R
XC4000E and XC4000X Series Field Programmable Gate Arrays
IF UNCONNECTED,
DEFAULT IS CCLK
DATA
RIP
READ_DATA
CLK
MD1
READBACK
OBUF
READ_TRIGGER
TRIG
MD0
X1786
IBUF
Figure 49: Readback Schematic Example
Readback Options
I/O
I/O
Readback options are: Read Capture, Read Abort, and
Clock Select. They are set with the bitstream generation
software.
PROGRAMMABLE
INTERCONNECT
Read Capture
When the Read Capture option is selected, the readback
data stream includes sampled values of CLB and IOB sig-
nals. The rising edge of RDBK.TRIG latches the inverted
values of the four CLB outputs, the IOB output flip-flops and
the input signals I1 and I2. Note that while the bits describ-
ing configuration (interconnect, function generators, and
RAM content) are not inverted, the CLB and IOB output sig-
nals are inverted.
rdbk
I/O
I/O
I/O
rdclk
X1787
Figure 50: READBACK Symbol in Graphical Editor
Violating the Maximum High and Low Time
Specification for the Readback Clock
When the Read Capture option is not selected, the values
of the capture bits reflect the configuration data originally
written to those memory locations.
The readback clock has a maximum High and Low time
specification. In some cases, this specification cannot be
met. For example, if a processor is controlling readback, an
interrupt may force it to stop in the middle of a readback.
This necessitates stopping the clock, and thus violating the
specification.
If the RAM capability of the CLBs is used, RAM data are
available in readback, since they directly overwrite the F
and G function-table configuration of the CLB.
The specification is mandatory only on clocking data at the
end of a frame prior to the next start bit. The transfer mech-
anism will load the data to a shift register during the last six
clock cycles of the frame, prior to the start bit of the follow-
ing frame. This loading process is dynamic, and is the
source of the maximum High and Low time requirements.
RDBK.TRIG is located in the lower-left corner of the device,
as shown in Figure 50.
Read Abort
When the Read Abort option is selected, a High-to-Low
transition on RDBK.TRIG terminates the readback opera-
tion and prepares the logic to accept another trigger.
Therefore, the specification only applies to the six clock
cycles prior to and including any start bit, including the
clocks before the first start bit in the readback data stream.
At other times, the frame data is already in the register and
the register is not dynamic. Thus, it can be shifted out just
like a regular shift register.
After an aborted readback, additional clocks (up to one
readback clock per configuration frame) may be required to
re-initialize the control logic. The status of readback is indi-
cated by the output control net RDBK.RIP. RDBK.RIP is
High whenever a readback is in progress.
The user must precisely calculate the location of the read-
back data relative to the frame. The system must keep track
of the position within a data frame, and disable interrupts
before frame boundaries. Frame lengths and data formats
are listed in Table 19, Table 20 and Table 21.
Clock Select
CCLK is the default clock. However, the user can insert
another clock on RDBK.CLK. Readback control and data
are clocked on rising edges of RDBK.CLK. If readback
must be inhibited for security reasons, the readback control
nets are simply not connected.
Readback with the XChecker Cable
The XChecker Universal Download/Readback Cable and
Logic Probe uses the readback feature for bitstream verifi-
cation. It can also display selected internal signals on the
PC or workstation screen, functioning as a low-cost in-cir-
cuit emulator.
RDBK.CLK is located in the lower right chip corner, as
shown in Figure 50.
6-56
May 14, 1999 (Version 1.6)
XILINX [ XILINX, INC ]
