Basic Logic Blocks

The first step in understanding the schematics is to understand this symbol which is used extensively. For a description of this I turn to Andrew Tower (his comments in italics).

"I should point out up front that I have no formal training in NMOS or any other chip design technology, hence the disclaimer attached to my notes! I believe the transistor-like symbols represent nMOS pass-gates, ie FETs used as a switchable bi-directional channel."

"The "L" block implements an inverter-based latch, using nMOS pass-gates driven by complimentary signals to switch between "follow" and "hold" mode. Research indicates that this is a very common latch used in VLSI CMOS design."

So, in the diagram above I would be the input, F the follow control, H the hold control, and Q/Qn the outputs. When F is high the output Q follows in input I. Qn will always be the complement of Q. When F goes low and H goes high the output Q will get looped back to the input of the first inverter thus holding the state of the input.


"The delay blocks (D1 - single input, D2 - dual input, D1R - single input with reset) and the delay latch DL are all driven by a bi-phase clock and operate as "dynamic" logic blocks.
Taking D1 as an example, when @1 (phase-1 clock line) [S1 in the diagram] is high the input pass-gate allows the input signal (ground or Vdd) to directly drive the first inverter. Assuming the inverter consists of a FET and a pull up resistor (usually implemented as a second FET hard-wired on), this will charge or discharge the "parasitic storage capacitor" formed by the FET gate inside the inverter.
As @1 [S1 in the diagram] goes low the input pass-gate goes into high impedance state leaving the inverter's gate capacitor in either a fully charged (representing "0") or fully discharged (representing "1") state. Since @2 (phase-2 clock line) [S2 in the diagram] is low the output pass-gate is also high impedance at this time. Although the gate capacitor is subject to leakage, for a short period it will hold its charge.
As long as @2 [S2 in the diagram] goes high before the gate charge drains away and the first inverter floats high, the second inverter will take on its current value and propagate it to the output of the logic block for a short time - long enough for the next clocked logic block to latch or make use of the data.

I'm not sure exactly what the * denotes but it only appears where these pass-gates are used (although not all gates have them). My original thought was it indicates where a data value is considered to be "stored" for a full bi-phase cycle, but it may indicate where pass-gates are built into the adjacent logic gate (I have seen this practice mentioned) or something else entirely!"

So to summarize the D1 (Delay 1 input) block, when S1 goes high the input I will be passed through the first inverter to the second FET. When S1 goes low and S2 goes high the input value will be passed through the second inverter to the output.

The D2 (Delay 2 input) block is the same as the D1 block except that is has two inputs. A high on either input will be passed though to the output.

 

The DIR (Delay 1 with Reset) block is similar to the D1 block but has a reset input. As long as R is held high the output will always be kept low. When R is held low, this will behave just like the D1 block.

The DL (Delay Latched) block is similar to the DIR block but with latching capability. When the input (I) is set high and S1 and S2 are clocked in sequence the high will appear on the output (O) and will also be fed back to the input so even if I goes low and S1 and S2 are clocked, the output will remain high. To clear the output R must be set high and S1 must be clocked high.

 



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Dan Boris - danlb_2000@yahoo.com