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The XScale PXA255 processor has been around for about 7 years now and is coming to the end of its production life with no direct replacement available. Originally made by Intel but then moved to Marvell the PXA255's last time buy date has been pushed out over the last few years. Now it is drawing to an end with the last orders being accepted no later than the end of June and final delivery at the end of 2010.


The PXA255 is the heart of the Snapper 255 designed by Bluewater Systems back in 2004-05.

It has been used in a number of products since then, with notable success coming from the .

It's out with the old and in with the new Snappers that are on their way. So cast your lines and hook into the new Snappers to be announced !

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In drawing up the schematics for a new design, it is very easy to add new components willy nilly as and when required. Then - once happy with the design - move on and produce a masterpiece of a PCB layout. After the PCB layout send the files out for quote and move on to purchasing components. It's at this point that you may find yourself confronted with a large BOM (Bill of Materials) to buy before you can build your board. The time to find and buy these components is not the end of it either. A large BOM has the following drawbacks:

  1. The more discrete part types in a design, the lower the volume of each will be, and the cost benifit of buying in bulk can not be realised
  2. The inavalibility of any part can stall a build. The more parts in a build, the more likely this is to happen
  3. More parts means longer machine setup times and more feeder requirements - possibly even a double pass through the machine. These all add to build cost
  4. More components to track and keep inventory of which costs time

On most boards there are a variety of very simple things that can be done for very low cost to mitigate these problems - the initial cost will more than pay for itself in reduced future costs and reduced risk.

The BOM rationalisation step should be performed after the schematic phase - NOT after the PCB phase. Often BOM simplification requires trivial changes in the schematics, but once the PCB layout is done changes may result in a lot of work needing to be redone.

There are many things that can be done to reduce the BOM count. Resistors and Capacitors are the simplest place to start: often there is a wide range of these in any design and these are by far the easiest target. There will often be many components of which there is only one or two used. Several things can be done in this case:

  1. Check if the odd-ball specific value is actually required, or can it be replaced by a more common component on the design. Pull-up resistors are a common culprit here
  2. If the value is required, see if it can be produced by a parallel or series combination of common components. It is quite suprising how often this technique can be used.
  3. Voltage dividers. These can be a special case of the above technique. In one recent design, it was discovered that a 100k, 120k and 330k could produce over 60 different divider ratios with one or two high side resistors, and one or two low side ones. The spread was quite even too.

Power supplies can often be simplified too:

  1. Using an adjustable version of a power supply, and tailoring it to several different voltage rails will use fewer different ICs than using the pre-programmed ones. When it comes to production, a shortage of any IC could put a halt to productio, so the fewer different IC types, the better
  2. Adjustable supplies normally have a voltage divider to set the output - take advantage of the voltage divider options as discussed above. Remember that a 3.3v supply doesn't need to be 3.300000000v: 3.27453v is more than acceptable for the vast majority of systems!
  3. Inductors can often be reused on switch mode supplies even though it is a different voltage rail. It may sacrifice a minor % of efficiency, or on the surface cost a few cents more - though doubling the volume will more than likely cancel this out.
  4. Don't be afraid to understand and modify the 'webbench' recommendation. TI, National, etc all have really easy to use web based tools for designing power supplies, but don't take the design they provide as the end of it. There is scope for adjustment and fine tuning to suit your particular board
  5. Some switch mode ICs can be configured as a DC/DC converter, or as a current source for driving an LED backlight. Look into these options at design time and use the same switch mode IC for both

Similar components can often crop up in designs. If there are multiple different types of MOSFETS, BJTS, or diodes these can be reviewed to ensure that there is a good reason for them to all be different. This is most often a problem when portions of several designs are combined to make a new one.

Use configurable logic gates - TI for one make some small ICs (SN74AUP1G57 for example) that can be wired up as an AND gate, an OR gate, or some other odd-ball gate. These can have their advantages if there are several different gate types in the design that can each be replaced by a different version of the single configurable gate.

Consider for some applications using transistors and resistors to replace a one-off logic IC.

There are many more things that can be done. The majority take very little effort at the schematic phase. It is easy to be of the mindset that that 12.7k resistor only costs a few cents, so why bother removing it, however the issue is not the cost of the component - it is the cost of supporting the component! The 5min to design it out pales when compared to the what it would cost to support it through the lifetime of the product through purchasing, stocking, machine setup, and risk of it being temporarily unavailable.

Here at Bluewater we strive to keep BOM counts down and this approach has produced dividends both in simpler prototype and production builds. The benefit is easier stock tracking and reduced time spent on component managment.

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Previously we spoke about our efforts to optimise the boot time of our Snapper 9260 module, getting it down to ~5s. Having done a bit more work on the hardware, software, and moving to the AT91SAM9G20 module, we now have this down to ~3s from initial power applied. This is a full boot, with peripherals initialised, reading from NAND Flash for its root filesystem etc. Having used a lot of embedded systems over the years, we are very pleased with this result, as it certainly out performs all the similar devices we have seen in its class.

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FPGAs are great devices for performance and functionality. FPGAs are primarily made up of flip flops, LUTs (Look Up Tables) and interconnect matrix. Additional to this there are often block rams, DSP blocks, fast adders, etc. LUTs themselves however offer some often unrealized design features. In most Xilinx FPGAs, the LUT is a 16x1 configuration. i.e. there are four lines in and 1 out. Internally there are 16 elements, each representing the output state for a specific input combination. Many people leave it at that and let the synthesis tools decide how best to use them. However upon closer inspection, the LUT can operate as a shift register through all 16bits, with a carry output and a programmable tap. -- see http://toolbox.xilinx.com/docsan/xilinx7/books/data/docs/lib/lib0370_356.html component SRL16E generic(INIT : bit_vector := X"0000"); port (D : in std_logic; CE : in std_logic; CLK: in std_logic; A0 : in std_logic; A1 : in std_logic; A2 : in std_logic; A3 : in std_logic; Q : out std_logic); end component; Encapsulated in modules I have made LUTs into the following * Small efficient fast counters: A single LUT can count (cycle) through 16 states. If the overall output of this is used to enable a second LUT counter, then overall one can count through 256 states. Three combined can do 1024, etc. This uses a fewer resources than if the counter were implemented in flip flops and also has the advantage that the fan in/out is less so can operate faster. To implement an arbitrary counter, a recursive module is used which takes k - the delay length and breaks it into k1*16+r1. k1 is a shorter counter that cycles 16 times, and when that is done triggers the final difference r1 (a short shift register). K1 then has the same algorithm applied to generate k2*16+r2, etc until kn is 0. * Delay paths: Delay paths are simply shift registers. Depending on the delay path, a counting implementation above may be used if only one thing can be in the path at a time. * FIFOs: the LUT shift registers form the basis of FIFOs. The FIFO length is the four address lines for the LUT * Control logic: With various control logic, the timing interdependencies can be programmed into a shift register. These implement very efficiently in a LUT. The shift register can be cyclic so it repeats forever. This has been used in an I2C controller where two parallel shift registers were used, one for the data and one for the start and stop bits. The cycle was initiated by a LUT based counter. Similar control logic has also been used for SDRAM timing which requires low latency and fan in/out. This LUT functionality can be hidden inside a module and if the code is moved to an architecture which doesn't support the LUTs being used as shift registers, a different implementation can be used

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Many devices are battery powered, and there is a wide range of battery technologies to choose from: Nicad, NiMH, Lithium, Alkali, etc. They all behave differently and have different caviets. We have designed many products to run off battery power. Often the battery is fixed for the product, however sometimes it is desirable to use a standard battery - such as an AA for example. The problem is there is no guaranteeing which AA a user will use. Nominally an AA battery is 1.5v and the current capacity can vary from 600mAh to 3000mAh. Furthermore rechargeable AA batteries are often 1.2v. A fresh Alkali AA battery can read as high as 1.7v. 3.3v is a common voltage for digital electronics, and it is quite tempting to connect up two AA batteries in series to produce 3.0v and assume that is the power supply sorted. Unfortunately, given the range of AA 'voltages' indicated above, and combined with the fact that battery voltage decreases based on load and capacity, this approach is not acceptable. What needs to be done instead in such a system is to have a boost converter to generate the system 3.3v. On a recent design we used a Linear Tech LTC3539. This works over an input range of 700mV to 5v and can provide up to 2A. If the input is above the system rail - as can happen with some fresh AA cells, the converter still regulates the output voltage (albeit not as efficiently). If the system rail is 3.3v, then the batteries will quickly fall beneath this threshold, so the power loss due to this is not great. If such a chip is used to generate a lower voltage rail, then system operating life will suffer more. Using such a chip to generate a 3v3 rail provides maximum battery life for the system and ensures the majority of energy is drawn from any AA cell before the batteries are 'flat'.

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