From: Jinto N.Jose (email@example.com)
Date: Thu May 31 2001 - 08:30:56 PDT
I have some more doubts regarding HSTL interfaces.
1. In HSTL standard (EIA/JESD8-6) series terminated load is shown only for
class 2 . Why can't HSTL class 1,3,&4 drivers can not drive series
terminated loads ?
2. For HSTL class 2, the parallel termination option shown is a 50 ohm to
Vddq/2 both at the driver end and receiver end (Symmetrically double
parallel terminated output load). What is the purpose of keeping a pull up
resistor near the driver ? (same thing is applicable for HSTL class 4 where
Vtt = Vddq) Will it work fine with a 50 ohm to Vddq or Vddq/2 ?
3. For HSTL class 3 the suggested load is 50 ohm to Vddq at the receiver
end. Will it work fine if the termination is 50 ohms to Vddq/2 ? My basic
doubt is that why termination is changing with the Class of the driver .
Both the schemes of termination 50 ohms to Vddq/2 and 50 ohms to Vddq are
identical as far as reflections are concerned . Both the schemes matches the
transmission line impedance (am I right ?). In that case is it necessary to
flow more than 20mA current ((1.5V -0.4V)/50 ohm) in the case of a class3
driver ? Or 50 ohm to Vddq termination is shown just to tell that a HSTL
class 3 driver can easily sink that much current . Similarly will a HSTL
class 4 driver work fine with a single 50 to Vddq/2 ?
4. How much current an HSTL input usually requires ? Is it in micro amps or
nano amps ?
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