User’s Manual 79
Transistors Q3 and Q4 are of opposite polarity so that a rail-to-rail voltage can be passed.
When the /CS1 voltage is low, Q4 will conduct. When the /CS1 voltage is high, Q3 con-
ducts. It takes time for the transistors to turn on, creating a propagation delay. This propa-
gation delay is typically very small, about 10 ns to 15 ns.
The signal that turns the transistors on is a high on the processor’s reset line, /RESET.
When the BL2000 is not in reset, the reset line will be high, turning on n-channel Q4
directly and p-channel FET Q3 by way of Q8. When the board is in reset both Q3 and Q4
are off, isolating /CSRAM and allowing it to be pulled to VRAM.
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