Eugene Hutorny

Previous examples illustrated VELOOS programming aspects. This post provides an example of using object oriented paradigm for designing VELOOS application. Unlike the previous examples which were purely programming exercises, this example is dedicated to a utilitarian home appliance application and provides a "reference design" for VELOOS applications.

Example3 – Designing MIHA – Multi Input Hall Automaton

Introduction

There are two common approaches for light switching in halls and staircases – off-timers and intermediate switching schemes (please excuse me if terminology is wrong). Off timers turn light on when user pushes a button and turn off when specified time elapsed. Intermediate switching schemes toggle lights when user toggles any switch (of two). Each approach has its own advantages and disadvantages, however, both approaches require special switches, which could be an undesired constraint for interior design. See, for example, Intermediate lighting switch, Stair case timer, Multifunction staircase timer

Requirements (user’s needs)

Implement a light-control appliance that:

• combines off-timer and intermediate switchers
• provides support for at least 3 switches
• supports regular light switches as well as push-buttons
• provides settable 0.5-10 min timer range with a simple user interface for entering value

MIHA Design Description

Key Design Decisions:

• Implement the appliance (named MIHA) as a VELOOS application running on a PIC microcontroller
• Store application configuration in EEPROM during device programming
• Use potentiometer for specifying timeout

Architecture
MIHA consists of the following units (see architecture diagram on Figure 1):

Mapping Architecture to Implementation. Mapping from architectural units to implementation units is straightforward – all MIHA units are directly mapped to operating system objects, inter-unit communications are mapped to messaging. All MIHA objects are configurable with EEPROM data. To make design flexible, object configuration embeds acceptor handle. The latter two decisions establish a generality which is captured as two abstract classes: PDO – a configurable Process Data Object PDD – a dynamically bound PDO (See class diagram on Figure 2)		Figure 1.
		Figure 2.

Moduling

Major advantage provided by VELOOS is ability to design and implement loosely coupled objects. The following practice can be applied to get full advantage of loose coupling:

• A unit should be made up of (at least) two files: (1) .inc containing interface and (2) .asm – implementation
• Each unit should implement one class or a hierarchy of closely related classes.
• A unit should not instantiate objects of reenterable or configurable classes. Instead it should publish class name in the interface.
• A unit may instantiate object of a non-reenterable class. In this case it should publish object name and encapsulate class name.

Sharing RAM among objects

Decoupling objects via messaging mechanism provides a very explicit and straightforward watershed for scoping variables – everything that should survive between two messages is a part of object state and should be allocated in udata section, everything that may be discarded after handling the message is a temporary variable and can be allocated in udata_ovr section.

Using Timer for Resource Access Arbitration

Class Potentiometer Parameter Input (PPI) implements time-quantified approach for sharing ADC among several instances – time is sliced on N quants, each instance may access hardware only if TIME mod N equals to channel number. To simplify computing TIME mod N, N can be chosen as power of 2. In this example N = 16.

Multi-input Debouncer On timer event Debouncer polls the port three times in a row and applies majority logic to determine current switch state. This approach eliminates sporadic spikes than may be present in a noisy environment. For each assigned pin it maintains a counter. This counter is incremented whenever pin level is high and decremented if low. Counter is maintained in range 0..16, e.g. when it reaches the boundary it is not changed. When counter reaches upper threshold (12) logical output state is changed to 1 and to 0 when it reaches lower threshold. This is a software model of an RC-integration connected to a Schmidt trigger. Few samples of switch bouncing are shown on figures on right. The switches are regular light switches manufactured by VIKO. Hardware Schematic MIHA schematic is shown on Figure 6. Relay, loaded on PIC12, should have actuating current up to 25 mA. Resistors R1-R4 are used for pull-up and can be roughly 1 kOhm. In an environment with strong EMI their can be reduced to avoid pickup. Package Content The package includes three projects to build this application for three different devices – PIC12F675, PIC16F690, PIC18F248. It is licensed to public under the Open Software License Download MIHA Sources v.1.0		Figure 3.
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		Figure 6.