Carry Output for n-Bit Cascading

Buffer-Type Outputs Drive Bus Lines

Directly

Choice of Asynchronous or Synchronous

Clearing and Loading

Internal Look-Ahead Circuitry for Fast

Cascading

Package Options Include Plastic

Small-Outline (DW) Packages, Ceramic

Chip Carriers (FK), and Standard Plastic (N)

and Ceramic (J) 300-mil DIPs

description

These binary counters are programmable and

offer synchronous and asynchronous clearing as

well as synchronous and asynchronous loading.

All synchronous functions are executed on the

positive-going edge of the clock.

The clear function is initiated by applying a low

level to either asynchronous clear (ACLR) or

synchronous clear (SCLR). ACLR (direct clear)

overrides all other functions of the device, while

SCLR overrides only the other synchronous

functions. Data is loaded from the A, B, C, and D

inputs by applying a low level to asynchronous

load (ALOAD) or by the combination of a low level

at synchronous load (SLOAD) and a

positive-going clock transition. The counting

function is enabled only when enable P (ENP),

enable T (ENT), ACLR, ALOAD, SCLR, and

SLOAD are all high.

A high level at the output-enable (OE) input forces the Q outputs into the high-impedance state, and a low level

enables those outputs. Counting is independent of OE. ENT is fed forward to enable the ripple-carry output

(RCO) to produce a high-level pulse while the count is maximum (15). The clocked carry output (CCO) produces

a high-level pulse for a duration equal to that of the low level of the clock when RCO is high and the counter is

enabled (ENP and ENT are high); otherwise, CCO is low. CCO does not have the glitches commonly associated

with a ripple-carry output. Cascading is normally accomplished by connecting RCO or CCO of the first counter

to ENT of the next counter. However, for very high-speed counting, RCO should be used for cascading because

CCO does not become active until the clock returns to the low level.

The SN54ALS561A is characterized for operation over the full military temperature range of –55°C to 125°C.

The SN74ALS561A is characterized for operation from 0°C to 70°C.