Download 4-Bit Counter

4-Bit Counter
Shanthan Mudhasani, ECE 533, University of Tennessee, Knoxville
Abstract—This paper presents a report on the design of a 4-bit Up Counter using J-K flipflop that has a clocked input with Reset. Performing simulations of various output parameters
including rise time, fall time, highlights the performance of the designed counter in Cadence.
INTRODUCTION:
The project aims to design a 4-bit counter using a Flip Flop. The design is done using cadence
and AMI C5N 0.6 µm Technology library. A JK-Flip Flop was used to design the counter.
DESIGN JUSTIFICATION
A. Counter Design Justification
•
A 4-bit has 16 states counting from 0 to 15.This means that to design a 4-bit counter
we need 4 Flip Flops.
•
The counter also has a reset pin that enables it to enter an all-zero state i.e. the
output of the counter is '0' if the reset is '1' irrespective of the clock and the current
state of the flip flops.
•
The counter also has CLA (carry look ahead) out pin that stores the carry. The CLA
pin can be used to modify the design. For example the counter can be upgraded to a
8-bit counter by adding an other 4-bit adder to the CLA output.
•
The outputs of the counter are named F0, F1, F2, and F3. These outputs also
represent current state of the flip-flops.
B. Choice of Flip-Flop
• The counter designed has 4 JK-Flip Flops. The JK-Flip Flop triggers at every
negative going edge of the clock signal.
• A latch is a level-sensitive device. The major problem with latch-sensitive devices
is that during the same level of the clock signal, a race around condition might
occur thereby making the device prone to glitches. This is avoided using the edgesensitive J-K flip-flop.
• Also, the rising/falling edge has to be very sharp. Hence a 1 ns delay is specified for
the clock signal transition from one state to the other.
• JK-Flip Flop is versatile. A reset can be easily implemented using the set-reset
mode of the JK Flip-Flop. And a D or a T Flip Flop can be easily implemented
using a JK-Flip Flop.
• The aspect ratio (W/L) of PMOS and NMOS transistors is taken to be (6µ/600n)
and (3µ/600n) respectively. The width of the PMOS transistor has been
approximately be set to 6µm, for the same gate lengths, to account for the slow hole
mobility compared to the electron mobility.
J-K FLIP-FLOP DESIGN
A J-K flip-flop in the Master-slave configuration was used to implement the 4-bit up counter.
As seen from the schematic of the J-K flip-flop in fig.1, two 3-input NAND gates, six 2-input
NAND gates and two inverters in a feedback loop. A change of state may occur when the flipflop senses a negative edge of the clock signal. Also, a reset pin is incorporated by the
inclusion of an inverter, AND and OR gate. The reset pin operates on active high logic, i.e. the
output Q is forced to ‘0’ irrespective of the input levels at J and K. Table 1 shows the truth
table of the J-K flip-flop with Reset. The schematic was laid out using Composer Schematic
Fig.1
and is as shown in fig. 2. It can be observed that the individual gates have been turned in order
for the routing to be more convenient and also to make the layout more compact.
J
O
0
1
1
K
O
1
0
1
Table 1
Qn+1
Qn
0
1
Q'n
Fig. 3
4-BIT UP COUNTER DESIGN:
Fig. 3 shows the block diagram implementation of the counter. Four J-K flip-flops are
connected in cascade and the outputs of each of the flip-flop forms the counter bits. The least
significant bit (LSB) of the counter is the output of the first J-K flip-flop while the last flipflop output forms the most significant bit (MSB). The inputs of the J-K flip-flop are tied
together to form a T flip-flop. The output of the last JK FF is connected to an AND gate to
produce the CLA (carry-look ahead) output bit. This pin can be used to cascade the counter to
increase the number of states that can be counted by the counter. Fig.4 shows the symbol for
the designed counter.
Fig.4
Table 2 below shows the states that can be counted by the counter. It is seen that the counter is
able to count the states only when the reset pin is held low.
Reset
R
CLA
0
0
Counter States
F3
F2
F1
0
0
0
Count
F0
0
0
0
0
0
0
0
1
1
0
0
0
0
1
0
2
0
0
0
0
1
1
3
0
0
0
1
0
0
4
0
0
0
1
0
1
5
0
0
0
1
1
0
6
0
0
0
1
1
1
7
0
0
1
0
0
0
8
0
0
1
0
0
1
9
0
0
1
0
1
0
10
0
0
1
0
1
1
11
0
0
1
1
0
0
12
0
0
1
1
0
1
13
0
0
1
1
1
0
14
0
0
1
1
1
1
15
1
1
0
0
0
0
0
Table 2. Counter Truth Table
Fig. 5 shows the Pre-layout simulation of the counter. . It is observed that the LSB of the
counter F0 alternates between 1 and 0 at every falling edge of the clock cycle, and this
transition is propagated through to the MSB F3 of the counter. Fig. 6 shows the layout of the
counter. Fig. 7 shows the extracted layout, which denotes the various capacitances between
the various nodes of the circuit. Fig. 8 shows the Layout vs. Schematic (LVS) matching
performed on the counter circuit. The ‘si.out’ file actually gives an account of all the nets,
instances and other vital information regarding the extracted layout. It matches each of these
essential parameters in both, the schematic and extracted layout to finally conclude that the
netlists match. Fig. 9 show the post-layout simulation of the counter applying the same input
as that with the pre-layout simulation schematic. It is observed that the outputs match closely.
Fig 5
A certain degree of non-linearity is observed in the post-layout simulation, which can be
attributed to the fact that the extracted layout takes into account all the various capacitances
between the circuit nodes.
Fig. 6
Fig 7
Fig 8
Fig 9
PERFORMANCE PARAMETERS:
The performance of the designed counter is then tested by measuring the rise and fall times of
the various output bits of the counter with zero load capacitance. Table 3 shows a qualitative
comparison of the rise and fall times of these bits.
Output
bits
Rise
tim
e
(ns)
Fall
tim
e
(ns)
F0
2.8
8.2
F1
2.8
6.4
F2
2.7
8.4
2.6
8.7
F3
Table 3. Rise and Fall time data
The rise time for each bit is less than the fall time for that bit. It means the discharging RC
constant is grater than that of the charging one. The rise and fall times are calculated between
10% to 90 % of the output voltage level.
Another critical performance measurement parameter is the propagation delay at each
individual bit of the counter. The propagation delay was measured as the time difference in
attaining the 50% of the maximum signal level between the clock cycle and each output bit.
The propagation delay varies greatly by changes in the capacitive load at the output of the
counter. Thus the delay was measured for different values of load capacitances as seen from
table 4. The delay times for ‘1’ to ‘0’ transitions are greater for each bit at every load than
those for ‘0’ to ‘1’ transitions. Also a plot of the Delay vs. Load capacitance shows that the
delay is linearly proportional to the increase in load capacitance.
Table 4 Propagation Delay for various output bits at different capacitive loads
Fig 10 Delay vs. Load capacitance
VI. PAD FRAME
Fig. 11 shows the connection of the counter to the pad frame layout. The input pins clk and R,
the output pins F3, F2, F1, F0 and CLA along with the power supply connections VDD and
ground are connected to individual pins on the pad frame. The connections to the pad frame
need to be routed carefully so that the metal1 and metal2 layers do not cross each other at
points where a connection is not required. The pad frame adds a significant amount of load
capacitance to the designed counter thereby increasing the rise and fall time of the circuit
considerably.
Fig 11
APPLICATIONS:
There are tremendous applications of a counter in the digital consumer electronics market. A
counter can play a vital role in several circuits ranging from a simple display to complex
microcontroller circuits. Some of the apparent applications of a counter are:
•
•
•
•
•
Frequency divider in phase-locked loops
Frequency synthesizers
Signal generation and processing circuits
Microcontrollers and digital memories
In digital clock and timing circuits
CONCLUSION:
The design of a 4-bit Counter has allowed us to implement the various digital VLSI concepts
learnt in the course to put to practical use and experience a very powerful VLSI modeling tool
in the form of Cadence. It is not only useful for laying out the actual circuit schematic that we
have built but also allows us to understand the various capacitances affecting the circuit when
laid out on a chip by means of showing them in the extracted layout. Also, by performing
simulations in Cadence, it is possible to understand the effect of the varying the transistor
sizes to obtain the desired output parameters. It is also useful to get know-how of the various
design rules learnt and how they should be avoided to ensure a good working design with
minimum capacitances and occupying the minimum chip area.