Re: Ftdi FT230X based receiver gives incorrect timings for received IR pulses.

Hi!

Thanks for a really qualified message, and sorry for not providing any 
answer until now. I wanted to see if anyone actually using these chips 
would show up first.

On 15/03/17 14:48, John Penfold wrote:
> Hi All,
>
> I have built a USB infra-red receiver based on ftdi's FT230X chip.
>
> Comparison between the mode2 outputs when using the ftdi receiver and the
> serial port receiver showed that the ftdi receiver pulses were twice the
> width. The signal going into the FT230X was the correct width.
>
> The device was built for receiving only, but I tried sending by connecting the
> transmit output pin to an oscilloscope, the pulses were half the width they
> should have been (with discontinuities in the modulation pulses). I tried it
> with the ftdix driver which gave the correct output.
>
> Looking through the code in ftdi.c i found in parsesamples() the following
> (from line 151)
> 		/* Convert number of samples to us.
> 		 *
> 		 * The datasheet indicates that the sample rate in
> 		 * bitbang mode is 16 times the baud rate but 32 seems
> 		 * to be correct. */
> 		usecs = (rxctr * 1000000LL) / (rx_baud_rate * 32);
> I wanted to verify this, and tried using the libftdi python bindings to write
> a small python program to do it. I found that if you set the baudrate before
> entering bitbang mode, the bitbang clock will be 16 times the baudrate you
> set, if you set the baudrate when in bitbang mode the clock will be 64 times
> the baudrate you set.
> This is caused by libftdi's ftdi_set_baudrate(), when you are in bitbang mode
> it multiplies the baudrate by 4 before setting the chip. I could not find any
> mention of this "feature" in the documentation.:
>
> In lirc the baudrate is set while in bitbang mode, the bitbang clock is
> therefore 64 times the baudrate you set. This suggests that the 32 in
> parsesamples() should be 64, which would halve the number of microseconds of
> the pulse, giving the correct result.
>
> Does the ftdi driver work correctly with any other ftdi chip (e.g. FT232R)? Is
> it only the FT230X that causes problems?

Until someone shows up with some data on this we just don't know.

However, you have verified a patch to apply if the chip actually is 
FT230X. If you could make a patch which tested this in runtime and 
applied the correct divisor for FT230X chips only I would be happy to 
apply this.

Doing so would leave an open issue for non-FT230X chips. We would have 
to live with that until it's tested.

An alternative would be to apply the correct divisor unconditionally, 
but add a driver option which makes it possible for users to reset it to 
the original value.

I would also appreciate if you opened a bug for this - bugs are easier 
to track, and we definitely need to track this.

Again: thanks for valuable input!


Cheers,

--alec

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