| ??? 08/22/08 04:47 Read: times |
#157652 - Where you measure the voltage makes a difference Responding to: ???'s previous message |
Muthuswamy Dikshit said:
The 1.78V which i mentioned was the Vmax for Lpt to sense a low. I used a potentiometer to give volatge to the lpt input pin. It toggled from low to high when voltage just crossed 1.78V. May this be the problem?
The output from lpt for low was almost Zero. Asim circuit uses 74HCT541 but since I could not get it I'm trying out with 74HC541. From the datasheets, I find that former accepts cmos input but latter does not. If you look at the datasheets, the HC541 accepts CMOS levels just fine. The 'T' in the HCT541 indicates that it accepts TTL levels, those having a somewhat lower minimal Vih. ISP programmer cable- How do i get this? Local shops don't have it. I checked ATMEL website, I think it also does not sell it. Pls help to get one. You'll have a lot less trouble with a Maxim/Dallas DS89C430/450 than with this ATMEL part, as the Maxim/Dallas DS89C430/450 (download and read and understand the datasheet and user guide from www.maxim-ic.com) as it has a dirt-simple serial port ISP interface for programming. It took me about an hour to build the circuit and I had a working program loaded in it within 5 minutes after completing assembly of the circuit. It's really not difficult. The programmer uses a MAX232A, a 74HCT125, and a crystal oscillator IC in addition to the MCU. Now this Maxim/Dallas DS89C430/450 isn't the part you intended to use, (I believe MAXIM will send you a sample if you ask for it) and I recommend you thoroughly read the datasheet for it, for the interface logic devices that you need in order to program it, and, most importantly, the datasheet for the computer parallel port you intend to use. The parallel port can vary widely in timing and voltage levels, though they're not supposed to do that. They do, typically, have series resistors in some of their signals, and that can confuse measurements made at various points in an attached circuit. Be sure you know what you're measuring. If you just blindly take someone else's circuit and attempt to apply it, you should first ensure that you have on hand all the exact components specified in the application circuit. If it says 100 ohms, then use 100 ohms, and if it says 74HCT541, then use that, and not what you can simply find in your desk drawer. The use of a solderless breadboard is often a fatal mistake. It may work on one day, after much effort, and not at all the next. I wouldn't waste a minute on a MCU circuit in a solderless breadboard. They really aren't suitable for electronics operating at much higher frequency than about speech-level audio. It will save you lots of stress if you simply solder the wires on a real prototype board with a "colander" ground plane on the wiring side. That way you can be certain that what you did today will still be there tomorrow. RE |
