[SI-LIST] HSTL classes and SSTL2
- From: rajat.chauhan@xxxxxx
- To: mittalr@xxxxxxxxxx
- Date: Mon, 16 Dec 2002 18:38:56 +0530
!
! Can anyone please tell me why we have so many different classes for
HSTL and SSTL?
Because we have different applications and we apply different termination
techniques for them depending upon our resources, margins, power etc.
If you see HSTL driver's specifications, each class is optimized for a
perticular termination method, to give optimum results in that perticular
enviorment (check AC characterisation load for each class in JEDEC std.).
Although each class of driver can be used with any other termination
method, but in this case it may perform badly(I do not say "NOT WORK").
HSTL class-I is a symmetrical driver for unidirectional signaling,
allowing only far end ~50 ohm termination and requires extra voltage
supply(VTT). It is too weak for both end ~50 ohm termination and will
have reduced output signal swing (and therefore reduced noise margin).
Therefore we have class-II. Class-II is usually used for bi-directional
signaling where both ends need to be terminated. Class-II is also a
symmetrical driver and need extra voltage supply(VTT=VDDQ/2).(for
symmetrical drivers, termination at VTT=VDDQ will give unequal rise and
fall times and delays)
If extra voltage supply is not desirable, we have Class III and IV
drivers. Class III and IV drivers are asymmetrical and does not require
extra voltage supply. Again Class III is for unidirectional signaling and
class-1V for bi-directional signaling.
Similar is the case with SSTL-2 Classes. Class-I for unidirectional
signaling and class-1I for bi-directional signaling.
! It seems to me that we started with HSTL class I and then we needed
class II bec
! ause class I doesn't allow series termination (if we want both source
and sink t
! ermination). Then we went to requiring class III and IV because that
saves one v
! oltage supply. However, anytime you go to a higher class, you require
more curre
! nt so many IC guys don't support all classes.
In General we rarely support HSTL class-IV, as it requires large driver
NMOS.
! Also you need more power
!
! Can anyone please confirm my reasoning. Also, there are no ac specs ie
tpd, tphl
! and tplh in the JEDEC spec. That seems very strange. Don't we need to
test that
! to make sure we can meet certain speeds
!
! thanks
! Rohit
!
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