SR4RLE, SR8RLE, SR16RLE

4-, 8-, 16-Bit Loadable Serial/Parallel-In Parallel-Out Shift Registers with Clock Enable and Synchronous Reset

SR4RLE, SR8RLE, and SR16RLE are 4-, 8-, and 16-bit shift registers, respectively, with shift-left serial input (SLI), parallel inputs (D), parallel outputs (Q), and three control inputs: clock enable (CE), load enable (L), and synchronous reset (R). The register ignores clock transitions when L and CE are Low. The synchronous R, when High, overrides all other inputs during the Low-to-High clock (C) transition and resets the data outputs (Q) Low. When L is High and R is Low, data on the D inputs is loaded into the corresponding Q bits of the register. When CE is High and L and R are Low, data on the SLI input is loaded into the first bit of the shift register during the Low-to-High clock (C) transition and appears on the Q0 output. During subsequent clock transitions, when CE is High and L and R are Low, the data is shifted to the next highest bit position as new data is loaded into Q0 (SLIQ0, Q0Q1, Q1Q2, and so forth).

Registers can be cascaded by connecting the last Q output (Q3 for SR4RLE, Q7 for SR8RLE, or 15 for SR16RLE) of one stage to the SLI input of the next stage and connecting clock, CE, L, and R inputs in parallel.

The register is asynchronously cleared, outputs Low, when power is applied. For CPLDs, the power-on condition can be simulated by applying a High-level pulse on the PRLD global net. FPGAs simulate power-on when global reset (GR) or global set/reset (GSR) is active. GR for XC3000 is active-Low. GR for XC5200 and GSR (XC4000, Spartans, Virtex) default to active-High but can be inverted by adding an inverter in front of the GR/GSR input of the STARTUP, STARTUP_SPARTAN2, or STARTUP_VIRTEX symbol.

Inputs						Outputs
R	L	CE	SLI	Dz - D0	C	Q0	Qz - Q1
1	X	X	X	X		0	0
0	1	X	X	Dz - D0		d0	dn
0	0	1	SLI	X		SLI	qn-1
0	0	0	X	X	X	No Chg	No Chg
z = 3 for SR4RLE; z = 7 for SR8RLE; z = 15 for SR16RLE dn = state of referenced input one setup time prior to active clock transition qn-1 = state of referenced output one setup time prior to active clock transition

Figure 10.9 SR8RLE Implementation XC3000, XC4000E, XC4000X, XC5200, XC9000, Spartan, SpartanXL, Spartan2, Virtex