FT2232 comment tweaks
Note that the FT4232 chips have four channels not two, and Elaborate on uses of the additional channels. Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
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@ -32,12 +32,24 @@
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* JTAG adapters based on the FT2232 full and high speed USB parts are
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* popular low cost JTAG debug solutions. Many FT2232 based JTAG adapters
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* are discrete, but development boards may integrate them as alternatives
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* to more capable (and expensive) third party JTAG pods. Since JTAG uses
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* only one of the two ports on these devices, on integrated boards the
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* second port often serves as a USB-to-serial adapter for the target's
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* console UART even when the JTAG port is not in use. (Systems which
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* support ARM's SWD in addition to JTAG, or instead of it, may use that
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* second port for reading SWV trace data.)
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* to more capable (and expensive) third party JTAG pods.
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*
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* JTAG uses only one of the two communications channels ("MPSSE engines")
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* on these devices. Adapters based on FT4232 parts have four ports/channels
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* (A/B/C/D), instead of just two (A/B).
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*
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* Especially on development boards integrating one of these chips (as
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* opposed to discrete pods/dongles), the additional channels can be used
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* for a variety of purposes, but OpenOCD only uses one channel at a time.
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*
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* - As a USB-to-serial adapter for the target's console UART ...
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* which may be able to support ROM boot loaders that load initial
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* firmware images to flash (or SRAM).
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*
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* - On systems which support ARM's SWD in addition to JTAG, or instead
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* of it, that second port can be used for reading SWV/SWO trace data.
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*
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* - Additional JTAG links, e.g. to a CPLD or * FPGA.
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*
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* FT2232 based JTAG adapters are "dumb" not "smart", because most JTAG
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* request/response interactions involve round trips over the USB link.
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