1. BUILDING BLOCKS OF BI-DIRECTIONAL BUFFER
Building blocks of a Bidirectional Buffer
A bi-directional buffer includes first and second unidirectional buffers connected for
retransmitting signals in opposite directions between first and second buses. When an
external bus driver pulls the first bus low, the first unidirectional buffer pulls the second bus
low and generates a signal inhibiting the second unidirectional buffer from actively driving
the first bus. When the external bus driver allows the first bus to return to the high logic
level, the first unidirectional buffer temporarily supplies a high charging current to the
second bus to quickly pull it up. Similarly, when an external bus driver pulls the second bus
low, the second unidirectional buffer pulls the first bus low and generates a signal inhibiting
the first unidirectional buffer from actively driving the second bus. When the external bus
driver allows the second bus to return to the high logic level, the second buffer temporarily
supplies a high charging current to the first bus to quickly pull it up. The bi-directional buffer
includes a register for storing and reading out data representing successive logic states of a
signal on the first bus, thereby providing a history of data appearing on the bus.

2. Bidirectional I/O Buffers are preferred because
i. Drive different load currents as per specification
ii. Minimize noise from the input signal
iii. Provide interface between ICs with different signal levels
iv. Protect internal circuit from extremely high voltages
v. Low to High and High to Low level shifting
Above figure shows the block diagram of a Bidirectional I/O cell, comprising of the
Input section and the Output section. Each block has been designed to carry out a specific
task to reduce the overall design complexity of the I/O device.
The bidirectional buffer mainly includes two sections:
1) Output section
2) Input Section
OUTPUT SECTION
The output section comprises of the following blocks:
a) Level Shifter (LOW TO HIGH) with a logic Circuit
b) Pre-driver
c) Driver
d) ESD protection
Driver
This block is used to drive the current at the PAD. The length and width of the circuit is
chosen as per device specifications of the project.
Pre-Driver
Pre-Driver is the controller block of the output section and it controls the Driver as it guides
the Driver about when to drive the current and how much current to drive. There are
basically two drivers (4m and 20m drivers). The two Pre-Drivers are connected parallelly.

3. Level Shifter
The output section needs a transition from low voltage to high voltage. The circuit used for
this transition is a CROSS-COUPLED C-MOS.
ESD
The Electro Static Discharge is a protection layer against the additional environmental
generated voltages which could happen due to the human touch or other such conditions.
INPUT SECTION
The input section comprises of the following blocks:
a) Low pass digital filter
b) Level Shifter (HIGH TO LOW) with a logic Circuit