Introduction

Systems designed by hardware engineers that do not appreciate the subtleties of interface implementation can impact software development in ways that range cause from anoyances to complete project failures.

Many of the common mistakes impact the ability to write reusable software:

• Register grouping
• Mixed functions
• Forced mutexes
• Forced polling
• Loss of synchronization between HW and SW
• 
  

ystems that involve custom hardware with software interfaces

The advent of the micro-processor allows digital hardware designers to avoid a great deal of design effort by plopping down a processor and leaving the problem to the software guys.

The result is inevitably that the

A system development should recognize the needs of both hardware and software aspects systems.

To summarize: get involved with the system design.

The State Change Pattern Interrupt Pattern. describes a best practice for implementing the interface for a set of state change indications.
 

• Group contiguously in memory map by low level function rather than by register type.
• Think modular. E.g. today I may only have one foo-bar, but in a future design I may have ten  foo-bars. Can I just add the ten foo-bars to the memory map, or must I change the register contents or locations in ways that cause the software to be changed, other than simply creating another instance of the driver?
• Separate control and status (e.g. avoid read-clear registers).
• Separate functions:
• E.g. UART
• TX,RX
• Modem Control
• Flow Control
• Avoid read-modify-write requirements if a register contains bits related to more than one low level function. Otherwise, software must take steps to ensure mutually exclusive access to the register by the drivers. These avoidable unnecessary software constraints affect not only the design of the software, but can be a source of latent software anomalies. Requiring software to provide the mutual exclusion causes the software to be written such that it is either dependent upon system services for mutexes that vary from system to system adversely affecting the reusability and reliability of the software across processors and operating systems. In addition, the overhead required for the mutual exclusion enforcement has adverse affects on performance characteristics such as interrupt latencies and processor throughput.
• Writes are typically more efficient than reads on the processor bus. For example Posted Writes vs Delayed Reads on a PCI bus.
• Polling sucks. Polling is a waste of processor bandwidth, but tt also has a negative impact on system scalability.
• Do not mix bit fields with different semantics in the same register.
• Hitel example of control and interrupt clear.
• Analyze Register Types
• Configuration
• Command
• Status/interrupt
• Mask,Enable,Disable
• Stateful/Stateless
• Edge/Level
• Interrupt per source
• Symmetry (Interrupt on All state changes, not Hitel asymmetric)
• Strive for symmetry. Symmetry is beauty.
• Contiguous memories. Simplify exception cases for buffer allocation.
• Consistent access rule memory spaces. E.g. External PCI bus masters access all program memory spaces (e.g. All RAM, ROM). Otherwise, software must make special provisions for buffer space, and will need to perform otherwise unneccessary copy operations that negatively affect processor throughput and utilization.