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SN74ACT7881-20FN Arkusz danych(PDF) 5 Page - Texas Instruments |
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SN74ACT7881-20FN Arkusz danych(HTML) 5 Page - Texas Instruments |
5 / 19 page SN74ACT7881 1024 × 18 CLOCKED FIRST-IN, FIRST-OUT MEMORY SCAS227E – FEBRUARY 1993 – REVISED APRIL 1998 5 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 Terminal Functions TERMINAL† I/O DESCRIPTION NAME NO. I/O DESCRIPTION AF/AE 33 O Almost-full/almost-empty flag. The AF/AE boundary is defined by the AF/AE offset value (X). This value can be programmed during reset, or the default value of 256 can be used. AF/AE is high when the FIFO contains (X + 1) or fewer words or (1025 – X) or more words. AF/AE is low when the FIFO contains between (X + 2) and (1024 – X) words. Programming procedure for AF/AE – The AF/AE flag is programmed during each reset cycle. The AF/AE offset value (X) is either a user-defined value or the default of X = 256. Instructions to program AF/AE using both methods are as follows: User-defined X Step 1: Take DAF from high to low. Step 2: If RESET is not already low, take RESET low. Step 3: With DAF held low, take RESET high. This defines the AF/AE using X. Step 4: To retain the current offset for the next reset, keep DAF low. Default X To redefine AF/AE using the default value of X = 256, hold DAF high during the reset cycle. DAF 27 I Define-almost-full. The high-to-low transition of DAF stores the binary value of data inputs as the AF/AE offset value (X). With DAF held low, a low pulse on RESET defines the AF/AE flag using X. D0–D17 26–19, 17, 15–7 I Data inputs for 18-bit-wide data to be stored in the memory. A high-to-low transition of DAF captures data for the AF/AE offset (X) from D8–D0. HF 36 O Half-full flag. HF is high when the FIFO contains 512 or more words and is low when the number of words in memory is less than half the depth of the FIFO. IR 35 O Input-ready flag. IR is high when the FIFO is not full and low when the device is full. During reset, IR is driven low on the rising edge of the second WRTCLK pulse. IR is then driven high on the rising edge of the second WRTCLK pulse after RESET goes high. After the FIFO is filled and IR is driven low, IR is driven high on the second WRTCLK pulse after the first valid read. OE 2 I Output enable. The Q0–Q17 outputs are in the high-impedance state when OE is low. OE must be high before the rising edge of RDCLK to read a word from memory. OR 66 O Output-ready flag. OR is high when the FIFO is not empty and low when the FIFO is empty. During reset, OR is set low on the rising edge of the third RDCLK pulse. OR is set high on the rising edge of the third RDCLK pulse to occur after the first word is written into the FIFO. OR is set low on the rising edge of the first RDCLK pulse after the last word is read. Q0–Q17 38–39, 41–42, 44, 46–47, 49–50, 52–53, 55–56, 58–59, 61, 63–64 O Data outputs. The first data word to be loaded into the FIFO is moved to Q0–Q17 on the rising edge of the third RDCLK pulse to occur after the first valid write. RDEN1 and RDEN2 do not affect this operation. Following data is unloaded on the rising edge of RDCLK when RDEN1, RDEN2, OE, and OR are high. RDCLK 5 I Read clock. Data is read out of memory on the low-to-high transition of RDCLK if OR, OE, RDEN1, and RDEN2 are high. RDCLK is a free-running clock and functions as the synchronizing clock for all data transfers out of the FIFO. OR also is driven synchronously with respect to the RDCLK signal. RDEN1 RDEN2 4 3 I Read enable. RDEN1 and RDEN2 must be high before a rising edge on RDCLK to read a word out of memory. RDEN1 and RDEN2 are not used to read the first word stored in memory. RESET 1 I Reset. A reset is accomplished by taking RESET low and generating a minimum of four RDCLK and WRTCLK cycles. This ensures that the internal read and write pointers are reset and that OR, HF, and IR are low, and AF/AE is high. The FIFO must be reset upon power up. With DAF at a low level, a low pulse on RESET defines AF/AE using the AF/AE offset value (X), where X is the value previously stored. With DAF at a high level, a low-level pulse on RESET defines the AF/AE flag using the default value of X = 256. WRTCLK 29 I Write clock. Data is written into memory on a low-to-high transition of WRTCLK if IR, WRTEN1, and WRTEN2 are high. WRTCLK is a free-running clock and functions as the synchronizing clock for all data transfers into the FIFO. IR also is driven synchronously with respect to WRTCLK. WRTEN1 WRTEN2 30 31 I Write enable. WRTEN1 and WRTEN2 must be high before a rising edge on WRTCLK for a word to be written into memory. WRTEN1 and WRTEN2 do not affect the storage of the AF/AE offset value (X). † Terminals listed are for the FN package. |
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