libftdi Archives

Subject: RE: MPSSE baudrate

From: Sivaram Gowkanapalli <sivaramreddy@xxxxxxxxxxx>
To: <libftdi@xxxxxxxxxxxxxxxxxxxxxxx>
Date: Mon, 4 Jan 2010 10:14:22 -0500

Hello Gerd,

I am sorry for giving the impression that FT4232H is an SPI slave. I am using it as an SPI master.

I just want to sample a 1.5 MHz signal at either 3Mhz, 6MHz or 12 MHz.

This is the configuration I have:

FT4232H -> Clock signal -> Flip flop clock
FT4232H <- DI signal <- Flip flop Data output
Flip flop Data Input <- 1.5 MHz signal
FT4232H -> DO signal -> 1Meg resistor -> GND
FT4232H -> CS signal -> 1Meg resistor -> GND

Can I assume that if I set the baudrate to 0.75 MHz, I will be sampling the signal at 3 MHz in MPSSE SPI mode?

I read about async bitbang mode but I surmised that it cannot work at those speeds? Can you please confirm from your experiences?

Any thoughts, please?


> From: gerd.von.egidy@xxxxxxxxxxxxx
> To: libftdi@xxxxxxxxxxxxxxxxxxxxxxx
> Subject: Re: MPSSE baudrate
> Date: Mon, 4 Jan 2010 11:15:19 +0100
> CC: sivaramreddy@xxxxxxxxxxx
> 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,
> Gerd
> --
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