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.
thanks
Andy
Jim
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