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Subject: Re: MPSSE baudrate

From: "Gerd v. Egidy" <gerd.von.egidy@xxxxxxxxxxxxx>
To: libftdi@xxxxxxxxxxxxxxxxxxxxxxx
Cc: Sivaram Gowkanapalli <sivaramreddy@xxxxxxxxxxx>
Date: Mon, 4 Jan 2010 11:15:19 +0100
Hi Siva,

> I am a beginner to using FTDI chips and want to use FT4232H for sampling a
>  1.5 MHz line. I am hoping that if I could run MPSSE SPI at a baud rate of
>  > 3Mhz or 6 Mhz, will give me a reliable sample.

Do you want to read data from an SPI communication or do you want to sample 
the data at a defined frequency without the FT4232H "understanding" the SPI - 
like a logic analyzer?

When you want the FT4232H to act as SPI slave and read data, the best way is 
to use the MPSSE. I haven't done this myself and currently we lack a good 
example in libftdi for that. But there is code out there which does it.

When you plan to use the FT4232H like a logic analyzer and sample the data at 
a defined baudrate, then the bitbang mode is what you want.
You need to enable the bitbang (e.g. async bitbang), set the baudrate you want 
and start to read continously.

Currently there is some uncertainty about how the bitbang baudrates really 
work. The datasheet says you need to set 1/16 the baudrate you want for 
bitbang - but we observed that it just gives you 1/4 of the speed you wanted. 
So the code currently multiplies the speed by 4.

Kind regards,


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