Why Nand Gates are considered more than NOR

EDAboard.com Forum Index -> ASIC Design Methodologies & Tools (Digital) -> Why Nand Gates are considered more than NOR

Author

Message

santuvlsi

Joined: 19 Aug 2007
Posts: 71
Helped: 3

28 Aug 2007 7:43 Why Nand Gates are considered more than NOR

Hai every body,

Nand Gates are more preferred than Nor gates,

becos in Nand gates NMOS connected in series.

What is the logic behind this?

santu

pratap_v

Joined: 06 Jan 2007
Posts: 69
Helped: 3

28 Aug 2007 8:09 Re: Why Nand Gates are considered more than NOR

The mobility of holes is lesser than that of electrons. Inorder to make the rise and fall times of a gates equal usually the width of PMOS xtor is made higher. So the resistance of it would be less and can attain equal rise and fall times.

In NAND gates the PMOS xtors are connected in parallel and there by its effective resistance decreases. So now one can achieve the same rise and fall times at lower widths of the PMOS. Inaddition you are reducing the cell height also.

santuvlsi

Joined: 19 Aug 2007
Posts: 71
Helped: 3

28 Aug 2007 8:45 Re: Why Nand Gates are considered more than NOR

dear V_pratap,

U mean to say is,if pmos are connected in parallel their

resistance decreases so that they can acheive faster speed if their

width is 2~3 times larger than Nmos, even though Nmos if connected in series

have reistance higher , but can attain speed since majority are electrons.

But in NOR gates same thing we cannot do since PMOS is in series, resistance more

even we increase width we cannot acheive speed.

What about the capacitance?

santu

A.Anand Srinivasan

Joined: 15 Oct 2005
Posts: 1792
Helped: 216
Location: India

28 Aug 2007 15:07 Re: Why Nand Gates are considered more than NOR

it is basically two different capacitances during the switching.... because for each there will be two parallel MOS in one case and one MOS in the other case.... so for the two MOS in parallel case of switching the capacitance will be high....

sekapr

Joined: 27 Jul 2006
Posts: 100
Helped: 5

29 Aug 2007 10:27 Re: Why Nand Gates are considered more than NOR

Digital circuits mostly use the the low-high transition for sampling data which faster in NAND compared with Nor

Google
AdSense
Google Adsense

29 Aug 2007 10:27 Ads

deh_fuhrer

Joined: 25 Jul 2006
Posts: 421
Helped: 28

29 Aug 2007 10:44 Re: Why Nand Gates are considered more than NOR

santuvlsi wrote:

Hai every body,

Nand Gates are more preferred than Nor gates,

becos in Nand gates NMOS connected in series.

What is the logic behind this?

santu

Consider you have basic inverter which have equal rise and fall time and its Aspect ratio is 2:1. So for symmetric NOR gate with both NMOS have ratio W/L each, you should have both PMOS with aspect ratio four times that of NMOS, i.e., 4W/L each.
But in NAND you can have symmetric rise and fall times with all NMOS and PMOS having aspect ratio W/L each. This is the reason why NAND are preferred over NOR gates..

((NMOS have lesser resistance than PMOS approximately half, so resistance of two NMOS devices (in series) will be equal to one PMOS device on Pull-up side...))

pichuang

Joined: 06 Apr 2006
Posts: 11
Helped: 1

29 Aug 2007 15:07 Re: Why Nand Gates are considered more than NOR

NAND gates is more preferred than NOR gates because of sizing.

NAND is NMOS in series and PMOS in parallel, while NOR is the other way around.
As people have already mentioned, the mobility of hole is less than that of the electron. Therefore, to achieve the same delay (current capability), PMOS needs to be approximately 3 times than NMOS (0.18um technology).

I am not sure if you have taken any digital IC course before, but in essence when you do the transistor sizing any transistor in series need to be sized up more (depending on the number of transistors in series). Therefore, we want to avoid PMOS transistors in series (because they take up more space than NMOS in series at same delay).

That is why NAND is a better choice than NOR.

Added after 43 seconds:

pichuang wrote:

NAND gates is more preferred than NOR gates because of sizing.

NAND is NMOS in series and PMOS in parallel, while NOR is the other way around.
As people have already mentioned, the mobility of hole is less than that of the electron. Therefore, to achieve the same delay (current capability), PMOS needs to be approximately 3 times bigger than NMOS (0.18um technology).

I am not sure if you have taken any digital IC course before, but in essence when you do the transistor sizing any transistor in series need to be sized up more (depending on the number of transistors in series). Therefore, we want to avoid PMOS transistors in series (because they take up more space than NMOS in series at same delay).

That is why NAND is a better choice than NOR.

kaka.alonso

Joined: 29 Aug 2007
Posts: 8

29 Aug 2007 16:46 Re: Why Nand Gates are considered more than NOR

sekapr wrote:

Digital circuits mostly use the the low-high transition for sampling data which faster in NAND compared with Nor

why Digital Circuits mostly use the 0->1 transition not the 1-> transition?

sohiltri

Joined: 16 Dec 2006
Posts: 90
Helped: 5

29 Aug 2007 18:10 Re: Why Nand Gates are considered more than NOR

It is not the issue of why dig circuit uses 0->1 or so....it is only for analysis purpose used by designer....basically dischargin of load capacitance is done by many path...the response of CMOS circuit depends on how fast and good u r able to charge load capacitance which happens only at 0->1 transition thru only single path ....supply to output path....

ameed

Joined: 28 Jun 2007
Posts: 119
Helped: 5
Location: INDIA

30 Aug 2007 10:52 Re: Why Nand Gates are considered more than NOR

hi all,

it is basically two different capacitances during the switching.... because for each Idea

there will be two parallel MOS in one case and one MOS in the other case.... so for Idea

the two MOS in parallel case of switching the capacitance will be high.... Idea

thanx.....

gck

Joined: 17 Oct 2006
Posts: 149
Helped: 5

04 Sep 2007 9:00 Re: Why Nand Gates are considered more than NOR

For Nand PMOS are in parallel whereas in case of NOR PMOS is in serial. As u know the mobility of hole is less than the mobility of electron, time taken to switching from ACTIVE state to Saturation state is higher. Same for reverse condition.

So NOR has delay while switching from one state to another state so prefered to use NAND over NOR

kpsai26779

Joined: 28 Jun 2007
Posts: 85
Helped: 6
Location: Bangalore, India.

04 Sep 2007 9:13 Why Nand Gates are considered more than NOR

Hi santu,

Nand Gates are more preferred than Nor gates, in CMOS technology only.

both resistance and capacitance effects the delay and this is reason for selecting NAND compared NOR gate