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976  Using Arduino / Project Guidance / Re: High acceleration with fast data logging on: November 29, 2012, 05:17:12 pm
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Would I be able to record data faster if I got a data logger shield and wrote directly to an SD card?

If you are talking about dropping a football, you are likely in need of recording over several ms range, but probably at low frequencies (5ksps for example. That means a SD/eeprom based solution isn't fast enough for you.

Once you figure out a way to trigger the logging, you may need to record to SRAM or FRAM devices.

Another alternative is to write to internal ram - but you need a mcu with large ram.
977  Using Arduino / Microcontrollers / Re: Advantage of Arduino on: November 29, 2012, 05:11:11 pm
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Advantage of Arduino

Arduino's biggest advantage is that it allows a user to code without looking at the datasheet;

Arduino's biggest disadvantage is that it allows a user to code without looking at the datasheet.
978  Using Arduino / General Electronics / Re: Op-Amps, regarding Inverting vs Non-Inverting modes. on: November 29, 2012, 05:10:01 pm
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Summing amplifiers (used in mixers) always invert.

It doesn't have to be that case.

979  Using Arduino / Project Guidance / Re: Electrical impedance of Dout port on: November 29, 2012, 05:06:54 pm
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I only need to put the right capacitor at the right place

Only if it were that simple. Two causes for those spikes:

1) gate capacitance: as long as you have mosfets, you have gate capacitance, especially when they are undampened;
2) non-atomic operation on the output pins.

Essentially, you took the more difficult route when easier ones are available.
980  Using Arduino / Project Guidance / Re: Electrical impedance of Dout port on: November 29, 2012, 11:18:11 am
Your solution doesn't have to be this complicated.

Take 5 resistors: tie your four output pins together through 4 reistors, R0..3, and from there tie them to ground through Rg. Buffer the output from where the resistors meet, with a jfet or an opamp.

The output voltage is = (V0/R0 + V1 / R1 + V2 / R2 + V3 / R3) / (1 / R0 + 1/R1 + 1 / R2 + 1 / R3 + 1 / Rg). V0..3 is either 0 or 5v, representing the output from your mcu.

All you need is to find the values of R0..R3 (in terms of Rg) to best approximate 16 points on a sine curve over 360 degrees (actually just over 90 degrees but that requires fancier software / hardware).
981  Community / Bar Sport / Re: -- new arduino on kickstarter? it is a fake? on: November 29, 2012, 08:38:24 am
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thats frankly not right.

That operates them up to a course of dealing argument.

Other than the "ARDUINO" mark on the pcb, I don't think the smARtDUINO guys made any mistake: their statements of having worked on the manufacturing of ARDUINO is true - I am sure when compelled by a court, ARDUINO would have to confirm that.

And assessing damages is even more interesting: a legal fight would force ARDUINO to open up its books. If ARDUINO made a lot of money, well, that will intensify competition here and raise a few eyebrows; If ARDUINO didn't make a lot of money, well, no damage.
982  Using Arduino / Project Guidance / Re: Electrical impedance of Dout port on: November 29, 2012, 08:16:14 am
Another alternative you can try is a summing amplifier (both inverting or non-inverting).

What you should realize is that the mcu pins can be viewed as switches: they switch in (Vcc or GND) or they switch out (high impedance).

That will make it easier for you to think through the various issues.
983  Using Arduino / General Electronics / Re: Op-Amps, regarding Inverting vs Non-Inverting modes. on: November 29, 2012, 07:30:04 am
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why you would want the polarity to be reversed in the Inverting mode?

Polarities rarely matters.

In cases where it does matter, you can simply swap the speaker terminals (you can achieve that through labeling on the amp's output terminals).

Most (audio) power amp's are configured as non-inverting (there are exceptions: gainclone for example is originally inverting because folks insisted it sounded better than non-inverting, until they insisted otherwise).
984  Using Arduino / Project Guidance / Re: Electrical impedance of Dout port on: November 29, 2012, 07:26:38 am
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Could you be more specific?

It is actually very simple:

When the gate is 0, the opamp is in negative feedback and outputs a 3v so its inverting input sits at 3v.

When the gate goes to 6v, there are two cases:

1) if the mosfet's Vgs(th) is sufficiently low (let's say 1v) for the mosfet to conduct, the inverting inputs will be at 6v (mcu's otuput) - Vgs = 5v. With the opamp's non-inverting input at 3v, the amp goes to 0v to try to bring down the voltage differential - it can never do that. or

2) if the mosfet's Vgs(th) is sufficiently high (let's say 4v) so the mosfet cannot be opened, the inverting input continues to sit at 3v, and the opamp outputs a 3v.

The analysis has nothing to do with the resistors in the drain, as you have noticed.

You could have easily reconfigured the circuit into a r2r and you wouldn't need to use those mosfets.

If you do want to use the existing topology, put a pnp mosfet there, between the resistors and 6v to switch the resistors into / out of the network.

Another alternative is to use a switched divider network.

Also, when you pick the mosfet, pick one with small capacitance (TO90 or IRF510) to avoid ringing on the rising edge.
985  Using Arduino / Project Guidance / Re: Electrical impedance of Dout port on: November 28, 2012, 08:18:08 pm
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What is the electrical impedance of the port?

50ohm is a good number to use.

Before you proceed, think about your design:

1) your circuit will not work, as is. It is not a r2r network and the switches are wrong. A typical r2r network can be implemented much easily, without using the switches.
2) If you really want to use your existing topology, think about a high-side switch.
3) you will run into issues with atomicity if those pins are not on the same port and you don't use port operations.

Unless you have a specific reason to use your design, there are far better ways to implement a dac.
986  Using Arduino / Programming Questions / Re: Why does uint64_t and unsigned long long not work? on: November 28, 2012, 07:48:55 pm
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It makes no sense why I should have to do that.

Pick up a good C book and read about "type promotion".
987  Using Arduino / Microcontrollers / Re: Interrupt types on: November 28, 2012, 05:14:17 pm
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whilst PCINT pins are interrupts that can only trigger on a change, not a rising or falling edge?

That's not a big deal: you just need to test for the levels of the pins in the isr to determine if it is rising or falling edge.

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I didn't know about pin change being a change on the port as a whole

On an avr, the pins can be individually configured to trigger a pcint. On some mcus where individual pins cannot be configured (STM8), you simply have to mask the port in the isr.
988  Using Arduino / Sensors / Re: Why did I melt my LM35DZ? on: November 28, 2012, 08:33:39 am
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I connected it directly to ground, 5V

Next time, put a small resistor to its +Vs pin, in case of wrong connection. Anything less than 1k will work. After that, you can take it out if you wish.
989  Using Arduino / Displays / Re: 16x2 lcd on: November 28, 2012, 08:11:39 am
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i can see only this on lcd....(image is below)

That means the device is getting powered up, has the right contrast, but not correctly initialized.
990  Using Arduino / Programming Questions / Re: LSB Vs MSB on: November 28, 2012, 08:08:26 am
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You need to wire 16 pins up to do the actual bit reversing, then on the Uno it might be:

That  may be two statements in C but it will likely translate into multiple assembler statements + overhead.

If you are going to use 16 pins, you can do so with two ports:

Code:
#define IN_PORT PIND
#define OUT_PORT PORTA

byte rev (byte in)
{
  OUT_PORT = in ;
  return  INPORT;
}

OUT_PORT.x is connected to IN_PORT.7-x
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