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-rw-r--r--src/jtag/drivers/ft2232.c24
1 files changed, 18 insertions, 6 deletions
diff --git a/src/jtag/drivers/ft2232.c b/src/jtag/drivers/ft2232.c
index b45e8a4d..8e835c59 100644
--- a/src/jtag/drivers/ft2232.c
+++ b/src/jtag/drivers/ft2232.c
@@ -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.