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Lecture 7

*Digital System Design

Resource: Xilinx, Aleksandra Kovacevic

Introduction

Comparison of Standard Logic Circuits and Programmable Logic Circuits

Evolution and Overview of PLC: PROM, PLA, PAL

CPLD

FPGA

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Logic Circuits

Standard Logic Circuits Programmable Logic Circuits

• Realize single function or set of functions, once defined and with no possibility of changing.

• Contains great number of standard logic circuits

• Possibility of realizing many various functions

• Hardware can configure any time by user programming.

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Standard Logic Circuits

• Appropriate for many applications because of possibility of realization in mass production for relative low cost.

• Standard logic circuits are sometimes the best choice in high-performance devices.

• Disadvantage: Not permitting design updates (function changes) with no hardware replacement necessary.

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Field- Programmable Logic Devices

• Component function is defined by users program.• Advantages:

- Ease of design changes- Reduce prototype-product time- Large scale integration (over 100 000 gates)- Reliability increased, low financial risk- Smaller device, low start-up cost

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FPLDs Representatives

• PLA - Programmable Logic Arrays

• PAL - Programmable Array Logic

• CPLD - Complex Programmable Logic Devices

• FPGA - Field Programmable Gate Arrays

PLD

Programmable logic device

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Evolution of PLD: Why not PROM?• A special device (called a burner),

used to put the information, supplies an electrical current to specific cells in the ROM that effectively blows a fuse in them = burning the PROM. From that point on, chip is read-only.

• PROM was the first type of user-programmable chip; address lines = logic circuit inputs data lines = logic circuit outputs

• PROMs are inefficient architecture for realizing logic circuit:

Logic functions rarely require more than few product terms

PROM contains a full decoder for its address inputs. 6/13

Evolution of PLD: PLA• PLA was the first

device developed for implementing

• Consist of two levels of logic gates - programmable “wired” AND-plane & OR-plane

),...,(),...,( 11 nn xxxxf

Note:

• Drawbacks:• Expensive to

manufacture

• Offered somewhat poor speed-performance 7/13

Evolution of PLD: PAL™• Overcame weaknesses of

PLA

• Single level of programmability - consists of a programmable “wired” AND-plane & fixed OR-gates

• Simpler to program and cheaper implementation

• Limited numbers of terms in each output

Note:PAL is a trademark of Advanced Micro Devices

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Evolution of PLD: Register PLA

• Contain flip flops connected to the OR gate outputs

• Importance:• Profound effect on

digital hardware design

• Basis for more sophisticated architectures

sequential circuits can be realized

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Evolution of PLD: CPLD

• Technology advanced

possibility to produce devices with higher capacity than SPLDs.• Structure grows too quickly in size

as the number of inputs is increased

• Integrating multiple SPLDs onto a single chip - the only feasible way to provide large capacity devices based on SPLD

• Programmably connect the SPLD blocks together

• Logic capacity up to the equivalent of about 50 typical SPLD devices

Logic Array Blocks

- Complex SPLD-like structure

Programmable

Interconnect Array

- Capable of connecting

any LAB input or

output to any other

LAB

10/13

... and finally...

11/13

Evolution of PLD: FPGA

• Difficult extending CPLDs architectures to higher densities - a different approach is needed

• FPGAs comprise an array of uncommited circuit elements, called logic blocks, and interconnect resources

• FPGA configuration is performed through programming by the end user.Xilinx FPGA

Configuration

contains a set of basic

functions (gates, FFs,

memory cells)

12/13

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*FPGA

*A field-programmable gate array (FPGA) is a logic device that contains a two-dimensional array of generic logic cells and programmable switches.

Resource1: FPGA Prototyping By Verilog Examples, Pong P. Chu, Wiley, 2008,

Resource2: Xilinx DS099 Design Specification,

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*FPGA

*A logic cell can be configured (i.e., programmed) to perform a simple function

*A programmable switch can be customized to provide interconnections among the logic cells

*A custom design can be implemented by specifying the function of each logic cell and selectively setting the connection of each programmable switch

*Once the design and synthesis are completed, we can use a simple adaptor cable to download the desired logic cell and switch configuration to the FPGA device

*Since this process can be done "in the field" rather than "in a fabrication facility (fab)," the device is known as field programmable.

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*Look-up table (LUT) based logic cell

* A logic cell usually contains a small configurable combinational circuit with a D-type flip-flop (DFF)

* The most common method to implement a configurable combinational circuit is a look-up table (LUT). An n-input LUT can be considered as a small 2n-by-1 memory

* By properly writing the memory content, we can use a LUT to implement any n-input combinational function

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*Macro cell

*Most FPGA devices also embed certain macro cells or macro blocks,

*These are designed and fabricated at the transistor level and their functionalities complement the general logic cells

*Commonly used macro cells include memory blocks, combinational multipliers, clock management circuits, and I/0 interface circuits

*Advanced FPGA devices may even contain one or more prefabricated processor cores

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*Xilinx Spartan3 devices

*Logic cell

* The most basic element of the Spartan-3 device is a logic cell (LC), which contains a four-input LUT and a DFF

*Slice

* In Xilinx terms, two logic cells are grouped to form a slice

*CLB

* Four slices are grouped to form a configurable logic block (CLB)

*Macro Cell

* The Spartan-3 device contains four types of macro blocks: combinational multiplier, block RAM, digital clock manager (DCM), and input/output block (IOB)

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*Xilinx Spartan3 devices

*Array Multiplier

* The combinational multiplier accepts two 18-bit numbers as inputs and calculates the product

*Block RAM

* The block RAM is an 18K-bit synchronous SRAM that can be arranged in various types of configurations

*DCM

* A DCM uses a digital-delayed loop to reduce clock skew and to control the frequency and phase shift of a clock signal

*IOB* An IOB controls the flow of data between the device's

I/0 pins and the internal logic. It can be configured to support a wide variety of I/0 signaling standards.

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*Spartan-3 Family Architecture

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*Spartan-3 Simplified IOB Diagram

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*Devices in the Spartan-3 family

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*Spartan-3 Device I/O Chart

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* Spartan-3 BGA Package Marking Example for Part Number XC3S1000-4FT256C

*Spartan-3 FPGA QFP Package for Part Number XC3S400-4PQ208C

25*DlGlLENT S3

BOARD

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*DlGlLENT S3 BOARD

1. Xilinx Spartan-3 XC3S200 FPGA device (XC3S200FT256)

2. 2M-bit Xilinx XCF02S platform flash configuration PROM

3. Jumper to select the configuration source

4. Two 256K-by-16 asynchronous SRAM devices (ISSI IS61LV25616AL-10T) VGA display port

6. RS-232 serial port

7. RS-232 transceiverl voltage-level convertor

8. Second RS-232 transmit and receive channel

9. PSI2 mouse/keyboard port

10. Four-digit seven-segment LED display

11. Eight slide switches

12. Eight discrete LED outputs

13. Four momentary-contact pushbutton switches

14. 50-MHz crystal oscillator clock source

15. Socket for an auxiliary crystal oscillator clock source

16. Jumper to select an FPGA configuration mode

17. Pushbutton switch to force FPGA reconfiguration

18. LED to indicate whether the FPGA is successfully configured

1 9. 40-pin expansion connector 1 (labeled B1)

20. 40-pin expansion connector 2 (labeled A2)

2 1. 40-pin expansion connector 3 (labeled A1 )

22. JTAG connector for Digilent download cable

23. Digilent low-cost download cable (included in the S3 kit but not shown in Figure 2.3)

24. JTAG port (to be used with the Xilinx Parallel Cable IV and MultiPRO Desktop Tool,which are not included in the S3 kit)

25. Power connector for an unregulated 5-V power supply (included in the S3 kit)

26. Power-on LED indicator

27. 3.3-V voltage regulator

28. 2.5-V voltage regulator

29. 1.2-V voltage regulator

30. Selector for PS2 port voltage supply (3.3 or 5 V)