HMI gets a makeover in 2012

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Human Machine Interfaces are having a make-over. That is not just the idea of putting a large TFT screen on your refrigerator, but every new electronic device from big to small, from your wrist watch to your twenty thousand euro stereo system is getting bitten by the bug! TV make-overs were big in the 90s.

In the “naughties” (2000-2009) the world was busy “pimping” their cars. Now it is the turn of your dishwasher and your coffee machine.

If you’ve read this far then you’re more than likely already thinking of the android or iOS phone which you have in your pocket driving the market.  This is certainly true. 

You only have to ask a toddler to play with your old Nintendo game-boy to know that this kind of graphics is just not going to be acceptable to the next generation of consumers. 

Interesting, although this market was triggered by “the iPhone generation” as they are being coined today, it is also these smart phones that are killing off certain markets. 

Five years ago every embedded market specialist was raving about the future of your “in home energy monitor” which would roll out into every home in Europe between now and 2020.  Although these displays have in some part already rolled out, it appears likely that a lot of them will simply be an app which you can download for your telephone or tablet PC. 

Your coffee machine on the other hand is a different matter, as much as it would be a pleasant user experience to control your machine via an app from your sofa such that a piping hot caramel latte macchiato was waiting for you at the next commercial break, we have not yet reached that level of integration.  Traditional white goods such as ovens, fridges, washing machines and dishwashers are starting to launch into the market integrating TFT based human machine interfaces and show no signs of stopping. 

Until recently design engineers were left with two very simple system choices; A microcontroller system or a processor system.  Both these solutions have drawbacks and some advantages for a system design.  The choice of which option tends to be driven very much by which size of display the engineers are using and what their marketing teams have requested as a future path.  

Processor Systems

A simple strip down of your set top box, or your television at home will reveal one simple conclusion, if you’re going to be driving a big display then a processor system is the standard approach.  The reasoning is simple: Memory use.  Even a medium sized screen such as the VGA (640 pixels by 480 pixels) needs 614kB of RAM as the picture buffer for a single frame. 

Common practise today is the use of double buffering whereby the current frame and the next frame are both simultaneously available in RAM such that there is no flickering between the two screens.  This then raises the picture buffer alone to 1.2MB of RAM, just for the picture data.  It is this fact that makes a microprocessor more attractive than an microcontroller initially. 

If your marketing team is also asking you for some video on the screen then system designers also have to think about providing a high performance solution.  MPEG4 decode on its own is unlikely to be possible from a device running at under 200MHz so designer move very quickly into the realms of using a processor system.

The most popular operating system in the HMI part of the processor market today is Linux.  Linux is a beautiful operating system which brings with it a lot of available open source software and benefits. 

It is also big!  Typical linux applications tend to require upwards of 32MB of RAM and often up to GB.  So Linux may be “free” but as any commercial RTOS vendor will tell you, the cost of Linux is in the external memory which you have to pay for.

So a typical processor system today will consist of the MPU, (For which Renesas has a full line up of devices), a significant amount of RAM, more often than not DDR2 or DDR3, and a large NOR flash block containing the code which will be transferred to RAM during the boot up routine.  Finally as we are now often talking about a 300-400pin BGA package, a typical design will likely be a multi-layer PCB. This set up can be seen in the diagram below.

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Microcontroller Systems

As previously stated, a microcontroller based system is the second obvious alternative (again we should state the Renesas is the worldwide number one microcontroller supplier).  This solution is often considered to be a low cost solution compared to the processor solution. 

It is clear that in such a design it will be impossible to drive particularly large screens, MCU solutions with embedded flash typically do not reach the performance level to provide video decode, and as the size of the flash and RAM is limited this brings with it additional sacrifices in terms of screen size, colour depth and even buffering.  The microcontrollers, either fitted with an IP for driving a TFT screen directly or interfacing to an external graphics chip provide a very simple and low cost solution in the market today. 

A typical microcontroller based system will consist of; The MCU, perhaps an external graphics IC, some external Flash (if the internal flash is not sufficient to store all the possible pictures) and some external RAM for use as the frame buffer for driving the screen. 

So both the microprocessor and microcontroller solutions have their drawbacks.  The microprocessor is an expensive system cost.  But it provides the flexibility and performance which you cannot achieve from an microcontroller based solution today. 

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So Renesas off ers something different.  The SH726x family is not a microcontroller in the sense that it does not have any embedded flash memory.  It is not however a microprocessor in that it does not have an MMU.  So the obvious question is: so what does it have?

Well the answer is:  The SH726x family claims to be the lowest bill of materials solution for a Human Machine Interface family in the market today.  It incorporates up to 2.5MB of SRAM.  This means that you have just cut up to 2 Euros off your system cost, because all the RAM you need for picture buffer is internal.

The SH726x family does not need any NOR flash.  You can boot directly from a new Quad Speed serial flash.  This will cut the cost of your design again by about 50 cents.

So the SH726x family is able to provide processor level performance at the low system cost.  But the cost saving is not done yet. 

The SH726x family clearly does not need to have an external bus interface, so although it is available on the devices, designers can select the lowest cost and smallest QFP packages.  This means that although you have processor levels of performance, you don’t need to have a BGA package and your PCB can be a low cost simple 2 layer PCB.

Another PCB saving comes from saving the DDR from the design too.  There is then no need to provide the dedicated 1.8V for this memory and again save PCB space and development time.

The Human Machine Market is one of the biggest growth areas of the embedded market today.  It is one where designers are faced with a tough decision early on in the system design.  A low cost microcontroller solution, or a high performance processor solution.  With the new SH726x family from Renesas designers can use the lowest BOM cost solution from  Renesas without having to sacrifice on performance.

Robert Kalman works for Renesas Electronics

Tags: embedded processors design, HMI, Renesas

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