Controlling Virtuoso schematic inputs with Digital Vector File

The utilization of a Vector File makes controlling multiple input and output signals independently possible. Its implementation is important for a variety of circuits which need different pulses at specific times, relieving the effort to manage PWL voltage sources at schematic.

Note: comments begins with “;” character

radix 1 1 4 4
; radix specifies the number of ports and number of bits for each port,
; the base number is also specified ( from binary '1' to hexadecimal '4' )

io i i i i

; io defines the state of each port, as input "i", output "o"
; or bidirectional "b"
vname Primeiro_Sinal Segundo_Sinal Terceiro_Sinal[0:3] Quarto_Sinal,Quinto_Sinal,Sexto_Sinal,Setimo_Sinal

; vname sets the name of each port

tunit 1ns

; tunit configures the time unit of simulation

trise 1ps
; trise defines the rise time of each vector

tfall 1ps
; tfall defines fall time of each vector

vih 5.0
; vih defines logical '1' voltage

vil 0.0
; vil defines logical '0' voltage
period 10.0

; period determines the time interval between each vector step, the
; number is multiplied by the previously time unit set

idelay 5 0 1 1 0

; idelay inserts a delay at the selected ports multiplied by the time unit

; The next lines will define the states of each port as the
; transient simulation happens, each line represents a time step

; The binary number can be represented with 0 or 1, and the
; hexadecimals with 0-F

0 0 0 0 ; All signals initially set to 0
1 0 F F ; First signal with a logical '1', second with a logical '0' , the other ones set to a logical '1'
0 1 C C ; And goes on
1 0 A A
1 1 3 3

The ports declaration and manipulation are indented by its position from left to right.

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SRAM write and read basics (1/2)

The Static Random Access Memory is widely used among digital circuits for its good trade-off between memory architectures. Being not large like register files, at the same time not slow as DRAM or hard disk management. It is also compatible with standard CMOS process.

sram_t

The standard SRAM cell architecture is shown above, a pair of pMOS and nMOS transistors create cross-coupled inverters that can hold its state without needing to have its input driven by an extern signal.

The word line “WL” enables the bit lines “BL and “!BL” to drive the net “Q” to a logical value (either 0 or 1) and at the same time, opening the cell data to be read by a special circuitry.

Sizing those transistors correctly is required. If the inverter transistors “M1 to M4” are considerably stronger than “M5” and “M6” access transistors, the current passing through the transistors “M5 and M6” will not be enough to flip the state of the memory cell. The stable state created by the cross-coupled inverters needs to be overpowered to change its logical level. Moreover, the opposite situation with unbalanced strength (size) causes the logical state to be changed from bit lines┬ánoise even with the word lines disabled. The same problem can happen during the read operation which is going to be presented further.

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