System on Chip Architecture Fundamentals: By Aviral Mittal avimit att yahu dat cam

1. I/O Pads:
For a SoC to be able to interact with the outside world, the least is needed are input/output pins.
Since these pins also called ports will carry information from inside the SoC to outside the SoC and vice-versa, these I/O ports are specially design circuits, which are called I/O pads. The electrical characteristics of off-chip metal is quite different to the electrical characteristics of on-chip metal, for example outside the SoC chip, these I/O pads have to drive very thick metal wires while inside the chip those wires are very thin.
So we need especially design I/O pads which special circuitry to deal with harsh outside world and relatively comfortable inside world.
A collection of these I/O pads for a chip is also often termed as I/O Pad Ring, as usually these pads are arranged in the form of a Ring around at the periphery of the chip surrounding the 'core' logic.
Each I/O pad can be either input pad, or an output pad or a bi-directional pad.
Then Each I/O pad can be either analog pad to carry analog signal or a digital pad to carry digital signal.
And then there are power pads such as VDD pad and GND pad for supplying power to the chip.

The I/O pad serve one more rather secondary purpose. This is to protect the chip-circuit from over-voltage pulses. This may be due to electrostatic charges, or due to human error while accidentally a user has applied higher than specified voltage to a pin.

What are pad drivers?
The output pads are required to drive relatively high amount of currents, as these will connect to off-chip metallic wires, which may connect to other circuits. For this purpose there are special purpose circuits designed to provide high current to the output pads. These are often called pad drivers.

What are GPIOs?
While some pads on the SoC has specific purpose, others can be general purpose. These general purpose pads are calld GPIOs. They do not have a defined function as such, but can be very useful to add functionality to the post-production device. These GPIOs can easily be made available to the SoC software which can then use it for a variety of purposes.

I/Os dedicated to DFT:
There is another function associated with the I/O pads. This is to help with the manufacturing test of the SoC. These pads usually do not participate in functionality of the SoC, but helps in the testing of the Chip. So the SoC has some core logic and I/O pads dedicated to what is called 'Design For Test' Logic or simply DFT logic. More about DFT is presented in later section(s).

Pull-up/Pull down functionality associated with I/Os

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