Messages in this thread | | | Date | Wed, 8 Oct 2008 14:40:32 +0800 | From | "Eric Miao" <> | Subject | Re: Toggling GPIO at 38Khz |
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On Wed, Oct 8, 2008 at 11:43 AM, Jon Smirl <jonsmirl@gmail.com> wrote: > I'm working on getring the LIRC IR subsystem integrated into the > kernel. One common IR hardware implementation is based on standard > serial ports. It uses the serial port's DCD and DTR as GPIO pins. > This common IR hardware does not have a transmit modulator and is > modulating the IR signal in software. Luckily is has hardware > demodulation. > > There are four common IR frequencies - 36Khz, 38Khz, 40Khz, 56Khz. I > need to create these frequencies in software. Data is then transmitted > as 400-600us burst of these frequencies. Around 10-30 clock pulses. > > So how do I reliably generate 38Khz without destroying latency in the > system? I also don't want to measure the speed of the code I'm > executing - same code has to run on 90Mhz ARM and 3Ghz Core2. If a do > a loop with nanosleep() I need to know how fast my code is to subtract > it's execution time form the sleep time. The existing LIRC code works > this way and measures it's timing loop. >
This isn't easy, 38KHz is quite a high frequency, I'd prefer to use an interrupt or fiq based on a accurate clock event source, but that will have impact on the system anyway.
> hrtimers look promising, but hrtimers doesn't have a periodic API. Is > there a technique for creating periodic timing that doesn't accumulate > error caused by rescheduling the timers? If I need to send 30 pulses > of 38Khz, it is more important that the total time be (1/38000)*30 = > 0.7894ms than it is for each pulse to be generated at exactly 26.3us > intervals. >
Make sure your clock source is accurate enough to generate this interval.
> Are there other techniques for generating pulse trains in the 36-56Khz > range on a simple GPIO pin? Anyone have some sample code? >
With a PWM possibly.
> -- > Jon Smirl > jonsmirl@gmail.com > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
-- Cheers - eric
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