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i A PHYSICAL IMPLEMENTATION WITH CUSTOM LOW POWER ...

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To solve this problem a ``Power-on Reset'' PMOS transistor<br />

is added to each of the<br />

decoder lines to rise all the power enable lines high and effectively block any of the memory<br />

blocks from charging until Vdd has fully powered up. This is shown for the power enable 0 line<br />

in Figure 8-4 below.<br />

Figure 8-4: Dynamic Decoder with power-on reset<br />

The Power-on Reset transistor is enabled with the “Power-on-Reset” signal held low until<br />

the power on reset circuit shuts off the transistor by setting “Power-on-Reset” high. This is<br />

assumed to happen well after the power supply is fully charged as shown in Figure 8-5 where<br />

Vdd is activated well after the power supply has powered up. This circuit causes all the power<br />

enable lines to rise at approximately the same speed as the power supply. Thus, the power<br />

enable transistor from Figure 8-1 has a Vgs ≈ 0 during power up and does not turn on.<br />

As<br />

POR (power-on-reset) circuits are a part of any digital system, the use of POR circuit<br />

does not cost additional power during the normal operation of the circuit. Ramos et.al. [31]<br />

gives an example of a low power power-on-reset circuit.<br />

146

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