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72255LA20PFGI Datasheet(PDF) 10 Page - Integrated Device Technology |
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72255LA20PFGI Datasheet(HTML) 10 Page - Integrated Device Technology |
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10 / 27 page  10 IDT72255LA/72265LA CMOS SuperSync FIFO™ 8,192 x 18 and 16,384 x 18 COMMERCIAL AND INDUSTRIAL TEMPERATURE RANGES SERIAL PROGRAMMING MODE If Serial Programming mode has been selected, as described above, then programming of PAE and PAF values can be achieved by using a combi- nation of the LD, SEN, WCLK and SI input pins. Programming PAE and PAF proceeds as follows: when LD and SEN are set LOW, data on the SI input are written, one bit for each WCLK rising edge, starting with the Empty Offset LSB and ending with the Full Offset MSB. A total of 26 bits for the IDT72255LA and 28 bits for the IDT72265LA. See Figure 13, Serial Loading of Programmable Flag Registers, for the timing diagram for this mode. Using the serial method, individual registers cannot be programmed se- lectively. PAE and PAF can show a valid status only after the complete set of bits (for all offset registers) has been entered. The registers can be reprogrammed as long as the complete set of new offset bits is entered. When LD is LOW and SEN is HIGH, no serial write to the registers can occur. Write operations to the FIFO are allowed before and during the serial programming sequence. In this case, the programming of all offset bits does not have to occur at once. A select number of bits can be written to the SI input and then, by bringing LD and SEN HIGH, data can be written to FIFO memory via Dn by toggling WEN. When WEN is brought HIGH with LD and SEN restored to a LOW, the next offset bit in sequence is written to the registers via SI. If an interruption of serial programming is desired, it is sufficient either to set LD LOW and deactivate SEN or to set SEN LOW and deactivate LD. Once LD and SEN are both restored to a LOW level, serial offset programming continues. From the time serial programming has begun, neither partial flag will be valid until the full set of bits required to fill all the offset registers has been written. Measuring from the rising WCLK edge that achieves the above criteria; PAF will be valid after two more rising WCLK edges plus tPAF, PAE will be valid after the next two rising RCLK edges plus tPAE plus tSKEW2. It is not possible to read the flag offset values in a serial mode. PARALLEL MODE If Parallel Programming mode has been selected, as described above, then programming of PAE and PAF values can be achieved by using a combination of the LD, WCLK , WEN and Dn input pins. ProgrammingPAE and PAF proceeds as follows: when LD and WEN are set LOW, data on the inputs Dn are written into the Empty Offset Register on the first LOW-to- HIGH transition of WCLK. Upon the second LOW-to-HIGH transition of WCLK, data are written into the Full Offset Register. The third transition of WCLK writes, once again, to the Empty Offset Register. See Figure 14, Parallel Loading of Programmable Flag Registers, for the timing diagram for this mode. The act of writing offsets in parallel employs a dedicated write offset register pointer. The act of reading offsets employs a dedicated read offset register pointer. The two pointers operate independently; however, a read and a write should not be performed simultaneously to the offset registers. A Master Reset initializes both pointers to the Empty Offset (LSB) register. A Partial Reset has no effect on the position of these pointers. Write operations to the FIFO are allowed before and during the parallel programming sequence. In this case, the programming of all offset registers does not have to occur at one time. One, two or more offset reg- isters can be written and then by bringing LD HIGH, write operations can be redirected to the FIFO memory. When LD is set LOW again, and WEN is LOW, the next offset register in sequence is written to. As an alternative to holding WEN LOW and toggling LD, parallel programming can also be interrupted by setting LD LOW and toggling WEN. Note that the status of a partial flag (PAE or PAF) output is invalid dur- ing the programming process. From the time parallel programming has begun, a partial flag output will not be valid until the appropriate offset word has been written to the register(s) pertaining to that flag. Measuring from the rising WCLK edge that achieves the above criteria; PAF will be valid after two more rising WCLK edges plus tPAF, PAE will be valid after the next two rising RCLK edges plus tPAE plus tSKEW2. The act of reading the offset registers employs a dedicated read offset register pointer. The contents of the offset registers can be read on the Q0- Qn pins when LD is set LOW and REN is set LOW. Data are read via Qn from the Empty Offset Register on the first LOW-to-HIGH transition of RCLK. Upon the second LOW-to-HIGH transition of RCLK, data are read from the Full Offset Register. The third transition of RCLK reads, once again, from the Empty Offset Register. See Figure 15, Parallel Read of Programmable Flag Registers, for the timing diagram for this mode. It is permissible to interrupt the offset register read sequence with reads or writes to the FIFO. The interruption is accomplished by deasserting REN, LD, or both together. When REN and LD are restored to a LOW level, reading of the offset registers continues where it left off. It should be noted, and care should be taken from the fact that when a parallel read of the flag offsets is performed, the data word that was present on the output lines Qn will be overwritten. Parallel reading of the offset registers is always permitted regardless of which timing mode (IDT Standard or FWFT modes) has been selected. RETRANSMIT OPERATION The Retransmit operation allows data that has already been read to be accessed again. There are two stages: first, a setup procedure that resets the read pointer to the first location of memory, then the actual retransmit, which consists of reading out the memory contents, starting at the beginning of memory. Retransmit setup is initiated by holding RT LOW during a rising RCLK edge. REN and WEN must be HIGH before bringing RT LOW. At least one word, but no more than D - 2 words should have been written into the FIFO between Reset (Master or Partial) and the time of Retransmit setup. D = 8,192 for the IDT72255LA and D = 16,384 for the IDT72265LA. In FWFT mode, D = 8,193 for the IDT72255LA and D = 16,385 for the IDT72265LA. If IDT Standard mode is selected, the FIFO will mark the beginning of the Retransmit setup by setting EF LOW. The change in level will only be noticeable if EF was HIGH before setup. During this period, the internal read pointer is initialized to the first location of the RAM array. When EF goes HIGH, Retransmit setup is complete and read opera- tions may begin starting with the first location in memory. Since IDT Stan- dard mode is selected, every word read including the first word following Retransmit setup requires a LOW on REN to enable the rising edge of RCLK. See Figure 11, Retransmit Timing (IDT Standard Mode), for the relevant timing diagram. If FWFT mode is selected, the FIFO will mark the beginning of the Retransmit setup by setting OR HIGH. During this period, the internal read pointer is set to the first location of the RAM array. When OR goes LOW, Retransmit setup is complete; at the same time, the contents of the first location appear on the outputs. Since FWFT mode is selected, the first word appears on the outputs, no LOW on REN is necessary. Reading all subsequent words requires a LOW on REN to |
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