linux@xxxxxxxxxxxxxxxxx wrote:
> Hi
>
> On 20/01/16 03:06, Jim Paris wrote:
> >linux@xxxxxxxxxxxxxxxxx wrote:
> >>Hi
> >>
> >>I am trying to get lirc working with my Adafruit FT232H device and
> >>having some problems with getting the clock speed (baud) correct.
> >>
> >>I'm trying to transmit a carrier frequency of 38KHz which is turned
> >>on and off based on the IR pattern to transmit.
> >>
> >>I've created a separate program from LIRC to investigate this which
> >>simply opens the device, sets bit bang mode, sets the baud rate, and
> >>writes a block of 4k alternating 0xff / 0x00.
> >>
> >>I'm driving an IR LED and I have a receiver which will print out the
> >>duration it detects the 38KHz carrier for if the frequency is close.
> >>
> >>I'm using libftdi-HEAD-c4c9f0a from a few days ago.
> >>
> >>If I provide a baud rate of 3800 to ftdi_set_baudrate then that for
> >>some reason gives me the required 72K bits / second transmission
> >>rate. This is completely at odds with any docs I can find or the
> >>LIRC code.
> >I'm not sure I exactly follow your numbers here.
> >
> >For a 38KHz carrier, wouldn't you want 76Kbits/second transmission
> >rate? I assume 72Kbits was a typo?
> Yes it was a typo, I seem to have a mental block about typing 76 for
> some reason.
> >You're saying that you need to provide a baudrate of 3800, which means
> >that it's apparently getting scaled up by (76000 / 3800) = 20
> >with the libftdi version.
> >
> >>As an experiment I tried building the same program against the FTDI
> >>D2XX libraries which the docs say multiplies the rate up by 16. But
> >>to get that to give me the correct baud rate, I have to pass 15200
> >>as the argument to FT_SetBaudRate.
> >By correct baud rate, you mean it's getting scaled up by (76000 /
> >15200) = 5 in the D2XX version? So you're not seeing the baudrate get
> >multiplied by 16 in either situation?
> Yes except that the FTDI D2XX example code and docs say that in bit
> bang mode it gets scaled up by 16 not 5.
> >>USB sniffing shows that ultimately both of these calls result in the
> >>same value being passed down to the chip.
> >What is the value?
> It is 0x4315 which according to the FTDI docs is a sub integer
> divisor of 0.5 and a divisor of 0x315.
>
> If I request 9600 baud, the values which appear here match the
> values in the FTDI docs for serial port baud rates.
> >>Can anyone help me understand why I have such a seemingly bizarre
> >>bit rate being used by this device?
> >>I had some eeprom issues initially with the device; is it possible
> >>that the eeprom contents can affect the baud rate like this?
> >>Or have I bought a fake FTDI device which doesn't operate as the
> >>genuine ones do?
> >Not sure, but it sounds like you have a factor of 4 discrepency
> >between D2XX (scaling by 5?) and libftdi (scaling by 20?). I also
> >saw this mystery factor of 4, with a FT230X in bitbang mode. For 38
> >KHz IR, my code uses:
> >
> >#define CARRIER 38400
> >
> > ftdi_set_bitmode(&ftdic, 1, BITMODE_BITBANG);
> > ftdi_set_baudrate(&ftdic, (CARRIER * 2 / 16) / 4);
> >
> >The / 4 was just something I determined experimentally and never dug
> >into.
> The libftdi code multiplies the baud you request by 4 if bitbang is
> enabled. Not quite sure how this relates to the *16 that the FTDI
> code but it does at least explain why the libftdi baud parameter is
> 1/4 of the equivalent D2XX baud parameter.
OK. So this doesn't seem like a libftdi issue then. Let's assume
we're talking about D2XX which doesn't multiply baudrates by 4. Is
this correct:
- If you request 9600 baud, it sends 0x4138 to the device which means
a divisor of 312.5, and the actual bitbang baudrate is 9600 * 16 = 153600
- If you request 15200 baud, it sends 0x4315 to the device which means
a divisor of 789.5, *but* the actual bitbang bitrate is 15200 * 5 = 76000
If that's true then yes, this does seem like it could be a hardware
issue like a fake chip...
Jim
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