======Programming the Wattmon======

"Programming" and "scripting" are used interchangeably throughout this documentation.

The [[hardware:wattmons|Wattmon]] is built on a scripting language that is syntactically identical to PHP((PHP is a server-side scripting language designed primarily for web development but also used as a general-purpose programming language. PHP stands for "PHP: Hypertext Preprocessor." This confuses many people because the first word of the acronym is the acronym (this type of acronym is called a recursive acronym.) )) - we call it uPHP for Micro-PHP.  It has a small but powerful subset of the functions and features found in the PHP language.

=====Why uPHP?=====

First perhaps it would be good to explain a bit about how and why we chose PHP.  The goal of the [[hardware:wattmons|Wattmon]] is to provide a highly flexible platform that can be customized for pretty much any application imaginable, keeping in mind very low memory availability.  It became clear that a scripting language would be required to implement this, since having custom firmwares for various applications would be very tedious.  The question then became: what would be the *best* scripting language that is well known today and can be deployed on a microcontroller with such limited resource?  The choices are actually very small.  Most modern scripting languages require quite a bit of RAM.  So, we decided to roll our own scripting language from scratch and base it off a well known language.

The [[hardware:wattmons|Wattmon]] interface is primarily browser based, and generates HTML pages on the fly from uPHP scripts.  The methodology of HTML-PHP integration is well understood by most programmers and hence it allows you to become more productive quickly.

====Challenges====

The biggest challenge with the scripting language is its memory limitation.  The [[hardware:wattmons:WattmonPRO]] and [[hardware:wattmons:WattmonMINI]] processors have a total of 128KB (yes, Kilobytes!) of RAM available to run a multitasking OS with a TCP/IP and USB stack, modbus polling engine, scheduler and more.  For these platforms during normal operation scripts have a maximum of about 30 KB of memory for variables, which means that as a programmer you need to be very conscious of memory allocation.  We have introduced some special non-PHP compatible variable types to help with this.  The [[uphp:functions:indexed_array()]] function lets you create arrays of a very specific type (int, byte, etc) which will reduce the amount of resources compared to hash arrays created with the [[uphp:functions:array()]] function.

The [[hardware:wattmons:WattmonMEGA]] processor has a total of 512KB of RAM (4 times as much as for the [[hardware:wattmons:WattmonPRO|PRO]] and the [[hardware:wattmons:WattmonMINI|MINI]]). Memory limitations are much less of a challenge when scripting for the [[hardware:wattmons:WattmonMEGA|MEGA]].

====Benefits====

Having a scripting language that lets you modify and edit your program remotely is a very powerful thing.  It lets you provide remote support to your clients and fix issues without needing to visit the site, and also makes your applications future-proof as you can at any time update the code base and integrate new features or protocols.

=====Further Information on uPHP=====

Of interest to those who want to dig deeper and advance into making tweaks and changes to the system:

[[uphp:uPHP Language Basics]] - Basic syntax and tips to get started

[[uphp:functions|uPHP Function Reference]] - The entire function list (advanced)

[[uPHP Special Variables]] - Special arrays that are populated automatically before a script is run