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VLIW Digital Signal Processor

VLIW Digital Signal Processor. Michael Chang . Alison Chen . Candace Hobson . Bill Hodges. Introduction. Functionality ISA Implementation Functional blocks Circuit analysis Testing Off Chip Memory Status. Things to look for. Design Tradeoffs Register file size Multiple word sizes

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VLIW Digital Signal Processor

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  1. VLIW Digital Signal Processor Michael Chang . Alison Chen . Candace Hobson . Bill Hodges

  2. Introduction • Functionality • ISA • Implementation • Functional blocks • Circuit analysis • Testing • Off Chip Memory • Status

  3. Things to look for • Design Tradeoffs • Register file size • Multiple word sizes • Instruction set and implementation • Data forwarding • Software-controlled on chip cache • Shared Address/Data bus for off-chip data memory

  4. Instruction Set Architecture • 24-bit instruction words pack 3 sub-instructions: • Ex: SUB R5 R3, LDM R3 R6, BNEZ R1 • Register file - 8 registers • 3 bit encoding * 5 Reg. IDs = 15 bits per IW • Simple but useful Instruction Set • Multiply, Add/Subtract, Branch, Jump, Load Memory, Load intermediate, Load CCM • 2 Branch delay slots

  5. Microarchitecture • In order, 4 stage pipeline • IF, ID, EX, WB • 3 cycle pipeline stage • Data forwarding • Eliminate RAW hazards (ELEC 320, 425) • 5 forwarding paths • Control Logic • PLA controls pipeline • Initialize pipeline, reset Program Counter • Cycle through three cycles of pipeline stage

  6. Implementation Double Wide Silicon Floorplan

  7. Array Multiplier Ripple-Carry Adder Longest Paths: Add/Subtract: 10.74 ns through MSB Multiply: 15.87 ns through 10th product term ALU Design

  8. Compiler Controlled Memory (CCM) • Small on chip software controlled cache • Similar to Commercial DSPs • Predictable access time in real time • Benefits over off chip memory: • Double bandwidth • Software configurability • Reduced register “spill”/ “fill” pressure • Easily extendable

  9. Implementation of CCM • 4 12-bit lines of memory on chip (8 words) • Two registers, R6 and R7, for loading and storing • Two instructions, LDC and STC • 9-bit instruction • Three bit opcode • Five bit word line • Single bit determines single/double access • Example instruction: LDC 1 00001 (Reads CCM Line 1 into R6 and R7)

  10. ORCA Test Vector Generation Process • Goal: Greater accuracy and shorter time to verify chip functionality Assembly Code assembler Vector translator Binary Code IRSIM vectors

  11. ORCA Vector Suite • Goal: Create functional vectors to isolate specific chip cells to aid in post-silicon debug. • Register File • Compiler Controlled Memory • ALU • Branch • Data Forwarding • Pipeline

  12. ORCA Obsbus State Machine • Goal: Increase internal test signals to the IO’s by implementing a MUX. The MUX is controlled by output signals generated from a state machine. • 16:1 MUX, 6 output observability pins, 1 input observability pin • Allows observation of up to 96 internal signals using 7 pins. • The state machine changes state on each toggle of the input pin. • < IRSIM Obsbus PLA OUTPUT HERE>

  13. Obsbus Signals • Goal: Track an instruction execution through each of the pipeline stages. Fetch Decode Execute Write Back RegF output Forwarding signals OpCodes ALU input/output CCM input/output OpCodes RegF inputs Program Counter Branch Address OpCodes

  14. Off Chip Memory • Instruction memory • Regular static RAM (used previously in 422) • 8 bit addressing, 8 bit data reads • 28 = 256 words possible = 85 VLIW instructions • 70 ns read time • One read every cycle • Output address on clock A, latch data on clock B • One read/cycle * 8bits * 3 cycles/pipeline state = 24 bit VLIW

  15. Off Chip Memory, continued • Data memory (DS1609) • Shared Address/Data bus • PLA carefully designed to control memory • Uses worst case propagation delays • Timed signals using two out of phase clocks • Default PLA output latching on clock B • External latching on clock A to properly time signals • 50 ns read time

  16. Current Status • Functionality of major blocks tested • Instruction Fetch in final stages • ALU instructions implemented and working, including data forwarding • Memory instructions just need to be routed • Crystal and HSPICE analysis fifty percent complete • Global power, clock, and pin routing allocated in floorplan

  17. Conclusion • Solid fundamental ISA gives a nice “baby DSP” • Modular implementation of fundamental blocks • Design Decisions are well justified • Register file size • Instruction word length • Implementation balances timing and space • Access to off chip memory

  18. Questions?

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