74HCT181

· Full carry look-ahead for high-speed arithmetic operation on long words
· Provides 16 arithmetic operations: add, subtract, compare, double, plus 12 others
· Provides all 16 logic operations of two variables: EXCLUSIVE-OR, compare, AND, NAND, NOR, OR plus 10 other logic operations
· Output capability: standard,
                   A=B open drain
· I_CC category: MSI

The 74HCT181 is high-speed Si-gate CMOS devices and are pin compatible with low power Schottky TTL (LSTTL). They are specified in compliance with JEDEC standard no. 7A.

The 74HCT181 is 4-bit high-speed parallel Arithmetic Logic Units (ALU). Controlled by the four function select inputs (S0 to S3) and the mode control input (M), they can perform all the 16 possible logic operations or 16 different arithmetic operations on active HIGH or active LOW operands (see function table).

When the mode control input (M) of the 74HCT181 is HIGH, all internal carries are inhibited and the device3 performs logic operations on the individual bits as listed. When M is LOW, the carries are enabled and the "181" performs arithmetic operations on the two 4-bit words. The "181" incorporates full internal carry look-ahead and provides for either ripple carry between devices using the C_n+4 output, or for carry look-ahead between packages using the carry propagation (P) and carry generate (G) signals. P and G are not affected by carry in.

When speed requirements of the 74HCT181 are not stringent, it can be used in a simple ripple carry mode by connecting the carry output (C_n+4) signal to the carry input (C_n) of the next unit.

For high-speed operation the 74HCT181 is used in conjunction with the "182" carry look-ahead circuit. One carry look-ahead package is required for each group of four "181" devices. Carry look-ahead can be provided at various levels and offers high-speed capability over extremely long word lengths.

The comparator output (A=B) of the 74HCT181 goes HIGH when all four function outputs (F0 to F3) are HIGH and can be used to indicate logic equivalence over 4 bits when the unit is in the subtract mode. A=B is an open collector output and can be wired-AND with other A=B outputs to give a comparison for more than 4 bits. The open drain output A=B should be used with an external pull-up resistor in order to establish a logic HIGH level. The A=B signal can also be used with the C_n+4 signal to indicate A > B and A < B.

The function table lists of the 74HCT181 the arithmetic operations that are performed without a carry in. An incoming carry adds a one to each operation. Thus, select code LHHL generates A minus B minus 1 (2s complement notation) without a carry in and generates A minus B when a carry is applied.

Because subtraction of the 74HCT181 is actually performed by complementary addition (1s complement), a carry out means borrow; thus, a carry is generated when there is no under-flow and no carry is generated when there is underflow.

As indicated, the "181" can be used with either active LOW inputs producing active LOW outputs or with active HIGH inputs producing active HIGH outputs.

For either case the table lists the operations that are performed to the operands.