DELAY

1 ) What is propagation delay ?2 ) How to decrease word line delay ?

Propagation Delay of NOR ROMWord line delay

Delay of a distributed rc-line containing M cellstword = 0.38(rword x cword) M2

= 0.38 (17.5 x (0.049 + 0.75) fF) 5122 = 1.4 nsec

Bit line delayAssuming (0.5/0.25) pull-down and (1.3125/0.25)

pull-up with reduced swing bit lines (2.5V to 1.5V)Cbit = 512 x (0.8 + 0.09) fF = 0.46 pF

tHL = 0.69 (13 k/2 || 31k/5.25) 0.46 pF = 0.98 nsec

(and tLH = 0.69 (31k/5.25) 0.46 pF = 1.87 nsec)

• Word line delay is factor of M**2 – quadratic term. Word line delay dominates due to the large resistance of the poly wire.

• Driving the line from both sides and using metal bypass lines (global word lines) would help speed it up.

• But the most effective approach is to carefully partition the memory into sub-blocks of adequate size that balance word and bit-line delay.

• Partitioning also helps to reduce the energy consumption attributed to driving and switching the word lines.

• The bit-line delay can be reduced as well by further reducing the voltage swing on the BL. Voltage swings of around 0.5V are quite common.

Decreasing Word Line Delay