Swiss Army knife （3）

Published:2011/8/1 21:15:00 Author:Phyllis From:SeekIC

By Jim Spence

...and USB

As you can see from the PCB artwork, the USB interface is connected to the MCU section by copper tracks. If you want to fit the USB interface at some distance from the MCU board, the PCB sections have to be separated by cutting or sawing and a small connection cable installed between the respective connectors.

Now for some bad news. The FT232BM chip only comes in a surface mount version and it’s a small one at that. You need to be brave to build this but it can be done with a simple soldering iron, solder wick and some solder paste.

We would urge you to have a go at this, it’s not a beginner task but it’s not impossible either.

Unfortunately, solder paste is expensive but it does make the process much easier. An alternative to using the paste is to use far too much solder and get rid of the excess with the solder wick.

The solder wick does such a good job at tidying up that it is not that important if the solder goes in all the wrong places at first. The most important thing to get right is the orientation of the IC1. It must be perfectly in line and square with all of the pads and remain there whilst the first heat is applied. If it slips then you’re in trouble.

We would recommend that you spend a lot of time lining IC1 up and when you are satisfied, solder just a few pins, (one pin if you can) the minimum coverage of the author’s fine tipped soldering iron is two to three pins. If all is well solder the rest of the pins, once in place it is almost impossible to remove. Remove all of the excess solder with solder wick and double check no short circuits exist between the pins.

The other components are not too bad. They were chosen for their large size (relatively speaking). The connector K2 is a pinheader that will probably need cutting from a larger one.

Before plugging in the device to the PC, download the FTDI device drivers and unzip to a suitable directory. If all is well when you first plug the device in you will be asked for the device driver. Take some time after building to inspect the circuit for shorts, use a meter if necessary. Plug the device into the USB port on the PC or preferably onto a hub to prevent any possible damage to the PC. Although the USB specification calls for short circuit protection, you never know, and a new hub is much cheaper than a PC. If it’s any consolation the prototype circuit was not checked well enough the first time it was plugged into a PC and all the USB devices suddenly stopped working, this included the mouse. It took a reset of the PC to restore things back to normal — not a pleasant experience.

Assuming all is well the PC will detect the new device and request the location of the device drivers. Install this just as you would any device driver. If you are not sure how to do this look on the FTDI website for information, installation instructions also come with the device drivers for Windows and other operating systems.

All being well you should now have a new COM port. To find out which port number has been allocated depends on the operating system. For Windows XP this is in the Control Panel→System →Hardware→Device Manager then open up the Ports tree by clicking on the -f sign. You should see a new port. If not, re-install the device driver and make a note of any error messages.

Complete circuit test

Connect the two boards together, use either the RS232 circuit or the USB circuit. Launch the free ByVac terminal utility, see the ’ByVac-TerrninaT inset. If using the RS232 board you need a straight-through (1:1) cable where pin 2 goes to pin 2 and pin 3 goes to pin 3 etc. In some cables the pins are crossed, check this with a meter. You also need to enable RTS in the ICP box. Press reset (SI), wait for a few seconds and you should see the sign-on message. If not, check that the sign on message is coming from ICl. If there is a signal and you still do not see the sign-on message check the settings, Baud rate etc., check the connectors, cable and wiring.

Here’s TCB

At the heart of this project is the TCB (Tiny Control BASIC) software that will incidentally run on any 89C8252 system with or without external RAM. If there is any RAM present it will automatically detect it, obviously there is no RAM in this project.

There are three ways you can get the TCB software onto ICl:
1. use the programming interface;
2. buy a pre-programmed chip, order code 030448-41 from Readers Services;
3. use an 89C8252 programmer.

Although possible, option 1 is not recommended, the programming interface and software being designed for short programs(<100 lines), also it will mean that you can’t properly test the circuits until you have done this so how do you know if the circuit has been built correctly? It is possible however but will take about 25 minutes, see the Assembly Code inset. Options 2 or 3 are recommended if you are building the circuit for the first time.

This processor and its architecture have been mentioned many times in various articles, however the memory aspect must be at least partly understood so it is briefly discussed here.

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