This modification is for the
Processor 1 Ver 2.0 board only, and should not be attempted if you have
any other processor board.
The Problem
Some Cybots, when seeking light,
work for a short while then suddenly stop for no apparent reason.
You then have to turn Cybot off then back on again to continue.
Apparently, not all Cybots have this problem, but if you do experience
this then this modification is for you.
The Reason
The pin out for the processor chip
is shown below:
Pin 20 is a Reset pin. If this
pin is brought to 0V, then the processor stops executing any instructions.
If the pin is then brought up to the supply voltage, then the processor
will start to run its program from the start. The reason for having
such a pin is so that an external circuit can cause the processor to
reset, or you can delay when the processor starts running to allow other
circuits to power up and initialise fully first.
The problem with Cybot is that this
pin is not connected to anything yet. I say yet, because in issue 13
another board will plug into the Light I/O board and so rectify this. So,
what happens if the pin is not connected to anything (which is called
'Floating' by the way)? Well, the internal circuitry within the
processor will monitor the reset pin and act accordingly. As there
is no connection to the pin, it can go either high or low. The state
can depend on a number of factors. Stray capacitance surrounding the
pins of the processor can put it high or low. Any noise from either
the motors or any other external source can also cause the pin to go high
or low. This is why any reset pin on any microprocessor,
microcontroller or logic circuit must be connected to something. In
the case of Cybot's processor, this pin must be connected high for the
processor to function.
The Solution
You can make a very small and cheap
modification. There are two ways of making the modification, and it
is up to you which one to select.
If you look back at the processor
circuit board, you will see that there is a 6 way connector on the
board. Pin 3 is connected to the +5V supply which is fed via
the 13 way connector. The RESET pin of the processor is fed to pin 6
of the 6 way connector. So, the solution is to connect pin 6 to pin
3. Now, the data sheet for the processor states that the reset pin
can be connected to the supply by any value resistor that is below 40K.
You can of course use a piece of wire, or a resistor. Either will
do. (The reason the manufacturers specify a maximum resistance is in
case you connect an external circuit to the Reset pin, in which case the
resistance of the external circuit must no be more than 40K). I will
show both methods using a resistor.
Method 1
This is the simplest method, and
possibly the preferred one also. When the board that comes with
issue 13 is fitted, you can easily remove the resistor before hand.
Take your resistor and bend it into
a 'U' shape. Bend the legs so that they are bent as close to the
resistor body as possible. Then cut the legs of the resistor in
half. You should then end up with a resistor that looks like the one
shown below:
Now, all you need to do is plug the
resistor into the 6 way connector pins 3 and 6 on the processor board as
shown:
This is a temporary solution, and
bear in mind that other circuit boards will connect to the same 6 way
connector. If this resistor is still required after the other
circuit boards are connected, and RR have not come up with another
solution by then, then the resistor will need to be fitted permanently, as
shown in method 2.
Method 2
This method is similar to method 1,
but this time the resistor is soldered directly onto the back of the
board and is a more permanent solution. I do not know at this stage
if the resistor will have any effect on the board that comes with Cybot. If you are not confident with soldering, then either find
someone who can do this for you, or just stick with method 1.
First, bend the resistor as in
Method 1. Now cut the leads so that they are 3 - 4 mm long.
Remove the processor board, and solder the resistor across pins 3 and 6 of
the 6 way connector. Now, carefully bend the resistor up so that
when the board is back on Cybot the resistor is pointing downwards.
If you don't bend it, then you won't be able to get the board back into
Cybot. Your board should look like that shown below:
Check and double check you have put
the resistor across the correct pins, then re-assemble the board back into
Cybot.
The only problem I can see with this
method is if the extra circuits that will fit to the 6 way connector are
affected by the resistor/link. If this is the case then the
modification will have to be undone. However, hopefully before then
RR will have a replacement board for you to use.
