U. Glaeser

FIGURE 10.83 A column circuit.
FIGURE 10.84 A write circuit.
Write Circuit
In the previous sub-section, a simple and fast circuit was presented. The write N transistor (and write select
transistor) are large enough to drive the highly capacitive bit line down and overcome the P pull-up transistor
inside the memory cell in the available write time. In that circuit the bit line is not driven to “1”. This is
because the bit line can not write a “1” to the memory cell and the bit line is already pre-charged to “1”.
To make sure that the bit line maintains the pre-charge value or if the pre-charge is not complete prior
to write, to save time, the write circuit must drive the “1” side also. Figure 10.84 is a more complex write
circuit that has only one N transistor (in series with the N column select transistor) and drives the high
side as well. During the read cycle, this circuit is in tri-state, so same column select might be used for
both read and write. Wen signal is either the write enable signal or a derivation of it.
Sense Amplifier
The sense amplifier is the last element considered in the column. In a read operation the bit lines are
pre-charged first. Then a word line is activated to let the ND transistor in the cell pull either Bit or Bit#,
depending on the data stored in the cell, down. The bit line is highly capacitive and the transistors in
the memory cell are small for density purpose, so it may take a long time for the cell to completely
discharge the bit line. The common practice is to let the cell develop only a limited differential voltage,
about ∼100 mV, on the bit lines and amplify it by a sense amplifier. Thus, reducing and matching the bit
line capacitance is important for a fast and correct read. Power consumption of a memory is also mainly
determined by the bit line capacitance.
Bit line capacitance components, contributed by each memory cell, include junction capacitance, bit
line to bit line coupling, bit line to word line, and bit line to substrate capacitance. Thus, each cell connected
© 2002 by CRC Press LLC
Wen
Bit Bit#
din din#
Readsel [1:0]
Writesel [1:0]
Pre-charge Pre-charge
p
din#[0] din[0] din#[1] din[1]
Bit Bit#
Sense amplifier