IS61WV25616FALL in a Medical Imaging Front End Build Step by Step

In an imaging front end, the SRAM is not storage. It is a timing device. The analog front end digitizes a row of detector channels at the rate the sensor demands, and the block behind it wants those samples in a different order, a different width, or at a different moment. The SRAM absorbs the mismatch.

The ISSI IS61WV25616FALL is 4 Mbit of asynchronous SRAM organized as 256K words by 16 bits, on a 1.65 V to 2.2 V rail, with a 10 ns access time at that rail. Active current is 35 mA maximum in the industrial grade, standby is 10 mA maximum in CMOS mode. It comes in a 44-pin TSOP II, a 48-ball mini BGA measuring 6 mm × 8 mm, or a 400-mil SOJ, and it needs 150 µs after power-up before the first access.

For a line buffer behind a 1.8 V FPGA bank, that is a clean fit, and the reason is the word "asynchronous". No clock, no refresh, no burst negotiation, so the latency of a read is the access time and nothing else. What it will not do is scale: the 8 Mbit part on the same rail runs at 20 ns, and the bus cannot be driven from 3.3 V logic without translation.

Stage 1 Defining the electrical envelope

The envelope comes from the sensor, not from the memory. A detector row is captured at a fixed rate, the FPGA needs a full row resident before it can run offset correction or bad-pixel replacement, and the buffer has to hold that row without stalling the capture path. Everything below follows from that, and every number is from the datasheet.

Two of those rows deserve a second look before anything else is decided. The first is the rail. 1.65 V to 2.2 V means a nominal 1.8 V supply with room for regulator tolerance, and it means the part sits directly on a 1.8 V FPGA bank. It also means a 3.3 V bank cannot drive it: the absolute maximum on any terminal is VDD + 0.5 V, and the input HIGH level is specified up to VDD + 0.2 V. A 3.3 V signal on a 1.8 V part is not marginal, it is outside the ratings.

The second is the cycle time. 10 ns is the number at 1.65 V to 2.2 V, and it is the number that holds across the full industrial range rather than at 25 °C. If the capture path needs a transfer every 10 ns, the buffer is doing one word per cycle with nothing left over for arbitration. Design the FPGA side so the SRAM is not the busiest thing in the pipeline.

Width and depth are coupled. 256K words at 16 bits is wide enough for a 12-bit converter padded to 16 bits, and deep enough for a row buffer without external paging. If a detector row does not fit, the answer is a different member of this family, which is the subject of Stage 2.

Stage 2 Picking the part and reading the datasheet

The first page advertises access times of 8 ns, 10 ns and 12 ns. All three are real, and not one of them applies to this part number at this voltage.

The operating range table on page 5 settles it: the FALL variant is 1.65 V to 2.2 V at 10 ns, and 8 ns belongs to the FBLL variant at 3.3 V ±10 %.

Budget 8 ns from the headline and you have a 2 ns hole in your timing on a 1.8 V rail.

The organization is 256K by 16, with separate upper and lower byte controls. UB# and LB# let you write one byte of the word, useful when a status byte has to land in the same word as a sample pair. Tie both low and every access moves the full word.

Current has two numbers and they mean different things. ICC is 35 mA maximum in the industrial grade at 10 ns with the part selected and cycling, and ISB2 is 10 mA maximum with CMOS levels and CS# high, against a 3 mA typical standby. In an imaging product the front end is idle between exposures, so the standby number is the one in the thermal budget.

No refresh and no clock is the whole reason this part is here. A DRAM or PSRAM needs refresh scheduling, and refresh steals cycles at unpredictable moments, which is the difference between a deterministic pipeline and one with an occasional extra cycle. What the asynchronous SRAM gives up is density per pin, and that trade is visible in the siblings.

IS61WV51216ALL is the direct density step: 512K words by 16 bits, 8 Mbit, on the same 1.65 V to 2.2 V rail. It pays in speed, because at that rail it is specified at 20 ns rather than 10 ns. Doubling the buffer doubles the cycle time, so check the capture path can absorb that before calling it an upgrade.

IS61WV10248ALL takes the other route to 8 Mbit: 1M words by 8 bits at 20 ns on the same rail. Twice the depth, half the bus, so a 16-bit path needs two devices and the FPGA pin count goes up.

IS61WV5128FALL keeps the 4 Mbit and cuts the bus instead: 512K by 8, 10 ns at 1.65 V to 2.2 V, in a 44-pin TSOP II or a 36-pin SOJ. If the detector outputs one byte lane, that is the same density with fewer FPGA pins.

IS61WV10248BLL and IS61WV5128FBLL are the same two dies on a 2.4 V to 3.6 V rail. They reach 10 ns there and 8 ns at 3.3 V, but they need the 3.3 V bank. Picking them for the faster headline is how a 1.8 V design ends up with level translators on every signal.

Stage 3 Layout and bring-up traps

The pinout is where bring-up time is lost. Address is A0 to A17, data is I/O0 to I/O15, control is CS#, OE#, WE#, UB# and LB#. Because the bus is 16 bits wide, A0 addresses a word, not a byte. Connect the FPGA's byte address A1 to the SRAM's A0 and every capture after a reset is shifted by one sample. It looks like a sensor fault and it is a netlist fault.

Do not leave UB# and LB# floating. A floating byte control gives writes that land on one lane or the other depending on the day. Tie both low if every access is a full word.

Do not use OE# as a chip select. OE# only gates the output buffers, so pulling it high during a write does not deselect the part, and the device stays active drawing ICC. Deselect with CS#, and use OE# to control when the FPGA drives the bus.

Power and ground arrive on multiple pins in the middle of each row, which is deliberate: the return path matters at this speed. Put the decoupling at those center pins rather than at the ends of the package, and keep the FPGA-to-SRAM traces short.

Watch the supply ramp. The device has an on-chip voltage sensor and needs 150 µs after VDD is stable before the first access. An FPGA that releases its memory controller early reads whatever the array contains, which shows up as a stripe in the first frame rather than as a bus error.

Finally, the input levels. At this rail, VIH is 1.4 V minimum and VIL is 0.4 V maximum, which a 1.8 V bank satisfies and a 3.3 V driver violates at the absolute maximum. Translate anything still running at 3.3 V before it reaches this bus.

Stage 4 What carries into production

The ordering table decides what you can actually buy. In the industrial range at 1.65 V to 2.2 V the listed numbers are IS61WV25616FALL-10BI, -10BLI, -10B2I, -10B2LI and -10TLI. B is the 48-ball mini BGA, T is the 44-pin TSOP II, and L marks lead-free. B2 is the switched-IO ballout, a different BGA pin arrangement on the same die, and B and B2 are not the same footprint.

Note what is missing. The commercial range table lists only IS61WV25616FBLL-10TL, the 3.3 V part. There is no listed commercial order code for the 1.8 V FALL variant, so that build has to be agreed with the supplier rather than assumed from the operating range table.

Above +85 °C the family continues as IS64WV25616FALL at −40 °C to +125 °C on the same 1.65 V to 2.2 V rail, and only at 12 ns. The automotive suffix changes the temperature and the speed together.

On consistency, the AC and DC tables are specified over the operating range, not at room temperature. A 10 ns part is 10 ns at −40 °C and at +85 °C, so the timing closure from the bench is the timing you ship.

Two things the datasheet does not answer. It has no packing section, so confirm tape and reel versus tray against the exact suffix, and it says nothing about lifecycle. Ask for the PCN and EOL position before a multi-year build, not after the last-time buy.

The vendor's datasheet excludes life support applications without a written agreement. An imaging front end is a diagnostic tool rather than a life support device, but that call belongs in your regulatory file, not in a purchasing decision.

Frequently asked questions

Sourcing note

The IS61WV25616FALL ships through Mandu with the full ISSI specification sheet. Send us the exact suffix, the packing form you need and your annual quantity, and we will confirm the orderable number, the tape and reel or tray option behind it and the lifecycle position before you place the first order.