CD4RE

4-Bit Cascadable BCD Counter with Clock Enable and Synchronous Reset

CD4RE is a 4-bit (stage), synchronous, resettable, cascadable binary-coded-decimal (BCD) counter. The synchronous reset input (R) is the highest priority input. When R is High, all other inputs are ignored; the Q outputs, terminal count (TC), and clock enable out (CEO) go to logic level zero on the Low-to-High clock (C) transition. The Q outputs increment when the clock enable input (CE) is High during the Low-to-High clock transition. The counter ignores clock transitions when CE is Low. The TC output is High when Q3 and Q0 are High and Q2 and Q1 are Low.

The counter recovers from any of six possible illegal states and returns to a normal count sequence within two clock cycles for FPGAs, as shown in the following state diagram. For XC9000, the counter resets to zero or recovers within the first clock cycle.

Larger counters are created by connecting the count enable out (CEO) output of the first stage to the CE input of the next stage and connecting the R and clock inputs in parallel. CEO is active (High) when TC and CE are High. The maximum length of the counter is determined by the accumulated CE-to-TC propagation delays versus the clock period. The clock period must be greater than n(t_CE-TC), where n is the number of stages and the time t_CE-TC is the CE-to-TC propagation delay of each stage. When cascading counters, use the CEO output if the counter uses the CE input; use the TC output if it does not.

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, STARTUP_SPARTAN2, or STARTUP_VIRTEX symbol.

Inputs			Outputs
R	CE	C	Q3	Q2	Q1	Q0	TC	CEO
1	X		0	0	0	0	0	0
0	1		Inc	Inc	Inc	Inc	TC	CEO
0	0	X	No Chg	No Chg	No Chg	No Chg	TC	0
0	1	X	1	0	0	1	1	1
TC = Q3•!Q2•!Q1•Q0 CEO = TC•CE

Figure 4.35 CD4RE Implementation XC3000, XC4000E, XC4000X, XC5200, Spartan, SpartanXL, Spartan2, Virtex

Figure 4.36 CD4RE Implementation XC9000