On Fri, Oct 1, 2010 at 1:21 PM, Drasko DRASKOVIC <[email protected]> wrote: > Hi all, > in the comment to the jtag_add_dr_scan() there is noted : > > /** > * Generate a DR SCAN using the fields passed to the function. > * For connected TAPs, the function checks in_fields and uses fields > * specified there. For bypassed TAPs, the function generates a dummy > * 1-bit field. The bypass status of TAPs is set by jtag_add_ir_scan(). > */ > void jtag_add_dr_scan(struct jtag_tap* tap, int num_fields, > const struct scan_field* fields, tap_state_t endstate); > > > Why is this dummy bit needed ?
That's how the JTAG standard works. When a TAP is in bypass it still takes up one clock, even if the data in/out is ignored/dummy, if you will. > When we shift out bits on TDO they will pass through the BYPASSed TAPs > and they wil all end up in our wanted TAP (which can be in the middle > of the scan chain), right ? It's a Bad Idea to use short-circuits, rather then standard way to send data on is to clock them through, so that's how they defined it in the JTAG standard... -- Øyvind Harboe US toll free 1-866-980-3434 / International +47 51 63 25 00 http://www.zylin.com/zy1000.html ARM7 ARM9 ARM11 XScale Cortex JTAG debugger and flash programmer _______________________________________________ Openocd-development mailing list [email protected] https://lists.berlios.de/mailman/listinfo/openocd-development
