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5641  Using Arduino / LEDs and Multiplexing / Re: mcp23018 not working on: April 29, 2012, 01:18:00 pm
The second line of the datasheet says "I/O pins default to input".  Do you need to set your output pins as outputs?

The outputs are Open Drain which means they switch between DISCONNECTED and GROUND.  Do you have the LED connected between +5 and the output (with an appropriate current limiting resistor in series)?
5642  Using Arduino / Project Guidance / Re: Teensy 2.0 and button lag on: April 29, 2012, 01:04:51 pm
If the button is between +5 and the Arduino pin you need a pull-down resistor between the Arduino pin and Ground.
5643  Using Arduino / Project Guidance / Re: CNC with servos-is it even possible? on: April 28, 2012, 07:44:31 am
If you have the rotating laser close to the sheet-goods being cut you will have problems with cutting at an angle.  If you have it far away you will have problems with resolution: hobby servos have about 1,000 steps on each axis.
5644  Using Arduino / Storage / Re: Arduino Uno R3 & CMOS Flash Memory on: April 28, 2012, 07:27:06 am
It has a 16-bit address bus so you will probably need some shift-register chips to get enough output pins to address it.

Hold Not-WriteEnable HIGH.  Hold Not-ChipEnable and Not-OutputEnable LOW.  Apply and address and read a byte of data.
5645  Using Arduino / Installation & Troubleshooting / Re: can somebody point me in the right direction on: April 28, 2012, 07:19:59 am
If the loopback test doesn't work it's the USB-to-Serial that is broken.  If you are lucky it's just the firmware that got fried and the hardware is still good.  If so you can use the DFU process to restore the USB-to-Serial:

http://arduino.cc/en/Hacking/DFUProgramming8U2
5646  Using Arduino / Installation & Troubleshooting / Re: This is a bad idea: Serial.print("!!!"); on: April 28, 2012, 07:16:44 am
Some versions of the bootloader have a "monitor" mode that is activated by sending three consecutive '!' characters.  This monitor mode allows you to read and write memory from a serial console.  Unfortunately the idiot person who implemented the code didn't put in any protection to prevent the monitor from being triggered during memory upload!  If your bootloader has this "feature" (typically the Mega and other larger processors) you have to remember to never use three '!' characters in a row. smiley-sad
5647  Using Arduino / Programming Questions / Re: Multiple SPI chip select question on: April 28, 2012, 07:11:57 am
You have to set Pin 53 (hardware Slave Select) as an OUTPUT even is you don't use it.  If it is set as an INPUT then the SPI hardware will be in Slave mode.
5648  Using Arduino / Programming Questions / Re: Simple question about turning on a led. on: April 28, 2012, 07:05:41 am
The problem is that you aren't keeping track of the previous position of your finger.  The result is that the LED will rapidly alternate between ON and OFF as long as the finger is in position.   You want to change the LED state only when the finger state changes from AWAY to CLOSE.
5649  Using Arduino / Sensors / Re: Ping Sensor Frequency on: April 27, 2012, 09:27:58 pm
It might be easier to just sample them in sequence.
5650  Using Arduino / Project Guidance / Re: need help for letting my program work on the Mega on: April 27, 2012, 11:04:17 am
I guess if the Arduino Mega pins 10-13 are set as inputs they won't be a problem.

It's not clear if the Slave Select signal will be on 10 or on 53.  Depends on how the libraries are written.

First try jumpers from 11-13 to 50-52 and set 11, 12, and 13 as INPUT pins.

If that doesn't work, add a jumper from 10 to 53 and set 10 as an INPUT as well.

If neither works you may have to invest in the Mega version of the board.
5651  Using Arduino / Storage / Re: Aduino Mega and SD Card on: April 26, 2012, 11:13:47 pm
The SD card use the SPI interface.  On the UNO that's pins 10-13.  On the Mega that's pins 50-53.

If you have a pin that goes to UNO pin 10 that's the "Slave Select" pin and should either got to Mega pin 10 or Mega pin 53 depending on how the SD library you  are using selects the SS pin.

The other three pins (UNO 11, 12 and 13) go to Mega pins 50, 51, and 52, respectively.
5652  Using Arduino / Programming Questions / Re: Improving thermistor resolution on: April 25, 2012, 09:00:22 pm
Would I have the same problem with the hole in the A/D range if I used external hardware to limit the range of the thermistor? 

If you can offset the voltage to read 0 at the low end an +5V at the high end you would be able to separate the range into 1024 steps.  Unfortunately the steps are probably not going to be linear so they would not all be 0.01°C steps.
5653  Using Arduino / Programming Questions / Re: Improving thermistor resolution on: April 25, 2012, 11:03:06 am
Over what range do you need 0.01°C resolution?    The A/D converter has 1024 states so your maximum range will be 10.24°C.  What is the low end of the range?  What is the resistance of the thermistor at the low end and the high end?

One problem you will have is that using the thermistor as part of a voltage divider you will be unable to get your voltage range to start at 0V.  That means that some portion of the A/D range will be lost.  That leaves you with a less than 10°C range, possible much less.

Perhaps and external A/D with 15 or 16 bit accuracy would give you the resolution you want. 
5654  Using Arduino / Project Guidance / Re: need help for letting my program work on the Mega on: April 25, 2012, 10:52:51 am
Several of the devices use the hardware SPI interface.  On the Mega the SPI pins are 50-53 instead of 10-13.

               UNO    Mega
Default SS  10       53
MOSI          11       50
MISO          12       51
SCK           13       52

You will probably have to at least cut pins 11, 12, and 13 and jumper those to 50, 51, and 52 respectively.  Set pinMode(53, OUTPUT).  That may be enough if the library you use doesn't play special tricks when it is compiled for the Mega.
5655  Using Arduino / Microcontrollers / Re: Using arduino board like programmer in AVR Studio on: April 25, 2012, 10:36:24 am
If you don't have a bootloader on the chip you need an In-circuit Serial Programmer (ISP or ICSP) device.  You can emulate one with an Arduino and the ArduinoISP sketch (Files->Exampels->ArduinoISP) or buy one from China for under $10 on eBay (look for "USBasp").
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