Quick question from a newbie.

Rick-S · Joined: 08 Jan 2009 Posts: 50

Is there any advantage to using an external crystal oscillator circuit on and ATtiny2313 or ATMega8 other than being able to achieve a higher clock speed? Just curious Confused

Rick

Luciano · Joined: 29 Nov 2004 Posts: 3149 Location: Italy

Hi,

With a crystal you can achieve any serial baud rate with almost 0% baud rate error
and the baud rate will be stable across a wide temperature range.
(Example a 11.0592MHz crystal).

The clock frequency will have the same accuracy of the used crystal.
When you need precise timing, you must use a crystal or an external
clock source.

Best regards,

Luciano

Rick-S · Joined: 08 Jan 2009 Posts: 50

Based on what you said, if my project isn't using serial communications and isn't timing dependent, then the internal oscillator is fine. However, if I need to do serial or have a timing dependent application I need to go with an external oscillator circuit to provide accurate timing and temperature stability.

Here's the project I'm working on and a bit of background.

I'm nearly 44yrs old and while I've been a computer enthusiast since the early 80's, I've only had limited formal training in electronics. Don't get me wrong, I can read a schematic, know what the various components are, and have no problem assembling circuitry and even designing simple circuits.

My programming knowledge comes from years ago and was self taught in GW-Basic, QuickBasic, and Visual basic for DOS. I have dabbled in current visual basic, 80's era borland pascal, and 90's era C+ (although very little in the pascal and C+).

The project I have at hand:

I have an inductive industrial joystick that provides an analog output of approximately 3.5vdc at the lower extreme, 4.5 at the center, and 5.5 at the upper extreme.

The joystick needs to control 2 things:

1) Provide 6 output lines at 24vdc to move the X, Y, and Z axis of a CNC milling machine. (These are on/off signals 24vdc toggles an input on the CNC control that moves the Axis 0vdc means no movement)

2) Provide an interface to a windows based PC to act as a mouse.

The two modes will be toggled by a single button. Press the button and the CNC output becomes active and all PC lines go low. Press it again and the CNC lines go low and the PC lines go active.

Currently I am reading the joystick into the ADC of an ATmega8. The plan is to simulate a quadrature output for X & Y and run that into a usb "mouse" board that is used in home brew arcade machines to hook up trac-balls. This seemed to work 'OK' when I tested it on an AT89c2051 but not great. I don't know how to get the quadrature signal to be consistent enough to avoid and occasional jitter. It was functional though. I'm making the move to the AVR's because of the true ADC and I need more than one channel. I haven't yet tried the quadrature on the Mega8,

Anyone have any good ideas for a project like this? Am I going about it in a method that will work? I know I don't have the expertise to program an HID device into a micro controller that is the reason for the secondary board to do this.

Sorry for the long post. This seems to be such a friendly forum I'm hoping someone can offer some insight or suggestions.

Keep in mind though... the simpler the better Wink

I am quite the newbie at this.

JC · Joined: 15 Dec 2007 Posts: 586 Location: Cleveland, OH

Sounds like a fun project.

If you are new to AVR's and such I would be inclined to take it a step at a time.

I would interface the joystick first, and either an LCD or an S-232 serial line to upload data to a PC Hyperterminal terminal emulator to read the joystick data. Do this until you have your joystick working as desired.

It wasn't clear to me why you need an analog input on the joystick if it is just being used for course positioning of the CNC head. Is the joystick just for course positioning, with the operator watching the head, and no position sensing? With simple On/Off output to the CNC controller it would be easier to use a simple joystick. On/Off for the various directions.

I don't recall how many AtoD inputs the Tiny2313 has...

Getting the uC USART to Max232 chip to Serial to PC Hyperterminal will be a big step in itself, and a good experience. You will use this knowledge in the future.

You can use a USB to Serial port converter if your PC doesn't have a true serial port.

You could go uC USART to FTDI chip to do logic level serial to USB prot on PC to Serial Com Port within PC software, if desired.

The uC to 24 V logic switching is a separate task unto itself.

JC

Rick-S · Joined: 08 Jan 2009 Posts: 50

mattcro · Joined: 03 Oct 2007 Posts: 327 Location: Scotland

If you have a spare COM port on the CNC PC (or a USB>RS232 adapter), you could use the AVR to emulate a serial mouse instead of generating 2 sets of quadrature outputs. The protocol for a standard (Microsoft) serial mouse is fairly simple and Windows should recognise it automatically. I can't remember the details offhand. You just need an RS232 level converter like the standard MAX232 to drive the serial port.

You can use the AVR hardware timer(s) to trigger an ADC read at a set interval (typically 40 per second), scale the ADC values to mouse coordinates (you'll have to experiment with the scaling values) and send the mouse position packet to the PC, plus set the CNC drive outputs according to the stick position.

Rick-S · Joined: 08 Jan 2009 Posts: 50