
Data Sheet
June 1999
ORCA Series 2 FPGAs
16
Lucent Technologies Inc.
Programmable Logic Cells 
(continued)
The set/reset operation of the latch/FF is controlled by 
two parameters: reset mode and set/reset value. When 
the global set/reset (GSRN) or local set/reset (LSR) are 
inactive, the storage element operates normally as a 
latch or FF. The reset mode is used to select a synchro-
nous or asynchronous LSR operation. If synchronous, 
LSR is enabled only if clock enable (CE) is active. For 
the Series 2 series, a new option called the LSR prior-
ity allows the synchronous LSR to have priority over the 
CE input, thereby setting or resetting the FF indepen-
dent of the state of CE. The clock enable is supported 
on FFs, not latches. The clock enable function is imple-
mented by using a two-input multiplexer on the FF 
input, with one input being the previous state of the FF 
and the other input being the new data applied to the 
FF. The select of this two-input multiplexer is clock 
enable (CE), which selects either the new data or the 
previous state. When CE is inactive, the FF output 
does not change when the clock edge arrives.
The GSRN signal is only asynchronous, and it sets/
resets all latches/FFs in the FPGA based upon the set/
reset configuration bit for each latch/FF. The set/reset 
value determines whether GSRN and LSR are set or 
reset inputs. The set/reset value is independent for 
each latch/FF. 
If the local set/reset is not needed, the latch/FF can be 
configured to have a data front-end select. Two data 
inputs are possible in the front-end select mode, with 
the LSR signal used to select which data input is used. 
The data input into each latch/FF is from the output of 
its associated QLUT F[3:0] or direct from WD[3:0], 
bypassing the LUT. In the front-end data select mode, 
both signals are available to the latches/FFs. 
For PLCs that are in the two outside rows or columns of 
the array, the latch/FFs can have two inputs in addition 
to the F and WD inputs mentioned above. One input is 
from an I/O pad located at the PIC closest to either the 
left or right of the given PLC (if the PLC is in the left two 
columns or right two columns of the array). The other 
input is from an I/O pad located at the closest PIC 
either above or below the given PLC (if the PLC is in 
the top or the bottom two rows). It should be noted that 
both inputs are available for a 2 x 2 array of PLCs in 
each corner of the array. For the entire array of PLCs, if 
either or both of these inputs is unavailable, the latch/
FF data input can be tied to a logic 0 instead (the 
default). 
To speed up the interface between signals external to 
the FPGA and the latches/FFs, there are direct paths 
from latch/FF outputs to the I/O pads. This is done for 
each PLC that is adjacent to a PIC. 
The latches/FFs can be configured in three modes:
1. Local synchronous set/reset: the input into the PFU’s 
LSR port is used to synchronously set or reset each 
latch/FF.
2. Local asynchronous set/reset: the input into LSR 
asynchronously sets or resets each latch/FF.
3. Latch/FF with front-end select: the data select signal 
(actually LSR) selects the input into the latches/FFs 
between the LUT output and direct data in.
For all three modes, each latch/FF can be indepen-
dently programmed as either set or reset. Each latch/
FF in the PFU is independently configured to operate 
as either a latch or flip-flop. Figure 18 provides the logic 
functionality of the front-end select, global set/reset, 
and local set/reset operations.
Note: CD = configuration data.
5-2839(F).a
Figure 18. Latch/FF Set/Reset Configurations
CE
D
S_SET
S_RESET
CLK
SET RESET
Q
LSR
GSRN
CD
CE
D
CLK
SET RESET
LSR
CD
CE
CE
D
CLK
SET RESET
CD
CE
CE
WD
LSR
GSRN
PDINLR
LOGIC 0
WD
F
LOGIC 0
WD
GSRN
Q
Q
PDINTB
F
PDINLR
PDINTB
F
LOGIC 0
WD
PDINLR
PDINTB