- #1
Jiggy-Ninja
- 309
- 1
Since I don't have a function generator, for some experiments at home with digital ICs I've used a 555 timer with 5V Vcc as the clock, and I noticed something problematic. It behaves rather erratically.
I've mostly been using it to trigger a 74LS90 asynchronous 4-bit counter. Sometime it works right (triggers on every positive edge), sometimes it triggers on both edges, and sometimes it triggers twice on each positive edge (counting by twos each time). Sometimes it works fine for a few numbers, then counts on each edge for a couple before going back to normal. And it's always on the same numbers in the count too, so something's weird.
Also, sometimes two units of the same chip (in two versions of more or less the same circuit, one soldered on a PCB the other on a breadboard) hooked up to the same timer will act differently. One will count by twos and the other will work fine.
Something that seems to make the problem go away is to load the timer with a couple LEDs, one Active LOW, the other Active HIGH. I've no idea why that works, since I have no idea what's going on to cause the problem in the first place.
At school there's boards like this http://www.elexp.com/tst_0dig.htm" [Broken] with a clock conditioning circuit using 7414 Schmitt Trigger Inverters. Would that make sure the problem goes away for good?
Unrelated small question: Is there any chip still being made that does the same thing as an XR-2206? It seems like such a cool little chip, but it looks like it's not being produced any more, and I can't find any ICs that make sine waves.
I've mostly been using it to trigger a 74LS90 asynchronous 4-bit counter. Sometime it works right (triggers on every positive edge), sometimes it triggers on both edges, and sometimes it triggers twice on each positive edge (counting by twos each time). Sometimes it works fine for a few numbers, then counts on each edge for a couple before going back to normal. And it's always on the same numbers in the count too, so something's weird.
Also, sometimes two units of the same chip (in two versions of more or less the same circuit, one soldered on a PCB the other on a breadboard) hooked up to the same timer will act differently. One will count by twos and the other will work fine.
Something that seems to make the problem go away is to load the timer with a couple LEDs, one Active LOW, the other Active HIGH. I've no idea why that works, since I have no idea what's going on to cause the problem in the first place.
At school there's boards like this http://www.elexp.com/tst_0dig.htm" [Broken] with a clock conditioning circuit using 7414 Schmitt Trigger Inverters. Would that make sure the problem goes away for good?
Unrelated small question: Is there any chip still being made that does the same thing as an XR-2206? It seems like such a cool little chip, but it looks like it's not being produced any more, and I can't find any ICs that make sine waves.
Last edited by a moderator: