SN74ACT2227, SN74ACT2229 DUAL 64 × 1, DUAL 256 × 1 FIRST-IN, FIRST-OUT MEMORIES SCAS220C – JUNE 1992 – REVISED OCTOBER 1997 D D D D D D D D D D Dual Independent FIFOs Organized as: 64 Words by 1 Bit Each – SN74ACT2227 256 Words by 1 Bit Each – SN74ACT2229 Free-Running Read and Write Clocks Can Be Asynchronous or Coincident on Each FIFO Input-Ready Flags Synchronized to Write Clocks Output-Ready Flags Synchronized to Read Clocks Half-Full and Almost-Full/Almost-Empty Flags Support Clock Frequencies up to 60 MHz Access Times of 9 ns 3-State Data Outputs Low-Power Advanced CMOS Technology Packaged in 28-Pin SOIC Package DW PACKAGE (TOP VIEW) 1HF 1AF/AE 1WRTCLK 1WRTEN 1IR 1D GND GND 1RESET 2Q 2OR 2RDEN 2RDCLK 2OE 1 28 2 27 3 26 4 25 5 24 6 23 7 22 8 21 9 20 10 19 11 18 12 17 13 16 14 15 1OE 1RDCLK 1RDEN 1OR 1Q 2RESET VCC VCC 2D 2IR 2WRTEN 2WRTCLK 2AF/AE 2HF description The SN74ACT2227 and SN74ACT2229 are dual FIFOs suited for a wide range of serial-data buffering applications including elastic stores for frequencies up to OC-1 telecommunication rates. Each FIFO on the chip is arranged as 64 × 1 (SN74ACT2227) or 256 × 1 (SN74ACT2229) and has control signals and status flags for independent operation. Output flags for each FIFO include input ready (1IR or 2IR), output ready (1OR or 2OR), half full (1HF or 2HF), and almost full/almost empty (1AF/AE or 2AF/AE). Serial data is written into a FIFO on the low-to-high transition of the write-clock (1WRTCLK or 2WRTCLK) input when the write-enable (1WRTEN or 2WRTEN) input and input-ready flag (1IR or 2IR) output are both high. Serial data is read from a FIFO on the low-to-high transition of the read-clock (1RDCLK or 2RDCLK) input when the read-enable (1RDEN or 2RDEN) input and output-ready flag (1OR or 2OR) output are both high. The read and write clocks of a FIFO can be asynchronous to one another. A FIFO data output (1Q or 2Q) is in the high-impedance state when its output-enable (1OE or 2OE) input is low. Each input-ready flag (1IR or 2IR) is synchronized by two flip-flop stages to its write clock (1WRTCLK or 2WRTCLK), and each output-ready flag (1OR or 2OR) is synchronized by three flip-flop stages to its read clock (1RDCLK or 2RDCLK). This multistage synchronization ensures reliable flag-output states when data is written and read asynchronously. A half-full flag (1HF or 2HF) is high when the number of bits stored in its FIFO is greater than or equal to half the depth of the FIFO. An almost-full/almost-empty flag (1AF/AE or 2AF/AE) is high when eight or fewer bits are stored in its FIFO and when eight or fewer empty locations are left in the FIFO. A bit present on the data output is not stored in the FIFO. The SN74ACT2227 and SN74ACT2229 are characterized for operation from – 40°C to 85°C. For more information on this device family, see the application report FIFOs With a Word Width of One Bit (literature number SCAA006). Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. Copyright 1997, Texas Instruments Incorporated PRODUCTION DATA information is current as of publication date. Products conform to specifications per the terms of Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters. POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 1 SN74ACT2227, SN74ACT2229 DUAL 64 × 1, DUAL 256 × 1 FIRST-IN, FIRST-OUT MEMORIES SCAS220C – JUNE 1992 – REVISED OCTOBER 1997 logic symbols† 9 1RESET 1WRTCLK 1WRTEN 1RDCLK 1OE 1RDEN 1D 3 4 27 28 26 23 2WRTEN 2RDCLK 2OE 2RDEN 2D 1RDCLK 1OE 1RDEN 1D 13 14 12 2RDCLK 2OE 2RDEN 2D OUT RDY 1 2 25 3 4 27 28 26 18 13 14 12 1HF 1AF/AE 1OR RDEN 1 24 1Q RESET 19 IN RDY WRTEN HALF FULL RDCLK ALMOST FULL/EMPTY EN2 15 16 11 OUT RDY 2IR 2HF 2AF/AE 2OR RDEN 10 RESET 2Q Φ FIFO 256 × 1 SN74ACT2229 IN RDY WRTCLK HALF FULL WRTEN ALMOST FULL/EMPTY RDCLK OUT RDY 5 1 2 25 1IR 1HF 1AF/AE 1OR EN1 RDEN 6 17 1IR EN1 1 24 1Q RESET 19 WRTCLK IN RDY WRTEN RDCLK HALF FULL 15 16 ALMOST FULL/EMPTY EN2 11 OUT RDY 2IR 2HF 2AF/AE 2OR RDEN 20 2 † These symbols are in accordance with ANSI/IEEE Std 91-1984 and IEC Publication 617-12. 2 5 2 23 2WRTEN ALMOST FULL/EMPTY RDCLK 20 2RESET 2WRTCLK WRTEN WRTCLK 18 9 1WRTEN HALF FULL 17 1RESET 1WRTCLK IN RDY WRTCLK 6 2RESET 2WRTCLK RESET Φ FIFO 64 × 1 SN74ACT2227 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 10 2Q SN74ACT2227, SN74ACT2229 DUAL 64 × 1, DUAL 256 × 1 FIRST-IN, FIRST-OUT MEMORIES SCAS220C – JUNE 1992 – REVISED OCTOBER 1997 SN74ACT2227 functional block diagram (each FIFO) OE D Location 1 RDCLK RDEN WRTCLK WRTEN Synchronous Read Control Synchronous Write Control Read Pointer Write Pointer Location 2 Dual-Port SRAM 64 × 1 Location 63 Location 64 Register Q AF/AE Status HF Reset Logic IR RESET OR SN74ACT2229 functional block diagram (each FIFO) OE D Location 1 RDCLK RDEN WRTCLK WRTEN Synchronous Read Control Synchronous Write Control Read Pointer Write Pointer Location 2 Dual-Port SRAM 256 × 1 Location 255 Location 256 Register Q AF/AE Status Reset Logic HF IR RESET OR POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 3 SN74ACT2227, SN74ACT2229 DUAL 64 × 1, DUAL 256 × 1 FIRST-IN, FIRST-OUT MEMORIES SCAS220C – JUNE 1992 – REVISED OCTOBER 1997 Terminal Functions TERMINAL NAME NO. I/O DESCRIPTION 1AF/AE 2AF/AE 2 16 O Almost-full/almost-empty flag. AF/AE is high when the memory is eight locations or fewer from a full or empty state. AF/AE is set high after reset. 1D 2D 6 20 I Data input GND 7, 8 1HF 2HF 1 15 O Half-full flag. HF is high when the number of bits stored in memory is greater than or equal to half the FIFO depth. HF is set low after reset. 1IR 2IR 5 19 O Input-ready flag. IR is synchronized to the low-to-high transition of WRTCLK. When IR is low, the FIFO is full and writes are disabled. IR is set low during reset and is set high on the second low-to-high transition of WRTCLK after reset. 1OE 2OE 28 14 I Output enable. The data output of a FIFO is active when OE is high and in the high-impedance state when OE is low. 1OR 2OR 25 11 O Output-ready flag. OR is synchronized to the low-to-high transition of RDCLK. When OR is low, the FIFO is empty and reads are disabled. Ready data is present on the data output when OR is high. OR is set low during reset and set high on the third low-to-high transition of RDCLK after the first word is loaded to empty memory. 1Q 2Q 24 10 O Data outputs. After the first valid write to empty memory, the first bit is output on the third rising edge of RDCLK. OR for the FIFO is asserted high to indicate ready data. 1RDCLK 2RDCLK 27 13 I Read clock. RDCLK is a continuous clock and can be independent of any other clock on the device. A low-to-high transition of RDCLK reads data from memory when the FIFO RDEN and OR are high. OR is synchronous with the low-to-high transition of RDCLK. 1RDEN 2RDEN 26 12 I Read enable. When the RDEN and OR of a FIFO are high, data is read from the FIFO on the low-to-high transition of RDCLK. 1RESET 2RESET 9 23 I Reset. To reset the FIFO, four low-to-high transitions of RDCLK and four low-to-high transitions of WRTCLK must occur while RESET is low. This sets HF, IR, and OR low and AF/AE high. Before it is used, a FIFO must be reset after power up. VCC Ground 21, 22 Supply voltage 1WRTCLK 2WRTCLK 3 17 I Write clock. WRTCLK is a continuous clock and can be independent of any other clock on the device. A low-to-high transition of WRTCLK writes data to memory when WRTEN and IR are high. IR is synchronous with the low-to-high transition of WRTCLK. 1WRTEN 2WRTEN 4 18 I Write enable. When WRTEN and IR are high, data is written to the FIFO on a low-to-high transition of WRTCLK. 4 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 SN74ACT2227, SN74ACT2229 DUAL 64 × 1, DUAL 256 × 1 FIRST-IN, FIRST-OUT MEMORIES SCAS220C – JUNE 1992 – REVISED OCTOBER 1997 RESET WRTCLK WRTEN D RDCLK RDEN OE Q OR AF/AE HF IR 1 2 3 4 1 2 ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎ ÎÎÎÎÎÎ ÎÎÎÎÎÎ ÎÎÎÎÎÎ ÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎ Don’t Care Don’t Care 1 2 3 4 Don’t Care Don’t Care Don’t Care Don’t Care Don’t Care Don’t Care Figure 1. FIFO Reset POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 5 SN74ACT2227, SN74ACT2229 DUAL 64 × 1, DUAL 256 × 1 FIRST-IN, FIRST-OUT MEMORIES SCAS220C – JUNE 1992 – REVISED OCTOBER 1997 1 0 RESET WRTCLK 1 0 WRTEN ÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎ ÎÎÎ ÎÎÎ ÎÎÎ ÎÎ ÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎ ÎÎÎ ÎÎÎ ÎÎÎ ÎÎ D RDCLK B1 B2 B3 B4 1 2 3 B10 A B 1 0 RDEN ÎÎÎÎÎÎÎÎÎÎÎ ÎÎÎÎÎÎÎÎÎÎÎ 1 0 OE B1 Q OR AF/AE HF IR DATA BIT NUMBER BASED ON FIFO DEPTH DATA BIT DEVICE A B C SN74ACT2227 B33 B57 B65 SN74ACT2229 B129 B249 B257 Figure 2. FIFO Write 6 C POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 SN74ACT2227, SN74ACT2229 DUAL 64 × 1, DUAL 256 × 1 FIRST-IN, FIRST-OUT MEMORIES SCAS220C – JUNE 1992 – REVISED OCTOBER 1997 1 0 RESET WRTCLK 1 2 WRTEN ÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ ÎÎ ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ D F RDCLK RDEN 1 0 OE Q B1 B2 B3 B9 B10 A B C D E F OR AF/AE HF IR DATA BIT NUMBER BASED ON FIFO DEPTH DEVICE DATA BIT A B C D E F SN74ACT2227 B33 B34 B56 B57 B64 B65 SN74ACT2229 B129 B130 B248 B249 B256 B257 Figure 3. FIFO Read POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 7 SN74ACT2227, SN74ACT2229 DUAL 64 × 1, DUAL 256 × 1 FIRST-IN, FIRST-OUT MEMORIES SCAS220C – JUNE 1992 – REVISED OCTOBER 1997 absolute maximum ratings over operating free-air temperature range (unless otherwise noted)† Supply voltage range, VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . – 0.5 V to 7 V Input voltage range, VI (see Note 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . – 0.5 V to VCC + 0.5 V Output voltage range, VO (see Note 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . – 0.5 V to VCC + 0.5 V Voltage applied to a disabled 3-state output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 V Input clamp current, IIK (VI < 0 or VI > VCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 20 mA Output clamp current, IOK (VO < 0 or VO > VCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 50 mA Continuous output current, IO (VO = 0 to VCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 50 mA Continuous current through VCC or GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 200 mA Package thermal impedance, θJA (see Note 2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78°C/W Storage temperature range, Tstg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . – 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 provided that the input and output current ratings are observed. 2. The package thermal impedance is calculated in accordance with JESD 51. recommended operating conditions VCC VIH Supply voltage VIL IOH Low-level input voltage High-level input voltage Low level output current Low-level TA Operating free-air temperature MAX 4.5 5.5 2 High-level output current IOL MIN UNIT V V 0.8 V Q outputs, flags –8 mA Q outputs 16 Flags 8 – 40 85 mA °C electrical characteristics over recommended operating free-air temperature range (unless otherwise noted) PARAMETER VOH VOL II IOZ ICC ∆ICC§ Ci Flags Q outputs TEST CONDITIONS VCC = 4.5 V, VCC = 4.5 V, IOH = – 8 mA IOL = 8 mA VCC = 4.5 V, VCC = 5.5 V, IOL = 16 mA VI = VCC or 0 VCC = 5.5 V, VI = VCC – 0.2 V or 0 VO = VCC or 0 VCC = 5.5 V, VI = 0, One input at 3.4 V, MIN 0.5 Other inputs at VCC or GND • DALLAS, TEXAS 75265 UNIT V 0.5 f = 1 MHz POST OFFICE BOX 655303 MAX 2.4 Co VO = 0, f = 1 MHz ‡ All typical values are at VCC = 5 V, TA = 25°C. § This is the supply current when each input is at one of the specified TTL voltage levels rather than 0 V or VCC. 8 TYP‡ V ±5 µA ±5 µA 400 µA 1 mA 4 pF 8 pF SN74ACT2227, SN74ACT2229 DUAL 64 × 1, DUAL 256 × 1 FIRST-IN, FIRST-OUT MEMORIES SCAS220C – JUNE 1992 – REVISED OCTOBER 1997 timing requirements over recommended ranges of supply voltage and operating free-air temperature (unless otherwise noted) (see Figures 1 through 4) MIN fclock Clock frequency tw Pulse duration tsu th Setup time Hold time 1WRTCLK, 2WRTCLK high or low 5 1RDCLK, 2RDCLK high or low 5 1D before 1WRTCLK↑ and 2D before 2WRTCLK↑ 4.5 1WRTEN before 1WRTCLK↑ and 2WRTEN before 2WRTCLK↑ 4.5 1RDEN before 1RDCLK↑ and 2RDEN before 2RDCLK↑ 4 1RESET low before 1WRTCLK↑ and 2RESET low before 2WRTCLK↑† 1RESET low before 1RDCLK↑ and 2RESET low before 2RDCLK↑† 6 1D after 1WRTCLK↑ and 2D after 2WRTCLK↑ 0 1WRTEN after 1WRTCLK↑ and 2WRTEN after 2WRTCLK↑ 0 1RDEN after 1RDCLK↑ and 2RDEN after 2RDCLK↑ 0 1RESET low after 1WRTCLK↑ and 2RESET low after 2WRTCLK↑† 1RESET low after 1RDCLK↑ and 2RESET low after 2RDCLK↑† 6 MAX UNIT 60 MHz ns ns 6 ns 6 † Requirement to count the clock edge as one of at least four needed to reset a FIFO switching characteristics over recommended ranges of supply voltage and operating free-air temperature, CL = 50 pF (unless otherwise noted) (see Figure 4) PARAMETER FROM (INPUT) fmax 1WRTCLK, 2WRTCLK, or 1RDCLK, 2RDCLK tpd tpd tpd tpd d tPLH tPHL tPLH tPHL ten tdis TO (OUTPUT) MIN MAX 60 UNIT MHz 1RDCLK↑, 2RDCLK↑ 1Q, 2Q 2 9 ns 1WRTCLK↑, 2WRTCLK↑ 1IR, 2IR 1 8 ns 1RDCLK↑, 2RDCLK↑ 1OR, 2OR 1 8 ns 3 14 3 14 2 12 ns 3 14 ns 1AF/AE, 2AF/AE 1 17 ns 1WRTCLK↑, 2WRTCLK↑ 1RDCLK↑, 2RDCLK↑ 1WRTCLK↑, 2WRTCLK↑ 1RDCLK↑, 2RDCLK↑ 1RESET 2RESET low 1RESET, 1AF/AE 2AF/AE 1AF/AE, 1HF 2HF 1HF, ns 1HF, 2HF 1 18 ns 1OE, 2OE 1Q, 2Q 0 8 ns 1OE, 2OE 1Q, 2Q 0 8 ns POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 9 SN74ACT2227, SN74ACT2229 DUAL 64 × 1, DUAL 256 × 1 FIRST-IN, FIRST-OUT MEMORIES SCAS220C – JUNE 1992 – REVISED OCTOBER 1997 PARAMETER MEASUREMENT INFORMATION 7V PARAMETER S1 ten 500 Ω From Output Under Test Test Point tdis tpd 500 Ω CL = 50 pF (see Note A) S1 tPZH tPZL tPHZ tPLZ tPLH tPHL LOAD CIRCUIT Open Closed Open Closed Open Open tw 3V 3V Timing Input 1.5 V 0V 0V VOLTAGE WAVEFORMS PULSE DURATION th 3V Data Input 1.5 V 3V 1.5 V 0V VOLTAGE WAVEFORMS SETUP AND HOLD TIMES 1.5 V 0V 1.5 V VOL Output Waveform 2 S1 at Open VOL + 0.3 V 1.5 V VOH – 0.3 V VOH ≈0V NOTE A: CL includes probe and jig capacitance. Figure 4. Load Circuit and Voltage Waveforms • DALLAS, TEXAS 75265 VOL tPHZ VOLTAGE WAVEFORMS ENABLE AND DISABLE TIMES VOLTAGE WAVEFORMS PROPAGATION DELAY TIMES POST OFFICE BOX 655303 tPLZ 1.5 V tPZH VOH 10 1.5 V ≈ 3.5 V Output Waveform 1 S1 at 7 V tPHL 1.5 V 1.5 V 0V 1.5 V tPLH Output Control tPZL 3V Output 1.5 V 1.5 V tsu Input Input SN74ACT2227, SN74ACT2229 DUAL 64 × 1, DUAL 256 × 1 FIRST-IN, FIRST-OUT MEMORIES SCAS220C – JUNE 1992 – REVISED OCTOBER 1997 TYPICAL CHARACTERISTICS SINGLE FIFO SUPPLY CURRENT vs CLOCK FREQUENCY I CC(f) – Supply Current – mA 40 VCC = 5.5 V 35 VCC = 5 V 30 25 VCC = 4.5 V 20 15 10 fI = 1/2 fclock TA = 75°C CL = 0 pF 5 0 0 10 20 30 40 50 60 fclock – Clock Frequency – MHz Figure 5 calculating power dissipation Data for Figure 5 is taken with one FIFO active and one FIFO idle on the device. The active FIFO has both writes and reads enabled with its read clock (RDCLK) and write clock (WRTCLK) operating at the rate specified by fclock. The data input rate and data output rate are half the fclock rate, and the data output is disconnected. A close approximation to the total device power can be found by Figure 5, determining the capacitive load on the data output and determining the number of SN74ACT2227/2229 inputs driven by TTL high levels. With ICC(f) taken from Figure 5, the maximum power dissipation (PT) of one FIFO on the SN74ACT2227 or SN74ACT2229 can be calculated by: PT = VCC × [ICC(f ) + (N × ∆ICC × dc)] + (CL × VCC2 × fo) where: N ∆ICC dc CL fo = = = = = number of inputs driven by TTL levels increase in power-supply current for each input at a TTL high level duty cycle of inputs at a TTL high level of 3.4 V output capacitive load switching frequency of an output POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 11 SN74ACT2227, SN74ACT2229 DUAL 64 × 1, DUAL 256 × 1 FIRST-IN, FIRST-OUT MEMORIES SCAS220C – JUNE 1992 – REVISED OCTOBER 1997 APPLICATION INFORMATION An example of concentrating two independent serial-data signals into a single composite data signal with the use of an SN74ACT2227 or SN74ACT2229 device is shown in Figure 6. The input data to the FIFOs share the same average (mean) frequency and the mean frequency of the SYS_CLOCK is greater than or equal to the sum of the individual mean input rates. A single-bit FIFO is needed for each additional input data signal that is time-division multiplexed into the composite signal. The FIFO memories provide a buffer to absorb clock jitter generated by the transmission systems of incoming signals and synchronize the phase-independent inputs to one another. FIFO half-full (HF) flags are used to signal the multiplexer to start fetching data from the buffers. The state of the flags also can be used to indicate when a FIFO read should be suppressed to regulate the output flow (pulse-stuffing control). The FIFO almost-full/almost-empty (AF/AE) flags can be used in place of the half-full flags to reduce transmission delay. SN74ACT2227 or SN74ACT2229 SYS_CLOCK +5 V 1HF Serial Data Stream 1WRTCLK 1RDCLK 1RDEN 1WRTEN 1Q 1D Serial Data Stream 2WRTCLK 2RDCLK 2WRTEN 2RDEN 2D 2Q 2HF READY_1 SELECT_1 DATA_1 Time-Division Multiplexer Composite Data Stream SELECT_2 DATA_2 READY_2 Figure 6. Time-Division Multiplexing Using the SN74ACT2227 or SN74ACT2229 12 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 IMPORTANT NOTICE Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue any product or service without notice, and advise customers to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extent TI deems necessary to support this warranty. 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