From: Austin Lesea
Subject: Re: HSTL standards
Date: Tue, 24 Dec 2002 07:33:41 -0800
X-Mailer: Mozilla 4.79 [en]C-CCK-MCD (WinNT; U)
Starting on page 178
You will find the topologies of the four interfaces. The class I and class
III are intended for unidirectional high speed interfaces, wheras classes
II and IV are intended for bidirectional high speed interfaces (mirror
symmetric so they would support a tristate IO for both TX and RX).
Classes I and II are weaker, and intended for shorter runs, and III and IV
are stronger and are intended for longer (more heavily loaded) runs.
Both 1.5V and 1.8 volt versions of all four classes of HSTL exist, and the
1.8 volt versions are just a bit faster than the 1.5 volt versions in most
multi-purpose IOs (ie programmable IOs). 1.8 volts came about when many
ASIC implementations just didn't work at the intended frequency, so the
voltage was increased to make it work.
All use a separate Vref supply of 1/2 Vcco at the receiver which is a high
All are parallel terminated standards that are suitable for multi-drop runs
(in the unidirectional case), and have excellent signal integrity
The lower voltage swing leads to less cross talk, and less EMI, but not
less ground bounce, as the currents are about as large as other strong IO
The disadvantage is that external resistors are required (unless you use an
internal termination feature, such as the Virtex II and II Pro DCI), and
that such parallel termination, internal or external burns power.
Many people use HSTL without the resistors on very short runs, but to do so
violates the standard, and one must simulate and test to be sure that you
will be safe, and the interface will work as intended.
John McMiller wrote:
> What is the main differences between the variuos HSTL I/O technologies: