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Unformatted text preview: Read-stability
- Assume Q=0, !Q=1
M2 and M6 are in CUT-OFF
is also CUT-OFF
pulling !Q to V_DD
is in SATURATION
is in TRIODE [Drawing]
Thus, the READ operation only involves M2, M4 and bitline BL:
I_D2 = 1/2•µ_n•C_ox•(W/L)4•(V_DD - V_Q - V_tn)^2
= µ_n•C_ox•(W/L)2•[(V_DD-V_tn)•V_Q - 1/2•V_Q^2] During a READ, both M2 and M4 draw current and so V_Q rises above
ground. Read stability is achieved by ensuring V_Q remains low
enough that the latch does not toggle its state.
Thus size M2 and M4 such that V_Q <= V_tn
I_D4 = 1/2•µ_n•C_ox•(W/L)4•(V_DD - V_Q - V_tn)^2
I_D2 = µ_n•C_ox•(W/L)2•[(V_DD-V_tn)•V_Q - 1/2•V_Q^2]
= (0.2)(W/L)2•[(2)0.5 - 0.5^2/2)]
Setting I_D4 = I_D2 0.225(W/L)4 = 0.175(W/L)2
1.3(W/L)4 <= (W/L)2
(with M1 and M3 identical to M2 and M4 by symmetry)
- Making M2 wider lowers V_Q
2) Sizing for READ speed
Speed of READ operation determined by discharging of bitline
capacitance C_BL. Must wait until the differential voltage
is large enough to determine which bitline was pulled low
I_D4 = C_BL ∆v/∆t Thus if C_BL=1pF and we want...
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This document was uploaded on 02/03/2014.
- Winter '09