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SN74ACT72231L35RJ Folha de dados(PDF) 6 Page - Texas Instruments |
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SN74ACT72231L35RJ Folha de dados(HTML) 6 Page - Texas Instruments |
6 / 21 page SN74ACT72211L, SN74ACT72221L, SN74ACT72231L, SN74ACT72241L 512 × 9, 1024 × 9, 2048 × 9, AND 4096 × 9 SYNCHRONOUS FIRST IN, FIRST OUT MEMORIES SCAS222 − FEBRUARY 1993 − REVISED JUNE 1993 6 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 POST OFFICE BOX 1443 • HOUSTON, TEXAS 77251−1443 flag programming (continued) Table 1. Writing the Offset Registers LD WEN1 WCLK† SELECTION 0 0 ↑ Empty offset (LSB) Empty offset (MSB) Full offset (LSB) Full offset (MSB) 0 1 ↑ No operation 1 0 ↑ Write into FIFO 1 1 ↑ No operation † The same selection sequence applies to reading from the registers. REN1 and REN2 are enabled and a read is performed on the low-to-high transition of RCLK. programmable flag (PAE, PAF) operation Whether the flag offset registers are programmed as described in Table 1 or the default values are used, the programmable almost-empty flag (PAE) and programmable almost-full flag (PAF) states are determined by their corresponding offset registers and the difference between the read and write pointers. The number formed by the empty offset least significant bit register and empty offset most significant bit register is referred to as n and determines the operation of PAE. PAE is synchronized to the low-to-high transition of RCLK by one flip-flop and is low when the FIFO contains n or fewer unread words. PAE is set high by the low-to-high transition of RCLK when the FIFO contains (n + 1) or greater unread words. The number formed by the full offset least significant bit register and full offset most significant bit register is referred to as m and determines the operation of PAF. PAF is synchronized to the low-to-high transition of WCLK by one flip-flop and is set low when the number of unread words in the FIFO is greater then or equal to (512 − m) for the SN74ACT72211L, (1024 − m) for the SN74ACT72221L, (2048 − m) for the SN74ACT72231L, and (4096 − m) for the SN74ACT72241L. PAF is set high by the low-to-high transition of WCLK when the number of available memory locations is greater than m (see Table 2). Table 2. Status Flags NUMBER OF WORDS IN FIFO OUTPUTS SN74ACT72211L SN74ACT72221L SN74ACT72231L SN74ACT72241L FF PAF PAE EF 0 0 0 0 H H L L 1 to n† 1 to n† 1 to n† 1 to n† H HL H (n + 1) to [512 − (m + 1)] (n + 1) to [1024 − (m + 1)] (n + 1) to [2048 − (m + 1)] (n + 1) to [4096 − (m + 1)] H HH H (512 − m)‡ to 511 (1024 − m)‡ to 1023 (2048 − m)‡ to 2047 (4096 − m)‡ to 4095 H LH H 512 1024 2048 4096 L L H H † n = empty offset (default value = 7) ‡ m = full offset (default value = 7) |
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