Chapter- 9 Programmable Logic Devices DHADUK ANKITA ENRL NO Noble Engineering College- Junagadh

PLDs  Programmable Logic Devices (PLD) General purpose chip for implementing circuits Can be customized using programmable switches  Main types of PLDs PLA PAL ROM CPLD FPGA  Custom chips: standard cells, sea of gates

PLD as a Black Box Logic gates and programmable switches Inputs (logic variables) Outputs (logic functions)

Programmable Logic Array (PLA)  Use to implement circuits in SOP form  The connections in the AND plane are programmable  The connections in the OR plane are programmable f 1 AND plane OR plane Input buffers inverters and P 1 P k f m x 1 x 2 x n x 1 x 1 x n x n

Gate Level Version of PLA f 1 = x 1 x 2 +x 1 x 3 '+x 1 'x 2 'x 3 f 2 = x 1 x 2 +x 1 'x 2 'x 3 +x 1 x 3

Customary Schematic of a PLA f 1 = x 1 x 2 +x 1 x 3 '+x 1 'x 2 'x 3 f 2 = x 1 x 2 +x 1 'x 2 'x 3 +x 1 x 3 f 1 P 1 P 2 f 2 x 1 x 2 x 3 OR plane AND plane P 3 P 4 x marks the connections left in place after programming

Limitations of PLAs  PLAs come in various sizes Typical size is 16 inputs, 32 product terms, 8 outputs  Each AND gate has large fan-in  this limits the number of inputs that can be provided in a PLA  16 inputs  3 16 = possible input combinations; only 32 permitted (since 32 AND gates) in a typical PLA  32 AND terms permitted  large fan-in for OR gates as well This makes PLAs slower and slightly more expensive than some alternatives to be discussed shortly

Programmable Array Logic (PAL)  Also used to implement circuits in SOP form  The connections in the AND plane are programmable  The connections in the OR plane are NOT programmable f 1 AND plane OR plane Input buffers inverters and P 1 P k f m x 1 x 2 x n x 1 x 1 x n x n fixed connections

Example Schematic of a PAL f 1 P 1 P 2 f 2 x 1 x 2 x 3 AND plane P 3 P 4 f 1 = x 1 x 2 x 3 '+x 1 'x 2 x 3 f 2 = x 1 'x 2 '+x 1 x 2 x 3

Comparing PALs and PLAs  PALs have the same limitations as PLAs (small number of allowed AND terms) plus they have a fixed OR plane  less flexibility than PLAs  PALs are simpler to manufacture, cheaper, and faster (better performance)  PALs also often have extra circuitry connected to the output of each OR gate The OR gate plus this circuitry is called a macrocell

Macrocell f 1 back to AND plane DQ Clock Select Enable Flip-flop OR gate from PAL 0101

Macrocell Functions  Enable = 0 can be used to allow the output pin for f 1 to be used as an additional input pin to the PAL  Enable = 1, Select = 0 is normal for typical PAL operation  Enable = Select = 1 allows the PAL to synchronize the output changes with a clock pulse  The feedback to the AND plane provides for multi- level design f 1 back to AND plane DQ Clock Select Enable 0101

CPLD  Complex Programmable Logic Devices (CPLD)  SPLDs (PLA, PAL) are limited in size due to the small number of input and output pins and the limited number of product terms Combined number of inputs + outputs < 32 or so  CPLDs contain multiple circuit blocks on a single chip Each block is like a PAL: PAL-like block Connections are provided between PAL-like blocks via an interconnection network that is programmable Each block is connected to an I/O block as well

Structure of a CPLD

Internal Structure of a PAL-like Block  Includes macrocells Usually about 16 each  Fixed OR planes OR gates have fan-in between 5-20  XOR gates provide negation ability XOR has a control input

More on PAL-like Blocks  CPLD pins are provided to control XOR, MUX, and tri-state gates  When tri-state gate is disabled, the corresponding output pin can be used as an input pin The associated PAL-like block is then useless  The AND plane and interconnection network are programmable  Commercial CPLDs have between PAL-like blocks

Programming a CPLD  CPLDs have many pins – large ones have > 200 Removal of CPLD from a PCB is difficult without breaking the pins Use ISP (in system programming) to program the CPLD JTAG (Joint Test Action Group) port used to connect the CPLD to a computer

Example CPLD  Use a CPLD to implement the function f = x 1 x 3 x 6 ' + x 1 x 4 x 5 x 6 ' + x 2 x 3 x 7 + x 2 x 4 x 5 x 7

Read-Only Memory (ROM)  A semi-conductor memory is a device where data can be stored and retrieved.  Logically, this memory device can be regarded as a table of memory cells (data). 0123::::n0123::::n Addresses 1-word data 1-bit data :::: word size

ROM  A ROM (Read Only Memory) has a fixed AND plane and a programmable OR plane  Size of AND plane is 2 n where n = number of input pins Has an AND gate for every possible minterm so that all input combinations access a different AND gate  OR plane dictates function mapped by the ROM

4x4 ROM  2 2 x4 bit ROM has 4 addresses that are decoded 3 d 2 d 1 d 0 2 -to-4 decoder a 0 a 1 d

Read-Only Memory (ROM)  A Read-Only Memory (ROM) is a memory device where data are read from, but not written to.  Writing is done at time of customisation, or, by special programming devices (programmable ROM).  Any Boolean expression can be implemented using ROM. Procedure: Obtain a truth table, treat the inputs as addresses and outputs as data.  Advantage: Boolean functions directly implemented.  Disadvantages: Don’t care conditions not used, and limited input variables (e.g. 10 inputs – 1K, 16 inputs – 64K, 20 inputs – 1M).

Read-Only Memory (ROM)  Different types of ROM devices available:  ROM: Read-Only Memory Data written into memory by mask programming during manufacturing time. Expensive start-up cost but economical for high volume. Cannot be erased after data are programmed in.  PROM: Programmable ROM This type of memory comes from the manufacturer without any data stored in it, i.e. Empty. The data pattern is programmed electrically by user using a special circuit known as PROM programmer. It can be programmed only once during its life time. Once programmed, the data cannot be altered. This type of memory is known as PROM. These are highly suited for high volume usage due to their low cost of production.

Read-Only Memory (ROM)  EPROM: Erasable PROM In this type of memory, data can be written any number of times, i.e. they are programmable. Before it is programmed, the contents already stored are erased by exposing the chip to ultraviolet radiation for about 30 minutes. This type of memory is referred to as EPROM. EPROMS are possible in MOS technology. Programming is done using a PROM programmer.  EEPROM: Electrically Erasable PROM This is another type of programmable memory in which erasing is done electrically rather than exposing the chip to ultraviolet radiation. It is referred to as EEPROM or electrically alterable ROM (EAPROM).

Programmable Read-Only Memory (PROM)  Devices with fixed AND array (which is a decoder) and programmable OR array.  The AND array (decoder) generates all 2 n possible minterm products of its n inputs (often referred to as n- to-2 n decoder).  n input lines, m output lines.  Bit combination of input variables – address.  Bit combination of output lines – word (each word contains m bits).

Programmable Read-Only Memory (PROM) I0I0 I1I1 I2I2 m0m0 m2m2 m3m3 m5m5 m4m4 m1m1 m7m7 Programmable OR array Fixed AND array X X XX X X X XX X XXXX X O1O2OkO1O2Ok X X X X X X X X X m6m Minterms Programmable read-only memory (PROM) can realize K functions f(I 2,I 1,I 0 ).

Programmable Logic Devices Fixed AND array Fused programmable OR array Fuses InputsOutputs Programmable Read Only Memory (PROM) Fixed OR array Fused programmable AND array Fuses InputsOutputs Programmable Array Logic (PAL) Inputs Fused programmable OR array Fuses Outputs FusesFused programmable AND array Programmable Logic Array (PLA)