IDT72T40108

The IDT72T40108 are exceptionally deep, extremely high speed, CMOS First-In-First-Out (FIFO) memories with the ability to read and write data on both rising and falling edges of clock. The device has a flexible x40/x20/x10 Bus-Matching mode and the option to select single or double data rates for input and output ports. These FIFOs offer several key user benefits:
• Flexible x40/x20/x10 Bus-Matching on both read and write ports
• Ability to read and write on both rising and falling edges of a clock
• User selectable Single or Double Data Rate of input and output ports
• A user selectable MARK location for retransmit
• User selectable I/O structure for HSTL or LVTTL
• The first word data latency period, from the time the first word is written to an empty FIFO to the time it can be read, is fixed and short.
• High density offerings up to 5Mbit
• High speed operation of up to 250MHz

Bus-Matching Double Data Rate FIFOs of the IDT72T40108 are particularly appropriate for network, video, telecommunications, data communications and other applications that require fast data transfer on both rising and falling edges of the clock. This is a great alternative to increasing data rate without extending the width of the bus or the speed of the device. They are also effective in applications that need to buffer large amounts of data and match buses of unequal sizes.

Each FIFO of the IDT72T40108 has a data input port (Dn) and a data output port (Qn), both of  which can assume either a 40-bit, 20-bit, or a 10-bit width as determined by the state of external control pins Input Width (IW), Output Width (OW), and Bus- Matching (BM) pin during the Master Reset cycle.

The input port of the IDT72T40108 is controlled by a Write Clock (WCLK) input and a Write Enable (WEN) input. Data present on the Dn data inputs can be written into the FIFO on every rising and falling edge of WCLK when WEN is asserted and Write Single Data Rate (WSDR) pin held HIGH. Data can be selected to write only on the rising edges of WCLK if WSDR is asserted. To guarantee functionality of the device, WEN must be a controlled signal and not tied to ground. This is important because WEN must be HIGH during the time when the Master Reset (MRS) pulse is LOW. In addition, the WSDR pin must be tied HIGH or LOW. It is not a controlled signal and cannot be changed during FIFO operation.

Write operations of the IDT72T40108 can be selected for either Single or Double Data Rate mode. For Single Data Rate operation, writing into the FIFO requires the Write Single Data Rate (WSDR) pin to be asserted. Data will be written into the FIFO on the rising edge of WCLK when the Write Enable (WEN) is asserted. For Double Data Rate operations, writing into the FIFO requires WSDR to be deasserted.  Data will be written into the FIFO on both rising and falling edge of WCLK when WEN is asserted.

The output port of the IDT72T40108 is controlled by a Read Clock (RCLK) input and a Read Enable (REN) input. Data is read from the FIFO on every rising and falling edge of RCLK when REN is asserted and Read Single Data Rate (RSDR) pin held HIGH. Data can be selected to read only on the rising edges of RCLK if RSDRis asserted. To guarantee functionality of the device, REN must be a controlled signal and not tied to ground. This is important because REN must be HIGH  during the time when the Master Reset (MRS) pulse is LOW. In addition, the RSDR pin must be tied HIGH or LOW. It is not a controlled signal and cannot be changed during FIFO operation.

Read operations of the IDT72T40108 can be selected for either Single or Double Data Rate mode. Similar to the write operations, reading from the FIFO in single data rate requires the Read Single Data Rate (RSDR) pin to be asserted. Data will be read from the FIFO on the rising edge of RCLK when the Read Enable (REN) is asserted. For Double Data Rate operations, reading into the FIFO requires RSDR to be deasserted. Data will be read out of the FIFO on both rising and falling edge of RCLK when and REN is asserted.

Both the input and output port of the IDT72T40108 can be selected for either 2.5V LVTTL or HSTL operation. This can be achieved by tying the HSTL signal LOW for LVTTL or HIGH for HSTL voltage operation. When the read port is setup for HSTL mode, the Read Chip Select (RCS) input also has the benefit of disabling the read port inputs, providing additional power savings.

There is the option of selecting different data rates on the input and output ports of the IDT72T40108. There are a total of four combinations to choose from, Double Data Rate to Double Data Rate (DDR to DDR), DDR to Single Data Rate (DDR to SDR), SDR to DDR, and SDR to SDR. The rates can be set up using the WSDRand RSDR pins. For example, to set up the input to output combination of DDR to SDR, WSDR will be HIGH and RSDR will be LOW. Read and write operations are initiated on the rising edge of RCLK and WCLK respectively, never on the falling edge. If REN or WEN is asserted after a rising edge of clock, no read or write operations will be possible on the falling edge of that same pulse.

An Output Enable (OE) input of the IDT72T40108 is provided for high-impedance control of the outputs. A read Chip Select (RCS) input is also provided for synchronous enable/disable of the read port control input, REN. The RCS input is synchronized to the read clock, and also provides high-impedance controls to the Qn data outputs. When RCS is disabled, REN will be disabled internally and the  data outputs will be in High-Impedance. Unlike the Read Chip Select signal however, OE is not synchronous to RCLK. Outputs are high-impedanced shortly after a delay time when the OE transitions from LOW to HIGH.

The Echo Read Enable (EREN) and Echo Read Clock (ERCLK) outputs of the IDT72T40108 are used to provide tighter synchronization between the data being transmitted from the Qn outputs and the data being received by the input device. These output signals from the read port are required for high-speed data communications. Data read from the read port is available on the output bus with respect to EREN and ERCLK, which is useful when data is being read at high-speed operations where synchronization is important.

The frequencies of both the RCLK and WCLK signals may vary from 0 to fMAX with complete independence. There are no restrictions on the frequency of one clock input with respect to another.

There are two possible timing modes of operation with these devices: IDT Standard mode and First Word Fall Through (FWFT) mode.

In IDT Standard mode of the IDT72T40108, the first word written to an empty FIFO will not appear on the data output lines unless a specific read operation is performed. A read operation, which consists of activating REN and enabling a rising RCLK edge, will shift the word from internal memory to the data output lines. Be aware that in Double Data Rate (DDR) mode only the IDT Standard mode is available.

In FWFT mode, the first word written to an empty FIFO is clocked directly to the data output lines after three transitions of RCLK. A read operation does not have to be performed to access the first word written to the FIFO. However, subsequent words written to the FIFO do require a LOW on REN for access. The state of the FWFT input during Master Reset determines the timing mode in use.

For applications requiring more data storage capacity than a single FIFO can provide, the FWFT timing mode permits depth expansion by chaining FIFOs  in series (i.e. the data outputs of one FIFO are connected to the corresponding data inputs of the next). No external logic is required.

These FIFOs of the IDT72T40108 have four flag pins,EF/OR (Empty Flag or Output Ready), FF/ IR (Full Flag or Input Ready), PAE (Programmable Almost-Empty flag), and PAF (Programmable Almost-Full flag). The EF and FF functions are selected in IDT Standard mode. The IR and OR functions are selected in FWFT mode. PAE and PAF are always available for use, irrespective of timing mode.

PAE and PAF flags of the IDT72T40108 can be programmed independently to switch at any point in memory. Programmable offsets mark the location within the internal memory that activates the PAE and PAF flags and can only be programmed serially. To program the offsets, set SEN active and data can be loaded via the Serial Input (SI) pin at the rising edge of SCLK. To read out the offset registers serially, set SREN active and data can be read out via the Serial Output (SO) pin at the rising edge of SCLK. Four default offset settings are also provided, so that PAE can be marked at a predefined number of locations from the empty boundary and the PAF threshold can also be marked at similar predefined values from the full boundary. The default offset values are set during Master Reset by the state of the FSEL0 and FSEL1 pins.

During Master Reset (MRS) of the IDT72T40108, the following events occur: the read and write pointers are set to the first location of the internal FIFO memory, the FWFT pin selects IDT Standard mode or FWFT mode, the bus width configuration of the read and write port is determined by the state of IW and OW, and the default offset values for the programmable flags are set.

The Partial Reset (PRS) of the IDT72T40108 also sets the read and write pointers to the first location of the memory. However, the timing mode and the values stored in the programmable offset registers before Partial Reset remain unchanged. The flags are updated according to the timing mode and offsets in effect. PRS is useful for resetting a device in mid-operation, when reprogramming programmable flags would be undesirable.

The timing of the PAE and PAF flags are synchronous to RCLK and WCLK, respectively. The PAE flag is asserted upon the rising edge of RCLK only and not WCLK. Similarly the PAF is asserted and updated on the rising edge of WCLK only and not RCLK.

This device includes a Retransmit from Mark feature that utilizes two control inputs, MARK and RT (Retransmit). If the MARK input is enabled with respect to the RCLK, the memory location being read at the point will be marked. Any subsequent retransmit operation (when RT goes LOW), will reset the read pointer to this "marked" location.

The IDT72T40108 can be configured with different input and output bus widths as previously stated. These rates are: x40 to x40, x40 to x20,x40 to x10, x20 to x40, and x10 to x40.

If, at any time, the FIFO is not actively performing an operation, the chip will automatically power down. Once in the power down state, the standby supply current consumption is minimized. Initiating any operation (by activating control inputs) will immediately take the device out of the power down state.

A JTAG test port of the IDT72T40108 is provided, here the FIFO has fully functional boundary Scan feature, compliant with IEEE 1449.1 Standard Test Access Port and Boundary Scan Architecture.

The Double Data Rate FIFO of the IDT72T40108 has the capability of operating in either LVTTL or HSTL mode. HSTL mode can be selected by enabling the HSTL pin. Both input and output ports will operate in either HSTL or LVTTL mode, but cannot be selected independent of one another.

The IDT72T40108 are fabricated using IDT's high-speed submicron CMOS technology.