Xilinx Virtex-6 Manual page 362

Chapter 4: About Design Elements
SRL16E
Primitive: 16-Bit Shift Register Look-Up Table (LUT) with Clock Enable
Introduction
This design element is a shift register look-up table (LUT). The inputs A3, A2, A1, and A0 select the output
length of the shift register.
The shift register can be of a fixed, static length or it can be dynamically adjusted.
• To create a fixed-length shift register -Drive the A3 through A0 inputs with static values. The length of
the shift register can vary from 1 bit to 16 bits, as determined by the following formula: Length = (8 x A3)
+(4 x A2) + (2 x A1) + A0 +1 If A3, A2, A1, and A0 are all zeros (0000), the shift register is one bit long. If
they are all ones (1111), it is 16 bits long.
• To change the length of the shift register dynamically -Change the values driving the A3 through A0
inputs. For example, if A2, A1, and A0 are all ones (111) and A3 toggles between a one (1) and a zero (0), the
length of the shift register changes from 16 bits to 8 bits. Internally, the length of the shift register is always 16
bits and the input lines A3 through A0 select which of the 16 bits reach the output.
The shift register LUT contents are initialized by assigning a four-digit hexadecimal number to an INIT attribute.
The first, or the left-most, hexadecimal digit is the most significant bit. If an INIT value is not specified, it defaults
to a value of four zeros (0000) so that the shift register LUT is cleared during configuration.
When CE is High, the data (D) is loaded into the first bit of the shift register during the Low-to-High clock (CLK)
transition. During subsequent Low-to-High clock transitions, when CE is High, data shifts to the next highest bit
position as new data is loaded. The data appears on the Q output when the shift register length determined by
the address inputs is reached. When CE is Low, the register ignores clock transitions.
Logic Table
Inputs
Am CE
0
Am
Am 1
m= 0, 1, 2, 3
362
CLK D
X X
↑
D
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Output
Q
Q(Am)
Q(Am - 1)
UG623 (v 14.5) March 20, 2013