two 18x picaxe in single circuit

[chris.celovic@hotmail.com]

Hi there

A couple of questions.

Can I use 2 18x's together, off the same power supply.

I am running 2 18x's because i need to send a cancel signal via the first chip (if needed) to stop the second... and - what would be the best way to output from the 1st and send it to the 2nd. I am having one of those bad "for the life of me, I can't think how" moments.

Cheers

 

[inglewoodpete]

Look at the big picture. A PICAXE chip is just a component. You can power many components in a circuit off a single power supply (look at a computer or TV set).

When connecting digital chips, like a PICAXE, it is wise to connect a 100nF capacitor as close as possible across the +ve & 0v connections to the chip.

When interconnecting PICAXE chips, in some circumstances, it can be possible for 1 chip to be outputting 5v and the other outputting 0v on the same interconnection. The resultant current could damage both chips, so always provide isolation or current limiting between the PICAXEs. The simplest form of current limiting would be a 1k ohm resistor between the output pin of 1 chip and the input pin of the other.

 

[krypton_john]

To stop it doing what? Shut it down altogether? Or send it a signal to do something?

Yes you can and should hook them to the same power supply as long as it can supply enough current.

You can just connect an output from one to an input on the other. The second one can check it's input or can interrupt on the input.

If you want to power up/down the second one I guess the output pin won't source enough current to run a whole 18X so you'd need to switch it's power on and off through a darlington pair or FET?

 

[chris.celovic@hotmail.com]

send a signal.

To stop it doing what? Shut it down altogether? Or send it a signal to do something?

Yes you can and should hook them to the same power supply as long as it can supply enough current.

You can just connect an output from one to an input on the other. The second one can check it's input or can interrupt on the input.

If you want to power up/down the second one I guess the output pin won't source enough current to run a whole 18X so you'd need to switch it's power on and off through a darlington pair or FET?

I use RF to send the second chip some data and it executes some code to do some timing, but i need to send a cancel signal to stop the code running (you cant do this on the chip that is running) so a third chip is employed to recieve the cancel signal from the first chip and viola! i have a serin within all the timing loops that checks for the cancel signal. That means they will be both running, not a problem as I have a constant power input for the second and third chip. thankyou

 

[chris.celovic@hotmail.com]

Look at the big picture. A PICAXE chip is just a component. You can power many components in a circuit off a single power supply (look at a computer or TV set).

When connecting digital chips, like a PICAXE, it is wise to connect a 100nF capacitor as close as possible across the +ve & 0v connections to the chip.

When interconnecting PICAXE chips, in some circumstances, it can be possible for 1 chip to be outputting 5v and the other outputting 0v on the same interconnection. The resultant current could damage both chips, so always provide isolation or current limiting between the PICAXEs. The simplest form of current limiting would be a 1k ohm resistor between the output pin of 1 chip and the input pin of the other.

I have asked this of a few people on this forum, but alas, I have not received an answer. What is a decoupling cap, why is it used?

 

[inglewoodpete]

A decoupling capacitor reduces "noise" on the power rail. This can be induced from other circuits or generated within the circuit esp by digital components. Digital components consume different amounts of current at different times, often at their clock speed, causing micro power surges on the power leads. This ends up being "noise", or high frequency fluctuations in the voltage and current in the power wiring. The capacitor evens out the fluctuations, reducing the influence it has on other components in the circuit.

A larger capacitor can also act as a "tank" of energy, as a charge, to minimise low frequency surges caused by demanding components like motors, relars and lamps. The function is similar to the above.

 

[moxhamj]

I am running 2 18x's because i need to send a cancel signal via the first chip (if needed) to stop the second

Does the second chip have to be an 18X? Now you have it working, you might be able to reduce costs etc by making the second chip an 08M. Have you now got 2 chips or 3?

 

[westaust55]

Decoupling in Digital Circuits

As an extension of what IWP has given, at a basic level all digital circuits switch between roughly Vss (0Volts) and Vdd/Vcc (5V).
The switching is fast and if thought of as a vertical rise or fall then it constitutes not only a change in the DC waveform level but looks like every odd harmonic/frequency summed together. (Lightning is basically the same). Harmonics are multiples of the main frequency which in the case of computers is fast. These harmonic are very high frequencies create noise in adjoining parts of the circuit through magnetic induction between conductors/traces on the circuit boards and also result in fluctuations on the power supply rails.
The capacitors have an internal resistance inversely proportional to the frequency: Z= 1/(2 x pi x f x C)
As a result they appear to be a close to a short circuit to the higher frequency noise/fluctuations and will filter these fluctuations out through the capacitor giving cleaner voltage supply rails.

 

[chris.celovic@hotmail.com]

great answers

As an extension of what IWP has given, at a basic level all digital circuits switch between roughly Vss (0Volts) and Vdd/Vcc (5V).
The switching is fast and if thought of as a vertical rise or fall then it constitutes not only a change in the DC waveform level but looks like every odd harmonic/frequency summed together. (Lightning is basically the same). Harmonics are multiples of the main frequency which in the case of computers is fast. These harmonic are very high frequencies create noise in adjoining parts of the circuit through magnetic induction between conductors/traces on the circuit boards and also result in fluctuations on the power supply rails.
The capacitors have an internal resistance inversely proportional to the frequency: Z= 1/(2 x pi x f x C)
As a result they appear to be a close to a short circuit to the higher frequency noise/fluctuations and will filter these fluctuations out through the capacitor giving cleaner voltage supply rails.

Trust a west Australian to come up with an explanation like that, however I do appreciate it, and to IWP for his succinct, but informative piece. I do appreciate the both of you taking the time to respond. Dr A, I have a few smaller chips, and yes I will reduce it, but I have all the space in the world for the RX unit (2 chips) but the hand held TX needs to be as small as one can get it, but I am limited by the size of the LCD, and batteries. BTW, if I used rechargables, what if anything do I have to do to charge them in situ. I have a fairly basic understanding of charging lead acid batteries, but I would think that charging Ni MH cells is a bit more complex?

Cheers

 

[chris.celovic@hotmail.com]

It Works

I have both 18xs doing every thing asked of them. now i just have to polish off the code and then draw it up.

 

[hippy]

I have asked this of a few people on this forum, but alas, I have not received an answer. What is a decoupling cap, why is it used?

Great question ... a chance to think up another 'bizarre' but hopefully useful analogy ...

Think of the power rail as a high-wire tight-rope with all the IC's hanging off it, bouncing up and down as they do whatever they do. You really wouldn't want to walk along that. The de-coupling capacitors are like big compression springs from the ground to the rope, dampening out the affect the operating IC's have.

Just like genuine de-coupling capacitors, the closer these springs are placed to the IC's which are affecting the stability of the rope the more effective they are.

The analogy is perhaps closer to the use of reservoir capacitors; big springs ( larger uF capacitors ) are best at taking out slow bounces, other springs ( smaller nF capacitors ) are better at taking out faster bounces and sudden tugs on the rope.

 

[BrendanP]

If you want to charge NiMH have a look at the MAX712, lead acids, UC3906.

Futurlec in Thailand carry both of these in DIP.

A read of the data sheets for both of these will help you to understand charging requirments of these two different battery types.