ian> When using an FTDI FT2232 based JTAG interface, what limits the JTAG speed? ian> [snip] [with the REV H (usb2.0) make it faster]?
The FTDI has a max output clock rate of 6mhz. However, that does not tell the whole story. (A) GDB talks to openocd via the GDB protocol. (B) GDB's protocol is ping-pong not a streaming window based DEFINED: ping-pong, I send you a packet, you send an ACK or NAK. (C) That ping-pong is layered on top of TCP/IP, which is ping-pong also (with some hacks) (D) OpenOCD must do many things and many state changes over jtag (E) USB is a ping-pong protocol also. (F) Each of these steps require 1 to N (where N is large) ping-pong USB transaction. (G) The JTAG clock must be some division of the CPU clock. Yes, there are 500mhz arms out there - but anything more then 8 to 9mhz does not do well on a 6 inch ribbon cable. The speed issues is all those steps in the middle. The USB2.0 "H" version should speed up step (E) the USB ping pong protocol, nothing more. A dongle and target that supports RTCK speeds up (G) - but only to a point (FT2232 does not support rtck) The real cost is (D) but is helped by using the DCC to download. OpenOCD supports this if you enable it and have a work area. -Duane. _______________________________________________ Openocd-development mailing list Openocd-development@lists.berlios.de https://lists.berlios.de/mailman/listinfo/openocd-development
