Since long time, I failed reading back data from the Spartan-6 configuration registers (STAT, COR1, COR2), even though the sequence is described in the Configuration Register Read Procedure (JTAG) section [UG380] (p.117, v2.5).
This part contains several incoherencies which led me not to follow each step blindly. Actually, I discovered today, that it is very important not to go back to RTI state in the JTAG TAP state machine during the sequence (Table 6-5), otherwise the data output is always zero.
Ready IDCODE(JTAG): 0x24001093 IDCODE(BITS): 0x24001093 CTL (BITS): 0x0081 STAT (BITS): 0x373C COR1 (BITS): 0x3D08 COR2 (BITS): 0x09EE BOOTSTS(BITS): 0x0001
Various status and configuration registers can now be read... and the internal FPGA configuration as well!
Extracts of the Xilinx documentation for later references
| [UG380] | Spartan-6 FPGA Configuration http://www.xilinx.com/support/documentation/user_guides/ug380.pdf |