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Lucent Technologies Inc.
Data Sheet
June 1999
ORCA Series 2 FPGAs
Special Function Blocks
Special function blocks in the Series 2 provide extra 
capabilities beyond general FPGA operation. These 
blocks reside in the corners of the FPGA array.
Single Function Blocks
Most of the special function blocks perform a specific 
dedicated function. These functions are data/configura-
tion readback control, global 3-state control (TS_ALL), 
internal oscillator generation, global set/reset (GSRN), 
and start-up logic.
Readback Logic
The readback logic is located in the upper right corner 
of the FPGA. 
Readback is used to read back the configuration data 
and, optionally, the state of the PFU outputs. A read-
back operation can be done while the FPGA is in nor-
mal system operation. The readback operation cannot 
be daisy-chained. To use readback, the user selects 
options in the bit stream generator in the ORCA
Foundry Development System.
Table 11 provides readback options selected in the bit 
stream generator tool. The table provides the number 
of times that the configuration data can be read back. 
This is intended primarily to give the user control over 
the security of the FPGA’s configuration program. The 
user can prohibit readback (0), allow a single readback 
(1), or allow unrestricted readback (U).
Table 11. Readback Options
The pins used for readback are readback data 
(RD_DATA), read configuration (
RD_CFG
), and configu-
ration clock (CCLK). A readback operation is initiated 
by a high-to-low transition on 
RD_CFG
. The 
RD_CFG
input must remain low during the readback operation. 
The readback operation can be restarted at frame 0 by 
driving the 
RD_CFG
 pin high, applying at least two ris-
ing edges of CCLK, and then driving 
RD_CFG
 low 
again. One bit of data is shifted out on RD_DATA at the 
rising edge of CCLK. The first start bit of the readback 
frame is transmitted out several cycles after the first ris-
ing edge of CCLK after 
RD_CFG
 is input low (see Table 
48, Readback Timing Characteristics in the Timing 
Characteristics section).
It should be noted that the RD_DATA output pin is also 
used as the dedicated boundary-scan output pin, TDO. 
If this pin is being used as TDO, the RD_DATA output 
from readback can be routed internally to any other pin 
desired. The 
RD_CFG
 input pin is also used to control 
the global 3-state (TS_ALL) function. Before and during 
configuration, the TS_ALL signal is always driven by 
the 
RD_CFG
 input and readback is disabled. After con-
figuration, the selection as to whether this input drives 
the readback or global 3-state function is determined 
by a set of bit stream options. If used as the 
RD_CFG
input for readback, the internal TS_ALL input can be 
routed internally to be driven by any input pin.
The readback frame contains the configuration data 
and the state of the internal logic. During readback, the 
value of all five PFU outputs can be captured. The fol-
lowing options are allowed when doing a capture of the 
PFU outputs.
1. Do not capture data (the data written to the capture 
RAMs, usually 0, will be read back).
2. Capture data upon entering readback.
3. Capture data based upon a configurable signal 
internal to the FPGA. If this signal is tied to 
logic 0, capture RAMs are written continuously.
4. Capture data on either options 2 or 3 above.
The readback frame has a similar, but not identical, for-
mat to the configuration frame. This eases a bitwise 
comparison between the configuration and readback 
data. The readback data is not inverted. Every data 
frame has one low start bit and one high stop bit. The 
preamble, including the length count field, is not part of 
the readback frame. The readback frame contains 
states in locations not used in the configuration. These 
locations need to be masked out when comparing the 
configuration and readback frames. The development 
system optionally provides a readback bit stream to 
compare to readback from the FPGA. Also note that if 
any of the LUTs are used as RAM and new data is writ-
ten to them, these bits will not have the same values as 
the original configuration data frame either.
Option
Function
0
1
U
Prohibit Readback
Allow One Readback Only
Allow Unrestricted Number of Readbacks