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During placement and routing, most of the placement tools, place/move logic cells based on floorplan specifications. Some of the important or critical cell's locations has to be pre-defined before actual placement and routing stages. The critical cells are mostly the cells related to clocks, viz. clock buffers, clock mux, etc. and also few other cells such as RAM's, ROM,s etc. Since, these cells are placed in to core before placement and routing stage, they are called 'preplaced cells'.
3. Let’s explore how a city is planned
Railway
Hospital
Station
Water
Playground
Storage Tank
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4. Basic necessities and utilities are pre-planned and positioned,
in a manner to have the best reachability by each citizen
Railway
Hospital
Station
Water
Playground
Storage Tank
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5. In a similar fashion, we start planning the chip area, by prioritizing the location
of critical cells.
Die
Core
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6. Crtical cells can be IP’s (memories, ALU, etc.) or std cells (clock Buffer, clock inverter, etc )
Die
Core
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7. Let us understand, what are IP’s, std cells and their architecture?
Die
Core
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8. IP is a reusable unit of logic, cell, or chip layout design that is the intellectual
property designed by any individual.
IP are also sometimes offered as generic gate-level netlist. i.e. standard cells,
and complex cells.
It consists of transistor level layout for logical cells and complex cells, which
are implemented using layout tools.
Lets have a look into internal of IP’s
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9. Consider one of the most commonly used IP i.e. CMOS Inverter
Vdd Vdd
In Inverter Out In Out
Vss Vss
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10. CMOS Inverter consist of P-MOS Transistor
Vdd
PMOS – P Diff
In Out
Vss
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11. CMOS Inverter consist of N-MOS Transistor
Vdd
PMOS – P Diff
In Out
NMOS – N Diff
Vss
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12. CMOS Inverter consist of Polysilicon Gate.
Vdd
Poly Gate
PMOS – P Diff
In Out
NMOS – N Diff
Vss
Note : At the component level, polysilicon has been used as the conducting gate
material in MOSFET and CMOS processing technologies.
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13. CMOS Inverter IN/ OUT Lines
Vdd
Poly Gate
PMOS – P Diff
In Out In
NMOS – N Diff
Vss
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14. CMOS Inverter IN/ OUT Ports
Vdd
Poly Gate
PMOS – P Diff
In Out In Out
NMOS – N Diff
Vss
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15. CMOS Inverter Vdd & Vss Ports.
Vdd
Vdd
Poly Gate
PMOS – P Diff
In Out In Out
NMOS – N Diff
Vss
Vss
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16. Lets draw preliminary layout of inverter using stick diagram
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17. Lets draw preliminary layout of inverter using stick diagram
Stick Diagrams are useful for planning the layout and routing of integrated circuits.
Every Line of a conducting material layer is represented by a line of a distinct color.
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18. Lets draw preliminary layout of inverter using stick diagram
Stick Diagrams are useful for planning the layout and routing of integrated circuits.
Every Line of a conducting material layer is represented by a line of a distinct color.
Polysilicon Gate
P Diffusion
N Diffusion
Metal
Contact
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20. N-MOS Transistor represented by a Apple Green Color line
PMOS – P Diff P Diff
NMOS – N Diff N Diff
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21. Polysilicon gates represented by Brown Color line
Poly Gate Poly
PMOS – P Diff P Diff
NMOS – N Diff N Diff
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22. Metal is represented by Bottle Blue Color line
Poly Gate Poly
PMOS – P Diff P Diff
In In
NMOS – N Diff N Diff
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23. Contacts are represented by Black Cross
Poly Gate Poly
PMOS – P Diff P Diff
In In
NMOS – N Diff N Diff
= Contact
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24. Metal is represented by Bottle Blue Color line
Poly Gate Poly
PMOS – P Diff P Diff
In Out In Out
NMOS – N Diff N Diff
= Contact
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25. Contacts are represented by Black Cross
Poly Gate Poly
PMOS – P Diff P Diff
In Out In Out
NMOS – N Diff N Diff
= Contact
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26. Metal is represented by Bottle Blue Color line
Vdd Vdd
Poly Gate Poly
PMOS – P Diff P Diff
In Out In Out
NMOS – N Diff N Diff
Vss Vss
= Contact
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27. Metal is represented by Bottle Blue Color line
Vdd Vdd
Poly Gate Poly
PMOS – P Diff P Diff
In Out In Out
NMOS – N Diff N Diff
Vss Vss
= Contact
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28. Stick Diagrams are converted to Cell Layout by assigning length and width to the Wire
Vdd Vdd
Poly Poly
P Diff P Diff
In Out In Out
N Diff N Diff
Vss Vss
= Contact = Contact
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29. Stick Diagrams are converted to Cell Layout by assigning length and width to the Wire
Vdd
Width (w)
Poly
P Diff
In Out
N Diff
Vss
= Contact
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30. Stick Diagrams are converted to Cell Layout by assigning length and width to the Wire
Vdd
Width (w)
Length (L)
Poly
P Diff
In Out
N Diff
Vss
= Contact
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31. Cell Layout is a Black Box for IP User.
Vdd
Vdd
Width (w)
Length (L)
Poly Poly
P Diff P Diff
In Out In Out
N Diff N Diff
Vss
Vss
= Contact = Contact
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32. In Black Box, internal architecture counts less compared to IP Functionality
Vdd Vdd
Poly Poly
P Diff P Diff
In Out In Out
N Diff N Diff
Vss Vss
= Contact = Contact
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33. IP’s serves the purpose of the Circuit design
i.e. Inverter in this case
Vdd
Vdd
Poly
P Diff
In Out In Out
N Diff
Vss
Vss
= Contact
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39. Complex blocks are also offered as IP’s
Vdd
In Out
Vss
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40. Complex blocks are also offered as IP’s
Buffer is nothing but two inverters connected back-to-back
Vdd
In Out
Vss
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41. Complex blocks are also offered as IP’s
Buffer is nothing but two inverters connected back-to-back
Vdd Vdd Vdd
In Out In Out
Vss Vss Vss
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42. Complex blocks are also offered as IP’s
Buffer is nothing but two inverters connected back-to-back
Vdd Vdd
In Out
Vss Vss
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43. Complex blocks are also offered as IP’s
Buffer is nothing but two inverters connected back-to-back
Vdd Vdd Vdd Vdd
Poly Poly
P Diff P Diff
In Out In In Out Out
N Diff N Diff
Vss Vss
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44. Complex blocks are also offered as IP’s
Buffer is nothing but two inverters connected back-to-back
Vdd Vdd Vdd
In Out In Buffer Out
Vss Vss Vss
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45. IP’s are offered in form of rectangular/square boxes
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46. IP’s are offered in form of rectangular/square boxes
For E.g. The Buffer IP, will be represented as below
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47. IP’s are offered in form of rectangular/square boxes
For E.g. The Buffer IP, will be represented as below
Vdd
In Buffer Out Buffer
Vss
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48. IP’s are offered in form of rectangular/square boxes
For E.g. The AND Gate IP, will be represented as below
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49. IP’s are offered in form of rectangular/square boxes
For E.g. The AND Gate IP, will be represented as below
Vdd
In1
AND Out AND
In2
Vss
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51. Commonly asked Question
How do we differentiate between Vdd and Vss ?
It is represented in below pattern.
A Cross line on the bottom left of the Block represents Vss and top corner Vdd
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52. Commonly asked Question
How do we differentiate between Vdd and Vss ?
It is represented in below pattern.
A Cross line on the bottom left of the Block represents Vss and top corner Vdd
Vdd
Buffer Buffer
Vss
Vdd
AND AND
Vss
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54. Complex blocks e.g. ALU will be represented as below IP Block
ALU
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55. • Memory is also a Complex IP used commonly.
• It is necessary to pre-define the geometrical location of these IP’s on a chip,
so that the automated PNR tools do not modify their locations
• These cells are referred to as Pre-placed cells
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