Re: Toggling GPIO at 38Khz

[Eric Miao]

On Wed, Oct 8, 2008 at 11:43 AM, Jon Smirl <jonsmirl@xxxxxxxxx> 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@xxxxxxxxx
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-- 
Cheers
- eric
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