Hardware Schematic

One of the prime considerations in the choice of the hardware components was the ease of construction. The easiest, and possibly the cheapest, solution would have been to use surface mount components for the PIC and the USB connectivity. However, not many 'amateurs' are happy with soldering SMD components and therefore I decided that the design had to be based around 'through hole' technology. I had already decided that I wanted to use an FTDI FT245 USB FIFO chip as it was capable of providing a 300kbytes per second transfer rate when used with the royalty free drivers provided by FTDI. The down side to this was that the FTDI chip only comes in SMD format so an alternative solution was required. I then discovered that a couple of companies were producing daughter boards containing the SMD FTDI chip and associated components with standard 24 by 0.6" and 32 by 0.6" " DIL plug-in connectivity. I decided to use a product made by DLP (www.dlpdesign.com/usb/) that utilise's a 24 pin DIP that would plug into a standard 0.6" wide 24 pin socket.

In choosing the appropriate PIC it was a trade off between the number of port pins required together with the code space needed to implement the design. I chose a 16F873 which has 3 more ports that the design requires together with 4Kbytes of code space (currently about 1400 bytes free).

The schematic below provides the wiring details. I have not bothered to produce a PCB for the design I have used a small piece of stripboard as the wiring required is relatively simple and should not present a user with soldering experience with too many problems. The connection for the LCD module is made from SIP pins and mates with SIP sockets soldered to the LCD display PCB. It must be recognised that the existing design is intended for the standard(?) 14/16 pin in-line connectors and if an LCD module containing two rows of 7/8 pins is used the layout will have to be changed. It is VITAL that users ensure that the LCD module they intend to use has the pins organised out as shown in the diagram below. If mistakes are made in the wiring of this part of the interface the probable result will be the destruction of the LCD display module.



The red dotted connection shows how a reset button can be connected if the user requres this feature. Construction of the project is very simple. I have made sockets for the USB Module and the PIC from turned pin SIL socket strips as these provide a reliable and inexpensive method, but if the user so desires, standard IC sockets can be used. As can be seen in the picture below only 7 additional components are required and the detailed component list is also shown. Also shown are the suppliers from whom I obtained the components together with that supplier's part number. I am sure that all components other than the USB module can be obtained from a variety of suppliers, most likely from your currently favoured company.

The value of R2 should be chosen to meet the maximum current requirement of the LED backligh fitted to the LCD module used. The 22ohm value used should be OK for most modules but could result in a lower intensity than some users might require.


Note 1: The maximum current that can be sourced from the USB +5v supply (using this design) is 100Ma. The interface requires a maximum of around 40Ma leaving 60Ma for driving the LCD logic and backlight. As the logic requirement is normally less than 5Ma it means that for all practcal puposes it can be ignored. In my test system, with the 22ohm resistor and 100ohm variable shown, the backlight cuurent was about 45Ma and so it was safe to link the V(5V) and VCC(5V) points together, but using other LCD modules that might not be the case. Users must ensure that an appropriate resistor is used to limit the backlight current to less than 55 - 60Ma if these points are to be linked. If the display requires more than 60Ma then an 'external' 5V supply must be used for the back light supply and in this case the V(5V) and VCC(5V) must not be linked and the external +5v supply should be connected to V(5V).
The USB interface specification does allow up to 500Ma to be drawn from the interface (if the USB cable is plugged directly int a PC or a powered hub) but only after the USB system has determined that it can safely supply it. It is possible to modify the design to allow this to achieved and over the next few weeks it is possible that I will provide details of how that can be undertaken using readily available components.

The Component List:

As will be seen most of the parts are shown as having come from Maplins but in fact most actually came from my 'junk box' and I have just looked at a Maplin catalogue to establish appropriate part numbers. The cheapest source that I could find for he PIC (UK based suppliers) was Crownhill. As far as I am aware the only UK based supplier of the DLP USB is Alpha Micro but they are available from Mouser and others in the USA (at $25.00!!). I have not checked to see if they ship to the UK nor have I checked delivery charges. When I purchased the module from Alpha Micro I found them to be very helpful and friendly and provided a swift delivery service!

I have used a 22 ohm 2 watt resistor (as I had one in the 'junk box') for the back light current limiter but that is almost certainly overkill - 1O ohms would probably be suitable for most devices but it is left to the user to establish if that is suitable for their display unit. I used a 2 watt resistor but in fact for most displays that I have looked at 1/2 watt would suffice. As Maplin do not supply 1/2 watt devices I have provided the part nummber for a 0.6 watt item. Once again it is the users responsibilty to determine the power requirement for their module and then calculate the appropriate wattage.

I have used modular connector strip (sockets) to form the sockets for the PIC and the USB module. A pin / socket combination has been used to link the interface 'PCB' to the LCD module's PCB. I cannot emphasise too much that users MUST ensure that the pin out on the LCD module matches the pin out used on the interface 'PCB'. Get this requirement wrong and you will almost certainly destroy the LCD display unit, the interface unit will probably survive.

The two pictures below shows the completed interface. There is no significance in the differing wire sizes I just used the first piece that came to hand.
In common with all subsequent pictures to get an enlarged view 'click' on the image.

The next picture shows the modular connector strip pin and socket arrangement used to join the display unit to the interface module. Note that in this prototype there is no other way to physically secure this connection and thus it is therefore possible to place undue strain on the connection, usually resulting in the copper strip lifting away from the board. It is strongly recommended that some sort of support be introduced, possibly using threaded rod and nuts and washers, or perhaps some type of threaded pillar can be employed. Also note that the strip board is not quite as wide as the pcb used on the display unit, this was due to an error on my part (I really should ensure that only one pencil mark is made!). The two boards should be the same size to allow for securely fixing the strip board to the PCB. Note that the strip board sizes shown in the parts list reflect the size of the 16 by 2 line LCD that was used. If a 4 line by 20 character display is used then the strip board should be be tha same size as the pcb on the LCD display, once again to allow for a secure fix between the LCD and the interface.

The picture below shows the finished project displaying my SETI statistics.

The next image shows the default bootstrap display - please note that I retain the copyright for the project.

The picture below shows a user defined bootstrap screen that I have created in order to show what can be done. Users can redefine the eight user defined characters and create their own display. More details of how this can be achieved are provided in the PC software section.