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CJ4RE, CJ5RE, CJ8RE

4-, 5-, 8-Bit Johnson Counters with Clock Enable and Synchronous Reset

XC3000
XC4000E
XC4000X
XC5200
XC9000
Spartan
SpartanXL
Virtex
Macro
Macro
Macro
Macro
Macro
Macro
Macro
Macro

figures/x4113n.gif

figures/x4115n.gif

figures/x4119n.gif

CJ4RE, CJ5RE, and CJ8RE are resettable Johnson/shift counters. The synchronous reset (R) input, when High, overrides all other inputs and causes the data (Q) outputs to go to logic level zero during the Low-to-High clock (C) transition. The counter increments (shifts Q0 to Q1, Q1 to Q2, and so forth) when the clock enable input (CE) is High during the Low-to-High clock transition. Clock transitions are ignored when CE is Low.

For CJ4RE, the Q3 output is inverted and fed back to input Q0 to provide continuous counting operation. For CJ5RE, the Q4 output is inverted and fed back to input Q0. For CJ8RE, the Q7 output is inverted and fed back to input Q0.

The counter is asynchronously cleared, output 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 or STARTUP_VIRTEX symbol.

Inputs
Outputs
R
CE
C
Q0
Q1
Q2
Q3
1
X

0
0
0
0
0
0
X
No Chg
No Chg
No Chg
No Chg
0
1

!q3
q0
q1
q2
q = state of referenced output one setup time prior to active clock transition

CJ4RE Truth Table

Inputs
Outputs
R
CE
C
Q0
Q1
Q2
Q3
Q4
1
X

0
0
0
0
0
0
0
X
No Chg
No Chg
No Chg
No Chg
No Chg
0
1

!q4
q0
q1
q2
q3
q = state of referenced output one setup time prior to active clock transition

CJ5RE Truth Table

Inputs
Outputs
R
CE
C
Q0
Q1 - Q7
1
X

0
0
0
0
X
No Chg
No Chg
0
1

!q7
q0 - q6
q = state of referenced output one setup time prior to active clock transition

CJ8RE Truth Table

Figure 4.40 CJ8RE Implementation XC3000, XC4000, XC5200, XC9000, Spartans, Virtex

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