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2 problems....7805 regulator getting hot and RCTIME giving strange results — Parallax Forums

2 problems....7805 regulator getting hot and RCTIME giving strange results

eagletalontimeagletalontim Posts: 1,399
edited 2009-01-03 05:08 in General Discussion
ok....here is the situation. I recently added a frequency crystal to my project board which I got from my local radioshack. The owner fixes small electronics and happened to have a bunch of crystals that I could have. On the crystal, it says 75.990Z. Since I got them for free, I was hoping to use them in my next project. All that is running from the chip is 3 7 segment displays and 2 pins are used for RCTIME to measure the resistance of a sensor. Before, I was just running the chip with the internal clock and it did not heat up at all. I am not sure what OSC setting to set it at so I did some research before hooking it up and found alot of people were using OSCHS2 at 50Mhz so that is what I went with. I also tried to add a 10uf cap across Vss and Vdd...that did not help at all. The displays have a 100 ohm resistor on the Anode. The RCTIME pins have a 10k and a 47k with a .01uf cap. I need help getting the heat down or eliminated. The final circuit will probably have 2 chips running from the 7805 and synced together with the same crystal if possible.

As for the RCTIME, I am having problems getting it to show accurate values. As I change the value of the sensor being tested, the output will go from 179 to 170, then to 181. When it gets to 189, it will go to 180, then to 191. Basically it counts 1....8, 9, 0, 11....18, 19, 10, 21..... Any ideas on this?

Comments

  • JonnyMacJonnyMac Posts: 9,214
    edited 2009-01-03 02:06
    Ever notice how when you run your car's engine really hard it heats up fast? Same thing with the SX: speed = heat.
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-01-03 03:02
    i figured that, but I also tried running it at 4mhz with the crystal and it still got hot. Still not sure about the RCTIME though. I am tinkering with some math functions to convert it to a 1 through 9 value if possible.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-01-03 03:05
    Any work I've done with the SX at 75 MHz has required not only an external oscillator in lieu of a crystal or resonator, but also a heatsink (glued onto the SX with JB Weld).

    Do you really need to run that fast? If so, get an external oscillator and a heatsink. If not, Parallax sells some ceramic resonators that will work at slower — but still respectable — speeds of 4, 8, 20, 32, or 50 MHz.

    Free is not always better!

    -Phil
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-01-03 03:10
    eaglealontim said...
    ... I also tried running it at 4mhz with the crystal ...
    "the" crystal? Which crystal?

    -Phil
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-01-03 03:20
    it's the crystal that says 75.990Z on it. Do you have to match the speed of the crystal in the program or can you run at a slower pace? I just put the program back to 4Mhz still running the external crystal and it heated up just as fast. The SX does not get hot at all....just the 7805 voltage regulator.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2009-01-03 03:25
    "Do you have to match the speed of the crystal in the program?"

    They must match.
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-01-03 03:46
    well dang. ok, I am going to go ahead and order a few crystals from here so now I need some recommendations on what I need to order. What I am expecting to have on the project when completed is a 1 wire LCD display (already ordered from here), an eeprom, and 2 SX28's (hopefully no more). One of the chips will just be to run the display, the other will be to check sensors and activate different components connected to the circuit. The SX's need to be synced to talk to each other and it does not have to run too fast for any part of the program. I was thinking of going with the 5Mhz crystal. I prefer to use a crystal instead of a resonator since a crystal only has 2 wires and can easily be placed on my small project board tongue.gif
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-01-03 03:48
    The resonant frequency of the crystal is what determines the clock rate — not anything you specify in the program. If you don't want to run at 75.990 Mhz, you will need to get a different crystal or resonator.

    -Phil
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2009-01-03 04:07
    The FREQ Directive establishes the reference for time_base (crystal, et al.),·letting the SX know what rate·it has coming in, very important for time-sensitive tasks.

    4MHz, 5MHz, that's all cool (hi_hi), just get it right with the FREQ Directive.· The higher the FREQ, the hotter the SX (potentially) will run.· One SX can be on a 4MHz and another on a 10MHz and they can exchange serial data and that like.
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-01-03 04:10
    ok, so with the 5mhz crystal, what should I set the DEVICE line to? I am ordering some now.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2009-01-03 04:11
    What, are you kidding?

    FREQ 5_000_000

    OSCHS1

    Post Edit -- DEVICE sx28, oschs1, TURBO, STACKX, OPTIONX
    FREQ· 5_000_000
  • eagletalontimeagletalontim Posts: 1,399
    edited 2009-01-03 04:21
    sorry....i am extremely new to running an external oscillator/crystal. Just wanted to make sure I get it right before I mess something up.
  • Shawn LoweShawn Lowe Posts: 635
    edited 2009-01-03 05:08
    Eagletalontim-
    I had the same problem in understanding the SX. Parallax sells resonators, and they are all the same price. The higher frequency you run the SX the more power it consumes, so if your using a wall wort they may not matter. But battery, yeah you have to look into that. Keep at it, I keep making rookie mistakes too.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Shawn Lowe


    When all else fails.....procrastinate!
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