channel DAC, called Goldilocks Analogue. The DAC can be used to generate variable DC voltage levels that might be used as part of a PID control system, and it can also generate AC voltages up to about 50kHz if it can be fed with sufficient samples to produce the required signal. To generate a 44.1kHz audio signal the DAC has to receive a stream of data, with a new sample every 22us without fail.
Finding an answer to the question of how to reliably stream data to the DAC is the background to this post.
Looking for a way to structure and assemble a combination of many WAV files on a host PC for storage onto to the AVR ATmega MCU, I needed a system that would support:
- Editing and assembly of files on a host PC (Linux, Windows, Mac), in to a package.
- Transferring a package of files to the AVR ATmega (Arduino) device very simply.
- Can read and write files to the storage medium very quickly, and without jitter.
- Simple implementation in the avr-libc environment.
Initially I was looking at using the FAT File System on a SD Card to provide the required capability, but I found that SD Cards are quite slow when writing data to their FLASH medium. Often taking 100ms or more to complete a write cycle. A SD Card read cycle also takes quite a long time, when the FAT file system must be inspected prior to reading or writing a specific block of information. The SD Card is great for storing Mega Bytes of information, but is not optimal for jitter free read and write applications.
So I started looking at chip storage based on the SPI bus as a mechanism to store large numbers of samples for playback, or to store large amounts of acquired data samples. There are many alternatives using different technologies for SPI storage devices. These range from EEPROM storage, through to SRAM and also newer FRAM technologies. Storage capabilities with up to 1Mbit seem to be quite good value. For my application 1Mbit of storage would allow about 16 seconds of reasonable quality audio to be retrieved with minimal issues for complexity, jitter, and delay.
So I redesigned the Goldilocks Analogue to incorporate space to have two SPI memory (EEPROM, FRAM, SRAM) devices on the board.
Implementing a method to read and write bytes to these storage devices is very straightforward. There are many libraries available supporting the SPI storage devices of various types. But none of them supported assembling a package of files on a host PC, and then transferring this to the AVR device in a simple manner. So the hunt for a solution to this issue brought me to the NASA EEFS solution.
NASA has been releasing their Core Flight System with Open Source licencing over the past few years. The Core Flight System (CFS) is a recognition that many satellite and deep space missions have very common core requirements and that successive missions were simply cloning previous mission software and then owning changes going forward, with learning being improved in a serial manner. The CFS enabled missions that were developing in parallel to push improvements in the platform CFS code back into the general solution for peer and successive missions to benefit from.
The CFS is layered and each layer hides its implementation, enabling the internals of the layer to be changed without affecting other layers’ implementation. Within the CFS Platform Abstraction Layer there is a module designed to support the management of flight software packages on non-volatile storage, called the EEPROM File System (EEFS).
The EEFS is a very small (approximately 2% of the flight software) piece of code that implements the storage and retrieval of all flight system software from flash storage devices. It was designed by NASA GSFC to support similar outcomes as what I needed for my application:
- Generate a flight software (or general embedded system) executable image on the development workstation. This feature allows the embedded file system to be generated with a known CRC and loaded on to the target processor as a single image. This is a big advantage over formatting a file system on the image, then transferring each file to the file system on the target.
- Prove that the file system is correct and reliable. Because the EEPROM file system is simple, the code size is small, making it easy to review and find errors.
- Patch the files in the file system. Due to the simple layout of the EEPROM file system, it is very easy to patch the files in the file system, if the need arises. This can be helpful in deeply embedded systems such as satellite data systems.
- Dump and understand the file system format. Because the EEPROM file system is simple, it is easy to dump the contents of the EEPROM or PROM memory and determine the contents of each file.
The EEFS is basically a configurable slot-based file system. The file system can be pre-configured with a certain number empty files of known sizes, or known files with specific “spare bytes”, and written with a CRC into an image. The File Allocation Table is a fixed size and contains a fixed number of file slots, together with the location and maximum size for each slot. The File Headers for each slot contain all the information about each File. Changing a file does not impact the FAT, and therefore does not affect other files in the File System.
An EEFS image is created with a tool called geneepromfs, which is a command line tool compiled for the respective host upon which it is used. It reads an input file specifying the files that are to be assembled into the EEFS image, together with the number of empty file slots and their size, and it outputs a complete EEFS image ready to be burnt on the EEPROM, FRAM, or SRAM storage device.
So the EEFS looks like a perfect solution to my requirements. Let’s go to Github and clone the EEFS repository, and get started.
AVR Implementation of EEFS
The EEFS code is supplied for VxWorks or RTEMS platforms, along with a standalone implementation design for bare metal designs. To get the standalone design to work with the AVR ATmega, and my freeRTOS platform of choice, there were two major pieces of work.
Firstly, to develop a generalised SPI interface layer that would allow me to select the actual SPI device installed on the Goldilocks Analogue at compile time. This was necessary because each individual SPI storage device has slightly different command requirements (EEPROM ready check, different address byte numbers), and it made good sense to unify the interface into a single function with compile time options.
Secondly, I needed to revise the pointer calculations inherent in the EEFS code. The NASA GSFC code is based on the availability of 32 bit pointers, and does 32 bit calculations to locate information within the file system. But, on the AVR ATmega platform the inherent pointer size is 16 bits, and many of the advanced pointer arithmetic calculations used in the code would fail.
When I finished the major work, I reduced the return values of most functions to 1 byte error codes, which shaved almost 2,000 bytes of program code off the end result. On the AVR ATmega platform, it is well worth saving 2,000 bytes.
I have built a simple FRAM test program that can write files from a SD Card to the EEFS SPI device, and then edit (read, modify, write) files on the EEFS SPI device for test purposes. This shows how the resulting EEFS library can be best used.