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CPE 528: Session #12

CPE 528: Session #12 . Department of Electrical and Computer Engineering University of Alabama in Huntsville. Outline. Introduction Actel Logic Modules Xilinx LCA Altera FLEX, Altera MAX Power Dissipation. Introduction. Basic internal structure PLB – Programmable Logic Blocks

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CPE 528: Session #12

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  1. CPE 528: Session #12 Department of Electrical and Computer Engineering University of Alabama in Huntsville

  2. Outline • Introduction • Actel Logic Modules • Xilinx LCA • Altera FLEX, Altera MAX • Power Dissipation UAH-CPE528

  3. Introduction • Basic internal structure • PLB – Programmable Logic Blocks • PI – Programmable Interconnect • Types of basic logic cells • (1) multiplexer based • (2) look-up table based, and • (3) programmable array logic UAH-CPE528

  4. Actel ACT • Actel ACT basic logic cell – Logic Module • Actel ACT 1 uses just one type of Logic Module • Actel ACT 2 and 3 use two different types of Logic Modules UAH-CPE528

  5. ACTEL Act 1 Logic Module • (a) Organization of the basic logic cells. (b) The ACT 1 Logic Module. (c) An implementation using pass transistors (without any buffering). (d) An example logic macro. (Source: Actel.) UAH-CPE528

  6. Shannon Expansion Theorem • ExpansionF = F · 1 = F · (A + A’)= F · A + F · A‘ • Example: expand F with respect to AF = A' · B + A · B · C' + A' · B' · C= A · (B · C') + A' · (B + B' · C) • cofactor F wrt A = B · C‘, cofactor F wrt A’ = B + B' · C • F with respect to BF = B · (A' + A · C') + B' · (A' · C) • We can continue to expand a function until we reach the canonical form – a unique representation that uses only minterms • minterm is a product term that contains all the variables of F UAH-CPE528

  7. An Example • F = (A · B) + (B' · C) + D= (A · B) + (B' · C) + [D ·(B + B’)]= B·(A+D) + B’·(C+D) = B·F2 + B’·F1 • F2 = A + D = A + D·(A + A’) = A + A’·D = A·1 + A’·D • F1 = C + D = C + D·(C + C’) = C + C’·D = C·1 + C’·D • Implementation • A0 = D, A1 = 1, SA = C (F1) • B0 = D, B1 = 1, SB = A (F2) • S0 = 0, S1 = B UAH-CPE528

  8. Multiplexer Logic as Function Generators • Logic functions of 2 variables • 10 functions of these 16 can be implemented using just one 2:1 multiplexer(See Table 5.1 of the textbook) UAH-CPE528

  9. Multiplexer Logic as Function Generators (cont’d) • Useful functions • INV. The MUX acts as an inverter for one input only. • BUF. The MUX just passes one of the MUX inputs directly to the output. • AND. A two-input AND. • OR. A two-input OR. • AND1-1. A two-input AND gate with inverted input, equivalent to an NOR-11. • NOR1-1. A two-input NOR gate with inverted input, equivalent to an AND-11. UAH-CPE528

  10. Multiplexer Logic as Function Generators (cont’d) Figure 5.3 The ACT1 logic module as a boolean function generator. (a) A 2:1 MUX viewed as a logic wheel. (b) The ACT1 logic module viewed as two function wheels. UAH-CPE528

  11. An Example • F = NAND(A, B) • F = (A · B)’ = A’ + B' = A’ + B’·(A’ + A) = A’ + B’· A = 1·A’ + B’·A • Implementation • Wheel 1 => 1 (A0 = 1, A1 = 1, SA = 1) • Wheel 2 => B’ (B0 = 1, B1 = 0, SB = B) • MUX (1, B’, A) => S0 = A, S1 = 0 • Note: We do not have to worry how to use Logic Modules to construct combinational logic functions – e.g., we use NAND2 gate symbol and software takes care of connecting the inputs in the right way to the Logic Module UAH-CPE528

  12. ACT 2 and ACT 3 Logic Modules Figure 5.4 The ACT2 and ACT3 logic modules. (a) The C-module. (b) The ACT2 S-module. (c) The ACT3 S-module. (d) The equivalent circuit of the SE. (e) The SE configured as a positive edge-triggered D flip-flop. UAH-CPE528

  13. Timing Model and Critical Path • Exact delay values in Actel FPGAs can not be determined until interconnect delay is known – i.e., place and route are done • Critical path delay between registers is: tPD + tSUD + tCO • There is also a hold time for the flip-flops - tH • The combinational logic delay tPD is dependent on the logic function (which may take more than one LM) and the wiring delays • The flip-flop output delay tCO can also be influenced by the number of gates it drives (fanout) UAH-CPE528

  14. Worst-case timing • Max delays in CMOS occur when • operating under minimum voltage • maximum temperature • slow-slow process conditions (process variation which results in slow p-channel and slow n-channel transistors) • Electronic Equipment classes • Commercial. VDD = 5 V ± 5 %, T A (ambient) = 0 to +70 °C. • Industrial. VDD = 5 V ± 10 %, T A (ambient) = –40 to +85 °C. • Military: VDD = 5 V ± 10 %, T C (case) = –55 to +125 °C. • Military: Standard MIL-STD-883C Class B. • Military extended: Unmanned spacecraft. • Tj – junction temperature => temperature of the transistors on the chip • to calculate this we need power dissipated and thermal properties of the package UAH-CPE528

  15. Xilinx LCA • Xilinix LCA (Logic Cell Array) Configurable Logic Blocks (CLBs) • bigger and more complex than the Actel cells =>coarse-grain architecture • XC3000 CLB inputs • five logic inputs (A-E) • common clock input (K) • asynchronous direct-reset input (RD) • enable clock (EC) • XC3000 CLB outputs • X, Y • Using programmable MUXes connected to the SRAM programming cells we can independently connect each of the two CLB outputs to the outputs of flip-flops (QX, QY) or to the output of the combinational logic (F, G) UAH-CPE528

  16. XC3000 CLB Figure 5.6 The Xilinx XC3000 CLB (configurable logic block). UAH-CPE528

  17. XC3000 CLB (cont’d) • LUT – Look-Up Table • 32-bit LUT stored in 32 bits of SRAM • Suppose we need to implement the function F,F = A·B·C·D·E • set the content of LUT cell number 31 to 1 • clear the content of all other LUT cells (0 – 30) • apply the input variables as an address to the SRAM • only when ABCDE = ‘11111’ the output F will be ‘1’ • CLB propagation delay is fixed, equal to LUT access time and does not depend on the function implemented UAH-CPE528

  18. XC3000 CLB (cont’d) • Combinational block inputs/outputs • CLB inputs (A-E) • flip-flop outputs (QX, QY) • outputs from the LUT (F, G) • Using LUT • use five of seven inputs with the entire 32-bit LUT => CLB outputs F and G are identical • split the 32-bit LUT in half to implement two functions (outputs F and G) of four variables each • we can choose four input variables (A-E, QX, QY) • we have to choose two from the five CLB inputs (A-E) • split the 32-bit LUT in half, using one of seven input variables as a select input for a 2:1 MUX that switches between F and G UAH-CPE528

  19. XC4000 Logic Block • Two four-input LUTs that feed a three-input LUT • Special fast carry logic hard-wired between CLBs • MUX control logic maps four control inputs C1-C4 into the four inputs: • LUT input (H1) • direct in (DIN) • enable clock (EC) • set/reset control for flip-flops (S/R) • Control inputs C1-C4 can also be used to control the use of the F’ and G’ LUTs as 32 bits of SRAM UAH-CPE528

  20. XC4000 Logic Block Figure 5.7 The Xilinx XC4000 CLB (configurable logic block). UAH-CPE528

  21. XC5200 Logic Block • Basic Cell is called a Logic Cell (LC) and is similar to, but simpler than, CLBs in other Xilinx families • Term CLB is used here to mean a group of 4 LCs (LC0-LC3) Figure 5.8 The Xilinx XC5200 LC (logic cell) and CLB (configurable logic block). UAH-CPE528

  22. Xilinx CLB Analysis • Using LUTs to implement combinational logic • Disadvantage: an inverter is as slow as a five input NAND • Advantage: simplifies timing UAH-CPE528

  23. Altera FLEX (8000) • Basic Cell is called a Logic Element (LE) and resembles the Xilinx XC5200 LC architecture • Altera FLEX uses the same SRAM programming technology as Xilinx Figure 5.10 The Altera FLEX architecture. (a) Chip floorplan. (b) LAB (Logic Array Block). (c) Details of the LE (logic element). UAH-CPE528

  24. Altera MAX • Two-level logic circuit (sum of products) • Using regular structure • vector of buffers • vector of AND gates • OR gates UAH-CPE528

  25. Altera MAX (cont’d) • (a) Two-level logic circuit (sum of products) • (c) Programmable Array Logic (PAL) • Horizontal line:product term line (bit line) • Vertical line: word line UAH-CPE528

  26. Altera MAX (cont’d) • Programmed EPROM transistor has no effect on the product-term line • Unprogrammed EPROM acts as a pull-down tran. UAH-CPE528

  27. Registered PAL Figure 5.12 A registered PAL with I inputs, j product terms, and k macrocells. UAH-CPE528

  28. Logic Expanders • A logic expander is an output line of the AND array that feeds back as an input to the array itself • Logic expanders can help implement functions that require more product terms than are available in a simple PAL • Consider implementing this function in in a three-wide OR array: F = A’ · C · D + B’ · C · D + A · B + B · C’ • This can be rewritten as a “sum of (products of products): F = (A’ + B’) · C · D + (A + C’) · B F = (A · B)’ (C · D) + (A’ · C)’ · B • Logic expanders can be used to form the expander terms (A · B)’ and (A’ · C)’ • Logic expanders require an extra pass through the AND array, increasing delay UAH-CPE528

  29. Logic Expander Implementation Figure 5.13 Expander logic and programmable inversion. UAH-CPE528

  30. Logic Expanders (cont’d) • Programmable inversion • programming one input of the XOR gate at the macrocell output allows you to choose whether or not to invert the output • this can reduce the required number of product terms Figure 5.14 Use of programmed inversion to simplify logic. (a) The function F = A·B’+ A·C’+ A·D’+ A’·C·D requires four product terms to implement while (b) the complement F’ = A·B·C·D+ A’·D’+ A’·C’ requires only three product terms. UAH-CPE528

  31. Altera MAX architecture • Macrocell features: • Wide, programmable AND array • Narrow, fixed OR array • Logic Expanders • Programmable inversion Figure 5.15 The Altera MAX architecture. (a) Organization of logic and interconnect. (b) A MAX family LAB (Logic Array Block). (c) A MAX family macrocell. UAH-CPE528

  32. Timing Model Direct path through the logic array and register UAH-CPE528

  33. Timing Model Using parallel expander UAH-CPE528

  34. Timing Model Using a shared expander (two passes through the logic array) UAH-CPE528

  35. Power Dissipation in Complex PLDs • AND arrays consume static power • AND arrays in any PLD built using EPROM or EEPROM transistors use a passive pull-up (a resistor or current source) • Altera uses a switch called Turbo Bit to control the current in the programmable-AND array in each macrocell • MAX 7000 static current • 1.4 mA – 2.2 mA per macrocell in high-power mode • 0.6 mA – 0.8 mA per macrocell in low-power mode • MAX 9000 static current • 0.6 mA – 0.8 mA per macrocell in high-power mode • 0.3 mA per macrocell in low-power mode • MAX 9000: 16 macrocells in LAB, 35 LABs =>static power dissipation in low-power mode = 840mW UAH-CPE528

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