Ca y est, j'ai testé toutes les permutations, et toujours :

Error:   jtag.c:1290 jtag_examine_chain(): JTAG communication failure, check con
nection, JTAG interface, target power etc.
Error:   jtag.c:1486 jtag_init(): trying to validate configured JTAG chain anywa
y...
Error:   jtag.c:1383 jtag_validate_chain(): Error validating JTAG scan chain, IR
 mismatch, scan returned 0x00
Error:   jtag.c:1383 jtag_validate_chain(): Error validating JTAG scan chain, IR
 mismatch, scan returned 0x00
Error:   jtag.c:1383 jtag_validate_chain(): Error validating JTAG scan chain, IR
 mismatch, scan returned 0x00
Error:   jtag.c:1383 jtag_validate_chain(): Error validating JTAG scan chain, IR
 mismatch, scan returned 0x00
Error:   jtag.c:1383 jtag_validate_chain(): Error validating JTAG scan chain, IR
 mismatch, scan returned 0x00
Error:   jtag.c:1383 jtag_validate_chain(): Error validating JTAG scan chain, IR
 mismatch, scan returned 0x00
Error:   jtag.c:1494 jtag_init(): Could not validate JTAG chain, exit