Policy Based SD card driver

Motivation

There are a lot of SD card drivers available online but most are tighly coupled with the framework they are developed for. I wanted a SD card driver that I could use anywhere, unit test from my desktop development machine, and not sacrafice in speed or code space.

Policy based design

Policy based design allows me to write the SD card driver without thinking too much about the underlying hardware implementation. Design descisons which are “orthogonal” can be based on policies provided at compile time. This is similar to how software should be modularized based on design decisions and not on process flow (Parnas, 1971)

design decisions:

I identified the following seperate design decisions that were then made into policy classes:

CRC: If the system is space limited then the CRC policy can calculate the CRC each time. If the system needs performance it can use a table based CRC. Some systems have hardware CRC peripherals, so that could also be used.
Timing: There are certain actions that require timout periods. Keeping track of time is delegated to a policy class. This enables very simple implementations. The default tick based policy simply tracks time by counting how many times the driver asks what time it is. This has been tested and works well on a system with no clock/counter that works well with timing.
SPI communication: This driver assumes SPI communication but the policy only needs a read/write/CS. SD cards expect weird CS behaviour so the CS pin could not also be abstracted.

design tradeoff:

Since the policy classes are obtained through template parameters there is the possibiliy of code bloat IF AND ONLY IF you have multiple SD cards with different policies. This seems like an acceptable tradeoff since it is unlikely that a system with two SD cards would use a different policy for CRC/timing for each card. since the SPI communication is a policy that is the most likely pain point if the SD cards are on different SPI busses.

Testing

The driver was tested with the I2C Driver and Aardvark SPI devices from a desktop PC during development. It was also tested running on a Atmel SAML21 custom board and FeatherM0. It passes all tests from the elem-chan FatFS library on all the systems.