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To: TimF who wrote (64234)	11/21/2001 4:48:23 PM
From: rsi_boy	Respond to of 275872

Anything can control the voltage. That's the beauty of transistors because they act like amplifiers. Consider a radio. A few waves rippling through the atmosphere disturb some electrons in an antenna. This whisper of a signal is directed to the gate of a transistor (actually there is a PLL involved which first latches onto the broadcast frequency and an amplifier consists of a bunch of transistors and other circuit components but let's not complicate things more than we have to). Small fluctuations in voltage at the gate of the transistor (originating from the radio wave) control large amounts of current flowing through the transistor. These large amounts of current can be used for interesting things like moving an electromagnet back and forth inside of a paper cone (this device is also known as a speaker). In a computer, the voltage at the transistor gate usually comes from some previous transistor. Each circuit elements has a certain number of inputs and a certain number of outputs. Each element's outputs have enough power to trigger a bunch of downstream inputs (exactly how many is called the fan out I think). If you follow all the way upstream to the very first transistor, it probably reads its voltage from something like the magnetic head of a disk drive (again, a tiny magnetic polarization on the surface of a disk is amplified by transistors and input into the digital logic). Alternately, a key press on the keyboard connects a circuit and some voltage from the power supply triggers a bunch of transistors on the motherboard to raise an interrupt (more transistors) to tell the CPU that some input has arrived.

To: TimF who wrote (64234)	11/21/2001 5:07:25 PM
From: tcmay	Respond to of 275872

"Also - "The gate controls that voltage" is just a bit backwards - it is the voltage that controls a gate. "What does control the voltage?" Many things can: 1. A varying voltage from some external voltage source, like an oscillator, square wave generator, even a pendulum making and breaking a contact with a battery. We call these "clocks." Inside the chip, this alternating pattern of ground and some voltage then gates other signals... 2. Physical switches sending voltages to the pins of the device. And then inside. And then through other chunks of logic. Clocked with the clock in #1 above. 3. Other logic devices on a board. They have pins wired to the input pins of other chips...this is what PCBs do. 4. And so on. Anything can be a source of these switched signals. There is nothing mystical about chains of gates and flip-flops and other logic blocks hooked up to each other. Nor is there any eschatological prooblem with "The First Gate" ("If God built the world, who built God? If there are sequences of gates, who controls the _first_ gate?") Literally, gates and ICs can be hooked up with Bell wire, or put on simple breadboard layout aids, and then "pulsed" with simple switches or whatever. Building a simple logic device out of NAND, NOR, etc. gates ("TTL") is a good exercise. Don Lancaster has authored a whole bunch of books on this stuff. BTW, buying a very inexpensive TTL circuit kit at Radio Shack is a pretty good way to get a handle on these ideas. --Tim May

To: TimF who wrote (64234)	11/21/2001 6:18:41 PM
From: Pravin Kamdar	Respond to of 275872

Tim, I wrote up something with an ASCII diagram for you, but the fonts used in SI won't display it properly (even their fixed font). If you want it, PM me with your email address. Pravin.

To: TimF who wrote (64234)	11/21/2001 6:55:26 PM
From: Ali Chen	Read Replies (1) \| Respond to of 275872

Tim, "What does control the voltage?" Tim, I will try to address some of your questions. Of course, even an introductory explanation would require few full-hour lectures. Anyway, I will try to link the long chain from MOSFET transistors to a computer/microprocessor. 1. The basic element of every microcircuit is an invertor. It takes two transistors connected in series between ground and Vcc, with their gates connected together. These two transistors have differnt switching features and are called "complimentary". Nice animated illustration of this can be found here: tech-www.informatik.uni-hamburg.de Another nice site, more advanced, shows how individual transistors are connected to form boolean logical functions, or "logical gates" (do not mix them up with transistor gates!!!): eng.uci.edu 2. In final design, every input of every gate is connected to output of another internal gate, or to some input buffers, to perform communication (I/O) with external world, usually via a collection of signals known as busses. A bus usually have several address lines ( to select external memory location), plus data and control lines. The internal gates form bunch of complex structures - flip-flops, bigger storage and control registers, even bigger logical blocks, local memories, etc. Every such structure is usually controlled by a a special self-contained circuitry called "state machine". Depending on variety of input signals, it uses input clock as "initial voltage switch", and hops between variety of states that control (send low or high voltages to) other logical blocks. See e.g. ee.ntu.edu.au The chip usually has several other external control signals that must be provided by a mainboard. The most important signals are RESET and abovementioned CLOCK. The RESET signal usually is generated once upon the power-on event, or can be issued manually. The main purpose of this signal is to ensure that the chip starts from a known pre-defined state, and is kept in this state until RESET goes off and all state machines start to use the CLOCK and logically analyze input signals. 3. For a microprocessor chip, the first thing it does it initiates a external read operation from a predefined memory location. In PC, the first thing a CPU does is a fetch of instruction from address FFFFFFF0. Always, that's all it can do alone. To force a CPU to do more meaningful job, a board designer has to place a memory chip on the bus, which would respond to this initial address, and make sure it contains a proper chain of instruction for the CPU. This memory chip usually contains a program that performs inital configurations of whatever needs to be configured, provides basic input/output subroutines to communicate with peripheral devices such as floppy and videocard, and therefors is called BIOS. 4. I'll stop here for now. For another drink. - Ali