X74_168

4-Bit BCD Bidirectional Counter with Parallel and Trickle Clock Enables and Active-Low Load Enable

X74_168 is a 4-stage, 4-bit, synchronous, loadable, cascadable, bidirectional binary-coded-decimal (BCD) counter. The data on the D - A inputs is loaded into the counter when the active-Low load enable (LOAD) is Low during the Low-to-High clock (CK) transition. The LOAD input, when Low, has priority over parallel clock enable (ENP), trickle clock enable (ENT), and the bidirectional (U_D) control. The outputs (QD - QA) increment when U_D and LOAD are High and ENP and ENT are Low during the Low-to-High clock transition. The outputs decrement when LOAD is High and ENP, ENT, and U_D are Low during the Low-to-High clock transition. The counter ignores clock transitions when LOAD and either ENP or ENT are High.

Inputs						Outputs
LOAD	ENP	ENT	U_D	A - D	CK	QA - QD	RCO
0	X	X	X	A - D		qa - qd	RCO
1	0	0	1	X		Inc	RCO
1	0	0	0	X		Dec	RCO
1	1	0	X	X	X	No Chg	RCO
1	X	1	X	X	X	No Chg	1
RCO = (Q3•!Q2•!Q1•Q0•U_D•!ENT) + (!Q3•!Q2•!Q1•!Q0•!U_D•!ENT) qa - qd = state of referenced input one setup time prior to active clock transition

The active-Low ripple carry-out output (RCO) is Low when QD, QA, and U_D are High and QC, QB, and ENT are Low. RCO is also Low when all outputs, ENT and U_D are Low. The following figure illustrates a carry-lookahead design.

Figure 11.27 Carry-Lookahead Design

The RCO output of the first stage of the ripple carry is connected to the ENP input of the second stage and all subsequent stages. The RCO output of second stage and all subsequent stages is connected to the ENT input of the next stage. The ENT of the second stage is always enabled/tied to VCC. CE is always connected to the ENT input of the first stage. This cascading method allows the first stage of the ripple carry to be built as a prescaler. In other words, the first stage is built to count very fast.

Figure 11.28 X74_168 Implementation XC3000, XC4000, XC5200, Spartans

Figure 11.29 X74_168 Implementation XC9000