SN74HC191

· Single Down/Up Count-Control Line
· Look-Ahead Circuitry Enhances Speed of Cascaded Counters
· Fully Synchronous in Count Modes
· Asynchronously Presettable With Load Control
· Package Options Include Plastic Small-Outline (D) and Ceramic Flat (W) Packages, Ceramic Chip Carriers (FK), and Standard Plastic (N) and Ceramic (J) 300-mil DIPs

Supply voltage range, V_CC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . . 0.5 V to 7 V
Input clamp current, I_IK (V_I < 0 or V_I > V_CC) (see Note 1) . . . . . . . .  . . . . . . . . . . . . . . . ±20 mA
Output clamp current, I_OK (V_O < 0 or V_O > V_CC) (see Note 1) . . . . . . . . . . . .  . . . . . . . . ±20 mA
Continuous output current, I_O (V_O = 0 to V_CC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±25 mA
Continuous current through V_CC or GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±50 mA
Package thermal impedance, _JA (see Note 2): D package . . . . . . . . . . . . . . . . . . . . . . 113°C/W
                                                                          N package . . . . . . . . . . . . . .  . . . . . . . . . 78°C/W
Storage temperature range, T_stg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65°C to 150°C

† Stresses beyond those listed under "absolute maximum ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under "recommended operating conditions" is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
NOTES: 1. The input and output voltage ratings may be exceeded if the input and output current ratings are observed.
2. The package thermal impedance is calculated in accordance with JESD 51, except for through-hole packages, which use a trace length of zero.

The 'HC191 are 4-bit synchronous, reversible, up/down binary counters. Synchronous counting operation is provided by having all flip-flops clocked simultaneously so that the outputs change coincident with each other when instructed by the steering logic. This mode of operation eliminates the output counting spikes normally associated with asynchronous (rippleclock) counters.

The SN74HC191 outputs of the four flip-flops are triggered on a low-to-high-level transition of the clock (CLK) input if the count-enable (CTEN) input is low. A high at CTEN inhibits counting. The direction of the count is determined by the level of the down/up (D/U) input. When D/U is low, the counter counts up, and when D/U is high, it counts down.

These SN74HC191 counters feature a fully independent clock circuit. Change at the control (CTEN and D/U) inputs that modifies the operating mode have no effect on the contents of the counter until clocking occurs. The function of the counter is dictated solely by the condition meeting the stable setup and hold times.

These SN74HC191 counters are fully programmable; that is, each of the outputs can be preset to either level by placing a low on the load (LOAD) input and entering the desired data at the data inputs. The output changes to agree with the data inputs independently of the level of CLK. This feature allows the counters to be used as modulo-N dividers by simply modifying the count length with the preset inputs.

Two outputs are available to perform the cascading function: ripple clock (RCO) and maximum/minimum (MAX/MIN) count. MAX/MIN produces a high-level output pulse with a duration approximately equal to one complete cycle of the clock while the count is zero (all outputs low) counting down, or maximum (9 or 15) counting up. RCO produces a low-level output pulse under those same conditions, but only while CLK is low. The counters can be easily cascaded by feeding RCO to CTEN of the succeeding counter if parallel clocking is used, or to CLK if parallel enabling is used. MAX/MIN SN74HC191 can be used to accomplish look ahead for high-speed operation.

The SN54HC191 is characterized for operation over the full military temperature range of 55°C to 125°C. The SN74HC191 is characterized for operation from 40°C to 85°C.