Programming Technologies in FPGA

Technology of Programmable Elements/ Programming Technologies

--Programming technology is Vary from vendor to vendor.
--All share the common property: Configurable in one of the two positions – ‘ON’ or ‘OFF’

Programming Technologies Can be classified into three categories:
1. SRAM based
2.Fuse based (Anti fuse)
3. EPROM/EEPROM/Flash based


Desired propertiesof Programming elements:

--Minimum area consumption
-- Low on resistance;
--High off resistance
-- Low parasitic capacitance to the attached wire
-- Reliability in volume production

1.SRAM Programming Technology


Features of SRAM promming Technologies are

--It Employs SRAM (Static RAM) cells to control pass transistors and/or transmission gates
-- SRAM cells control the configuration of logic block as well
-- Volatile
-- Needs an external storage
--Needs a power-on configuration mechanism
-- In-circuit re-programmable
-- Lesser configuration time
--Occupies relatively larger area

Pass Transistor using SRAM cell is shown ia fig.A
Multiplexer using SRAM cell is shown in fig.B









2.Anti-fuse Programming Technology

Properties of anti-fuse programming technology are

-- Though implementation differ, all anti-fuse programming elements share common property
-- Uses materials which normally resides in high impedance state
-- But can be fused irreversibly into low impedance state by applying high voltage

Anti-fuse Programming element is shown in fig

-- Very low ON Resistance (Faster implementation of circuits)
-- Limited size of anti-fuse elements; Interconnects occupy relatively lesser area
(Offset : Larger transistors needed for programming)
--One Time Programmable
-- Cannot be re-programmed
-- Design changes are not possible
-- Retain configuration after power off

3.EPROM, EEPROM or Flash Based Programming Technologies


(i)EPROM Programming Technology

EPROM Programming element(PE) is shown in fig
--It has Two gates: Floating and Select gate

-- Normal mode:
- No charge on floating gate
- Transistor behaves as normal n-channel transistor

-- Floating gate charged by applying high voltage
- Threshold of transistor (as seen by gate) increases
- Transistor turned off permanently

-- Re-programmable by exposing to UV radiation
--can be Used as pull-down devices
--Consumes static power
-- No external storage mechanism
-- Re-programmable (Not all!)
--Not in-system re-programmable
-- Re-programming is a time consuming task

EEPROM Programming Technology
 Two gates: Floating and Select
 Functionally equivalent to EPROM; Construction and structure differ
 Electrically Erasable: Re-programmable by applying high voltage
(No UV radiation expose!)
 When un-programmed, the threshold (as seen by select gate) is negative!

(ii)EEPROM Programming Technology

EEPROM Programming technology is

--It also has Two gates: Floating and Select gate
--It is Functionally equivalent to EPROM;
-- Construction and structure differ
-- Electrically Erasable --Re-programmable by applying high voltage
(No UV radiation expose!)

-- When un-programmed, the threshold (as seen by select gate) is negative!


--Re-programmable; In general, in-system re-programmable
-- Re-programming consumes lesser time compared to EPROM technology
--Multiple voltage sources may be required
-- Area occupied is twice that of EPROM!


for further details refer the links below..

http://books.google.co.in/books?id=8s4M-qYOWZIC

http://books.google.co.in/books?id=-jHkdnr8ezIC&printsec=frontcover&dq=antifuse+
programming+technology+in+fpga#PPP1,M1