1 COMP541 Memories, Part I Montek Singh Feb 27, 2007

2Topics  Overview of Memory Types ROMs: PROMs, FLASH, etc. ROMs: PROMs, FLASH, etc. RAMs RAMs  Random-Access Memory (RAM) Static today Static today Dynamic next Dynamic next

3 Properties of Memory  What would you want?

4 Non-Volatile Memory Technologies  Mask (old)  Fuses (old)  Electrically erasable

5 Details of ROM  Memory that is permanent  k address lines  2 k items  n bits

6 Notional View of Internals

7 Programmed Truth Table

8 Resulting Programming  In truth, they’re laid out in 2D (row, col)

9 Mask ROMs  Oldest technology  Originally “mask” used as last step in manufacturing Specify metal layer (connections) Specify metal layer (connections) Used for volume applications Used for volume applications Long turnaround Long turnaround Used for applications such as embedded systems and, in the old days, boot ROM Used for applications such as embedded systems and, in the old days, boot ROM

10 Programmable ROM (PROM)  First ones had fusible links  High voltage would blow out links  Fast to program  Single use

11 UV EPROM  Erasable PROM  Common technologies used UV light to erase complete device Took about 10 minutes Took about 10 minutes  Holds state as charge in very well insulated areas of the chip  Nonvolatile for several (10?) years

12EEPROM  Electrically Erasable PROM  Similar technology to UV EPROM  Erased in blocks by higher voltage  Programming is slower than reading  Some called flash memory Digital cameras, MP3 players, BIOS Digital cameras, MP3 players, BIOS Limited life Limited life Some support individual word write, some block Some support individual word write, some block One on Xess board has 5 blocks One on Xess board has 5 blocks Has a boot block that is carefully protected Has a boot block that is carefully protected

13 Random Access Memories  So called because it takes same amount of time to address any particular location This is not quite true for modern DRAMs This is not quite true for modern DRAMs  First look at asynchronous static RAM  Ones on Xilinx are synchronous Data available at clock edges, like registers Data available at clock edges, like registers

14 Simple View of RAM  Of some word size n  Some capacity 2 k  k bits of address line  Maybe have read line Strictly speaking may not need Strictly speaking may not need  Have a write line

15 1K x 16 memory  Variety of sizes From 1-bit wide From 1-bit wide  Issue is no. of pins  Memory size often specified in bytes This would be 2KB memory This would be 2KB memory  10 address lines and 16 data lines

16Writing  Sequence of steps Setup address lines Setup address lines Setup data lines Setup data lines Activate write line (maybe a pos edge) Activate write line (maybe a pos edge)

17Reading  Steps Setup address lines Setup address lines Activate read line Activate read line Data available after specified amt of time Data available after specified amt of time  Some use a clock

18 Chip Select  Usually a line to enable the chip  Why?

19Writing

20Reading

21 Static vs Dynamic RAM  SRAM vs DRAM  DRAM stores charge in capacitor Disappears after short period of time Disappears after short period of time Must be refreshed Must be refreshed  SRAM easier to use Uses transistors (think of it as latch) Uses transistors (think of it as latch) Faster Faster More expensive per bit More expensive per bit Smaller sizes Smaller sizes

22 Structure of SRAM  Control logic  One memory cell per bit Cell consists of one or more transistors Cell consists of one or more transistors Not really a latch made of NANDs/NORs, but logically equivalent Not really a latch made of NANDs/NORs, but logically equivalent

23 Bit Slice  Cells connected to form 1 bit position  Word Select gates one latch from address lines  Note it selects Reads also  B (and B’) set by R/W, Data In and BitSelect  Funny thing here when you write. What is it?

24 Bit Slice can Become Module  Basically bit slice is a X1 memory  Next

25 16 X 1 RAM  Now shows decoder

26Row/Column  If RAM gets large, there is a large decoder  Also run into chip layout issues  Larger memories usually “2D” in a matrix layout  Next Slide

27 16 X 1 RAM as 4 X 4 Array  Two decoders Row Row Column Column  Address just broken up  Not visible from outside on SRAMs

28 Change to 8 X 2 RAM  Minor change in logic  Also pinouts  What’s different?

29 Realistic Sizes  Imagine 256K memory as 32K X 8  One column layout would need 15-bit decoder with 32K outputs!  Can make a square layout with 9-bit row and 6-bit column decoders

30 SRAM Performance  Current ones have cycle times in low nanoseconds (say 2.5ns)  Used as cache (typically onchip or offchip secondary cache)  Sizes up to 8Mbit or so for today’s chips

31 RAM on FPGA  Ours has 10 4Kb blocks for a total of 40Kbits They call it block RAM They call it block RAM Can also use LUTs as RAM Can also use LUTs as RAM Block RAM: Two ports, and 5 possible layouts Block RAM: Two ports, and 5 possible layouts

32 Using from Verilog  Instantiate a block (here called R1) RAMB4_S8_S8 R1 (.DOA (data_a), RAMB4_S8_S8 R1 (.DOA (data_a),.DOB (data_b),.DOB (data_b),.ADDRA (addr_a),.ADDRA (addr_a),.ADDRB (addr_a),.ADDRB (addr_a),.CLKA (clk),.CLKA (clk),.CLKB (clk),.CLKB (clk),.DIA (data_in),.DIA (data_in),.DIB (data_in),.DIB (data_in),.ENA (ena),.ENA (ena),.ENB (enb),.ENB (enb),.RSTA (rsta),.RSTA (rsta),.RSTB (rstb),.RSTB (rstb),.WEA (wea),.WEA (wea),.WEB (web));.WEB (web));

33 Synthesizer can Infer  Careful how you specify (see ISE/XST manual) module inferRAM(clk, addr, data, we); input clk; input [8:0] addr;// 512 locations output [7:0] data;// by 8 bits input we; reg [7:0] mem [511:0]; reg [8:0] ra; (posedge clk) beginif(we) mem[addr] <= data; ra <= addr; end assign data = mem[ra]; endmodule

34 Can Initialize Block RAM  Have to do it two ways, one for simulator, another for hardware //synthesis attribute INIT_00 of R1 is "08192A3B4C5... total of 256 bits (64 hex characters)..." //synthesis attribute INIT_01 of R1 is "08192A3B4C5D6E7F08192A3B4C5D6E7F08192A3B4C5D6E7F08192A3B4C5D6E7F“ // Up to INIT_0F  Above is for hardware (next software)

35 For Simulation //synopsys translate_off defparam R1.INIT_00 = 64'h08192A3B4C5D6E7F08192A3B4C5D6E7F08192A3B4C5D6E7F08192A3B4 C5D6E7F; // 256-bit hex value defparam R1.INIT_01 = 64'h08192A3B4C5D6E7F08192A3B4C5D6E7F08192A3B4C5D6E7F08192A3B4 C5D6E7F; // 256-bit hex value... defparam R1.INIT_0F = 64'h08192A3B4C5D6E7F08192A3B4C5D6E7F08192A3B4C5D6E7F08192A3B4 C5D6E7F; // up to INIT_0F //synopsys translate_on

36 Look at Test Code  A RAM example Posted online (under Labs) Posted online (under Labs)  Note how memory values are specified Addresses go right-to-left, top-to-bottom Addresses go right-to-left, top-to-bottom See the Constraints Guide and Library manuals in Xilinx docs See the Constraints Guide and Library manuals in Xilinx docs

37 Wider Memory  What if you don’t have enough bit width?

38 Deeper Memory  Adding chips to increase storage, but keep same width  Need decoder

39Today  Fast look at non-volatile memory  Learned about Static RAM Specifics: synchronous RAM in FPGA Specifics: synchronous RAM in FPGA  Next: Dynamic RAM Complex, largest, cheap Complex, largest, cheap Much more trouble to use Much more trouble to use