Details, datasheet, quote on part number: 74AC161SCX
DescriptionSynchronous Presettable Binary Counter
CompanyFairchild Semiconductor
DatasheetDownload 74AC161SCX datasheet
Cross ref.Similar parts: CD74AC161M96, CD74ACT163, SN74F163A, CD74AC163, 74AC161MTR, 74AC161M, CD74AC161M, CD74AC161M96, CD74AC161MX
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Features, Applications

The AC/ACT161 are high-speed synchronous modulo-16 binary counters. They are synchronously presettable for application in programmable dividers and have two types of Count Enable inputs plus a Terminal Count output for versatility in forming synchronous multistage counters. The AC/ACT161 has an asynchronous Master Reset input that overrides all other inputs and forces the outputs LOW.


s ICC reduced 50% s Synchronous counting and loading s High-speed synchronous expansion s Typical count rate of 125 MHz s Outputs source/sink s ACT161 has TTL-compatible inputs

Order Number 74ACT161MTC 74ACT161PC Package Number MTC16 N16E Package Description 16-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-012, 0.150" Narrow Body 16-Lead Small Outline Package (SOP), EIAJ TYPE II, 5.3mm Wide 16-Lead Thin Shrink Small Outline Package (TSSOP), JEDEC MO-153, 4.4mm Wide 16-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300" Wide 16-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-012, 0.150" Narrow Body 16-Lead Small Outline Package (SOP), EIAJ TYPE II, 5.3mm Wide 16-Lead Thin Shrink Small Outline Package (TSSOP), JEDEC MO-153, 4.4mm Wide 16-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300" Wide

Device also available in Tape and Reel. Specify by appending suffix letter "X" to the ordering code.

Pin Names CEP CET Q0Q3 TC Description Count Enable Parallel Input Count Enable Trickle Input Clock Pulse Input Asynchronous Master Reset Input Parallel Data Inputs Parallel Enable Inputs Flip-Flop Outputs Terminal Count Output

The AC/ACT161 count in modulo-16 binary sequence. From state 15 (HHHH) they increment to state 0 (LLLL). The clock inputs of all flip-flops are driven in parallel through a clock buffer. Thus all changes of the Q outputs (except due to Master Reset of the AC/ACT161) occur as a result of, and synchronous with, the LOW-to-HIGH transition of the CP input signal. The circuits have four fundamental modes of operation, in order of precedence: asynchronous reset, parallel load, count-up and hold. Five control inputs--Master Reset, Parallel Enable (PE Count Enable Parallel (CEP) and Count Enable Trickle (CET)-- determine the mode of operation, as shown in the Mode Select Table. A LOW signal on MR overrides all other inputs and asynchronously forces all outputs LOW. A LOW signal on PE overrides counting and allows information on the Parallel Data (Pn) inputs to be loaded into the flip-flops on the next rising edge of CP. With PE and MR HIGH, CEP and CET permit counting when both are HIGH. Conversely, a LOW signal on either CEP or CET inhibits counting. The AC/ACT161 use D-type edge-triggered flip-flops and changing the PE, CEP, and CET inputs when the is in either state does not cause errors, provided that the recommended setup and hold times, with respect to the rising edge of CP, are observed. The Terminal Count (TC) output is HIGH when CET is HIGH and counter is in state 15. To implement synchronous multistage counters, the TC outputs can be used with the CEP and CET inputs in two different ways. Figure 1 shows the connections for simple ripple carry, in which the clock period must be longer than the to TC delay of the first stage, plus the cumulative CET to TC delays of the intermediate stages, plus the CET to CP setup time of the last stage. This total delay plus setup time sets the upper limit on clock frequency. For faster clock rates, the carry lookahead connections shown in Figure 2 are recommended. In this scheme the ripple delay through the intermediate stages commences with the same clock that causes the first stage to tick over from max to min in the Up mode, or min to max in the Down mode, to start its final cycle. Since this final cycle requires 16 clocks to complete, there is plenty of time for the ripple to progress through the intermediate stages. The critical timing that lim-

its the clock period is the to TC delay of the first stage plus the CEP to CP setup time of the last stage. The TC output is subject to decoding spikes due to internal race conditions and is therefore not recommended for use as a clock or asynchronous reset for flip-flops, registers or counters. Logic Equations: Count Enable = CEP CET Q2 Q3 CET

PE CET CEP Action on the Rising Clock Edge ( Reset (Clear) Load (PnQn) Count (Increment) No Change (Hold) No Change (Hold) )

H = HIGH Voltage Level L = LOW Voltage Level X = Immaterial
FIGURE 1. Multistage Counter with Ripple Carry
FIGURE 2. Multistage Counter with Lookahead Carry

Please note that this diagram is provided only for the understanding of logic operations and should not be used to estimate propagation delays.


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