Coppied the origional posting. I didn't know where he got the 55ms nor why he thought it would be important. So I mentioned the PC's 55ms thing. Bill ----- Original Message ----- From: "R. I. Nelson" To: "Microcontroller discussion list - Public." Sent: Monday, November 28, 2005 11:19 AM Subject: [PIC] newbie question on timing > Using a basic compiled program using a for-next loop, a flash PICchip. > can I expect toget a pulse every 55 ms. or about 18pulses per second? > > I know a lot of it will depend on what speed I am running the chip at. > > -------------------------------------------------------------------------------- >----- Original Message ----- From: "Dave Tweed" To: Sent: Wednesday, November 30, 2005 8:29 PM Subject: Re: [PIC] newbie question on timing > Russell McMahon wrote: >> I wrote: >> > > 55ms (or 55.55555ms to be more exact) is about 18 pulses per >> > > second. >> >> > ... 4.772727 MHz. >> >> which was chosen because that's the NTSc color burst frequency and the >> crystals are cheap. > > Nope. The NTSC color-burst frequency is 3.579545 MHz, which is > 14.31818 MHz divided by four, not three. > > The PC used the 14.3 MHz crystal partly because it was cheaply available > as a result of its wide use in the TV industry, and partly because the > early video cards wanted to be compatible with TV monitors. The early > 8088s could run at up to 5 MHz, and they figured 4.77 MHz was "close > enough" without requiring a separate oscillator. > >> > If you want 55.55555 ms, that's *exactly* 18 pulses/second. >> >> In engineering at least, 'about' includes 'exactly' :-) > > Yes, but I was emphasizing the point that this rate is not the PC's rate > of 18.2 Hz. I wasn't sure if that's what the OP was looking for or if he > cared about the difference. > > -- Dave Tweed > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist