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IDT70125L45J Datasheet(PDF) 11 Page - Integrated Device Technology |
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IDT70125L45J Datasheet(HTML) 11 Page - Integrated Device Technology |
11 / 12 page 6.10 11 IDT 70121/70125S/L HIGH-SPEED 2K x 9 DUAL-PORT STATIC RAM WITH BUSY & INTERRUPT COMMERCIAL TEMPERATURE RANGE FUNCTIONAL DESCRIPTION The IDT70121/125 provides two ports with separate control, address and I/O pins that permit independent access for reads or writes to any location in memory. The IDT70121/125 has an automatic power down feature controlled by CE. The CE controls on-chip power down circuitry that permits the respective port to go into a standby mode when not selected ( CE high). When a port is enabled, access to the entire memory array is permitted. INTERRUPTS If the user chooses to use the interrupt function, a memory location (mail box or message center) is assigned to each port. The left port interrupt flag ( INTL) is asserted when the right port writes to memory location 7FE (HEX), where a write is defined as the CE = R/W = VIL per the Truth Table. The left port clears the interrupt by access address location 7FE access when CER = OER= VIL,R/W is a "Don't Care". Likewise, the right port interrupt flag ( INTR) is asserted when the left port writes to memory location 7FF (HEX) and to clear the interrupt flag ( INTR), the right port must access the memory location 7FF. The message (9 bits) at 7FE or 7FF is user-defined, since it is an addressable SRAM location. If the interrupt function is not used, address locations 7FE and 7FF are not used as mail boxes, but as part of the random access memory. Refer to Table I for the interrupt operation. BUSY LOGIC Busy Logic provides a hardware indication that both ports of the RAM have accessed the same location at the same time. It also allows one of the two accesses to proceed and signals the other side that the RAM is “Busy”. The busy pin can then be used to stall the access until the operation on the other side is completed. If a write operation has been attempted from the side that receives a busy indication, the write signal is gated internally to prevent the write from proceeding. The use of busy logic is not required or desirable for all applications. In some cases it may be useful to logically OR the busy outputs together and use any busy indication as an interrupt source to flag the event of an illegal or illogical operation. If the write inhibit function of busy logic is not desirable, the busy logic can be disabled by using the IDT70125 (SLAVE). In the IDT70125, the busy pin operates solely as a write inhibit input pin. Normal operation can be programmed by tying the BUSY pins high. Once in slave mode the BUSY pin operates solely as a write inhibit input pin. Normal operation can be programmed by tying the BUSY pins high. If desired, unintended write operations can be prevented to a port by tying the busy pin for that port low. The busy outputs on the IDT70121/125 RAM in master mode, are push-pull type outputs and do not require pull up resistors to operate. If these RAMs are being expanded in depth, then the busy indication for the resulting array requires the use of an external AND gate. WIDTH EXPANSION WITH BUSY LOGIC MASTER/SLAVE ARRAYS When expanding an IDT70121/125 RAM array in width while using busy logic, one master part is used to decide which side of the RAM array will receive a busy indication, and to output that indication. Any number of slaves to be addressed in the same address range as the master, use the busy signal as a write inhibit signal. Thus on the IDT70121 RAM the busy pin is an output of the part, and the busy pin is an input of the IDT70125 as shown in Figure 3. If two or more master parts were used when expanding in width, a split decision could result with one master indicating busy on one side of the array and another master indicating busy on one other side of the array. This would inhibit the write operations from one port for part of a word and inhibit the write 2654 drw 15 MASTER Dual Port RAM BUSYL BUSYR CE MASTER Dual Port RAM BUSYL BUSYR CE SLAVE Dual Port RAM BUSYL BUSYR CE SLAVE Dual Port RAM BUSYL BUSYR CE BUSYL BUSYR IDT70121 IDT70121 IDT70125 IDT70125 Figure 3. Busy and chip enable routing for both width and depth expansion with IDT70121 (Master) and IDT70125 (Slave) RAMs. operations from the other port for the other part of the word. The busy arbitration, on a master, is based on the chip enable and address signals only. It ignores whether an access is a read or write. In a master/slave array, both address and chip enable must be valid long enough for a busy flag to be output from the master before the actual write pulse can be initiated with either the R/ W signal or the byte enables. Failure to observe this timing can result in a glitched internal write inhibit signal and corrupted data in the slave. |
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