User guide

116 www.xilinx.com Spartan-6 FPGA Configuration User Guide
UG380 (v2.7) October 29, 2014
Chapter 6: Readback and Configuration Verification
Readback Command Sequences
Spartan-6 FPGA configuration memory is read from the FDRO (Frame Data Register -
Output) configuration register and can be accessed from the JTAG, SelectMAP, and ICAP
interfaces. For the JTAG and SelectMAP interfaces, readback is possible while the FPGA
design is active or in a shutdown state, although block RAMs cannot be accessed by the
user design while they are being accessed by the configuration logic.
Accessing Configuration Registers through the SelectMAP Interface
To read configuration memory through the SelectMAP interface, users must set the
interface for write control to send commands to the FPGA, and then switch the interface to
read control to read data from the device. Write and read control for the SelectMAP
interface is determined by the RDWR_B input: the SelectMAP data pins are inputs when
the interface is set for Write control (RDWR_B = 0); they are outputs when the interface is
set for Read control (RDWR_B = 1).
The CSI_B signal must be deasserted (CSI_B =1) before toggling the RDWR_B signal,
otherwise the user causes an abort (refer to SelectMAP ABORT, page 153 for details).
The procedure for changing the SelectMAP interface from Write to Read Control, or vice
versa, is:
1. Deassert CSI_B.
2. Toggle RDWR_B.
RDWR_B = 0: Write control
RDWR_B = 1: Read control
3. Assert CSI_B.
4. CSI_B is synchronous to CCLK.
5. This procedure is illustrated in Figure 6-1.
Configuration Register Read Procedure (SelectMAP)
The simplest read operation targets a configuration register such as the COR0 or STAT
register. Any configuration register with read access can be read through the SelectMAP
interface, although not all registers offer read access. The procedure for reading the STAT
register through the SelectMAP interface follows:
1. Write a dummy word and a synchronization word to the device followed by at least
one no operation command (NOOP).
X-Ref Target - Figure 6-1
Figure 6-1: Changing the SelectMAP Port from Write to Read Control
CSI_B
RDWR_B
DATA[0:7]
UG380_c6_01_042909
WRITE
Byte n Byte nByte 0
CCLK
Byte 0
READ