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FT2232 comment tweaks

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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>
This commit is contained in:
David Brownell 2010-03-19 10:31:44 -07:00
parent 7373d1c342
commit 3b310dbac5
1 changed files with 18 additions and 6 deletions
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24
src/jtag/drivers/ft2232.c
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@ -32,12 +32,24 @@
* JTAG adapters based on the FT2232 full and high speed USB parts are
* popular low cost JTAG debug solutions. Many FT2232 based JTAG adapters
* are discrete, but development boards may integrate them as alternatives
* to more capable (and expensive) third party JTAG pods. Since JTAG uses
* only one of the two ports on these devices, on integrated boards the
* second port often serves as a USB-to-serial adapter for the target's
* console UART even when the JTAG port is not in use. (Systems which
* support ARM's SWD in addition to JTAG, or instead of it, may use that
* second port for reading SWV trace data.)
* to more capable (and expensive) third party JTAG pods.
*
* JTAG uses only one of the two communications channels ("MPSSE engines")
* on these devices. Adapters based on FT4232 parts have four ports/channels
* (A/B/C/D), instead of just two (A/B).
*
* Especially on development boards integrating one of these chips (as
* opposed to discrete pods/dongles), the additional channels can be used
* for a variety of purposes, but OpenOCD only uses one channel at a time.
*
* - As a USB-to-serial adapter for the target's console UART ...
* which may be able to support ROM boot loaders that load initial
* firmware images to flash (or SRAM).
*
* - On systems which support ARM's SWD in addition to JTAG, or instead
* of it, that second port can be used for reading SWV/SWO trace data.
*
* - Additional JTAG links, e.g. to a CPLD or * FPGA.
*
* FT2232 based JTAG adapters are "dumb" not "smart", because most JTAG
* request/response interactions involve round trips over the USB link.