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USB Encoding Scheme NRZI with Bit StiffIn this series, USB requests and descriptors have been already discussed. With the knowledge of USB requests, descriptors, transfer types and transactions it is now easy to understand how data is communicated over USB interface.  Another aspect of the USB communication is line states and encoding, which is generally ignored by the system designers as a USB device can be designed without knowing the Signaling and Encoding schemes. The signaling and encoding schemes are implemented at the lower levels (usually on the controller IC) and usually gets abstracted during system designing. However, understanding the terms related to bus states and encoding would help in getting comprehensive view of the USB System.

In the previous tutorial, it was mentioned that primary memory can be of two types - Random Access Memory and Read Only Memory. These memories are essential part of any processor or controller based system. In this tutorial, these memories will be discussed in detail. 



In the previous tutorials, it was  learnt that sequential circuits require memory elements to retain previous states of a digital system. The flip-flops and registers were then introduced as building blocks of memory in a sequential circuit. The registers are enough to store runtime data in small microcontrollers and ASICs. The complex computing systems require digital memory to not only store runtime information (as stored by registers in a microcontroller) but to store programs and data permanently within the digital system. So, they require dedicated memory either internal or external.