SAS 3.3V LED driver characteristics - Feedback by 8/2/02 please!

Dave Davis Dave_Davis at uk.xyratex.com
Mon Jul 22 11:03:30 PDT 2002


* From the T10 Reflector (t10 at t10.org), posted by:
* "Dave Davis" <Dave_Davis at uk.xyratex.com>
*
Alvin et al,
	I want to express a concern here on the brightness of LEDs that this spec might support. Most manufacturers use some type of lightpipe to transfer the light from the LED to the front of the enclosure. All of these suffer loss, some due to alignment and most due to material characteristics. Whilst the LEDs are improving there is still a challenge in the materials that will push the use of brighter, higher current LEDs. Whilst these could be driven using additional transistors, all of us are striving for passive backplanes for increased reliability and hence are not desirable. 

Has this been considered in the recommendation ? I believe that we currently run high brightness LEDs at around 19mA and that a reduction to 10mA would be too difficult to meet.

Regards , Dave Davis , Product Architect,
Storage Solutions Development, Dept 459 ,
Locn 1000/61, XYRATEX ,UK, mailto:dave_davis at uk.xyratex.com
TEL:Int 6466 Ext +44 (0)2392 496466 FAX: +44 (0)2392 492284
Http://www.xyratex.com

-----Original Message-----
From: John Lohmeyer [mailto:lohmeyer at t10.org] 
Sent: 22 July 2002 16:57
To: Alvin.Cox at seagate.com
Cc: T10 Reflector
Subject: Re: SAS 3.3V LED driver characteristics - Feedback by 8/2/02 please!

* From the T10 Reflector (t10 at t10.org), posted by:
* John Lohmeyer <lohmeyer at t10.org>
*
Alvin,

I wholeheartedly agree with moving to 3.3-volt LED driver specifications.

However, I have two issues with the Drive LED on specification:

1. The sign in the IOL inequality for the Drive LED on specification is 
backwards to normal practice for chip specs. This inequality should be less 
than or equal to 10 mA, not greater than 10 mA. (This says the chip can 
sink up to 10 mA over the VOL range.)

2. The 0.15 VOL spec is aggressive. Do you really want your chip vendors to 
scrape chips that can only sink 10 mA at 0.16 Volts on the LED driver pin, 
but are otherwise perfectly good parts?

John




At 7/19/2002 03:23 PM, you wrote:
>* From the T10 Reflector (t10 at t10.org), posted by:
>* Alvin.Cox at seagate.com
>*
>The majority of the SAS PHY working group desires to change the LED driver
>characteristics to be 3.3V compliant rather than 5V compliant. As a new
>interface, this seems to be the best opportunity to change. IC supply
>voltages are lower and LED efficiencies are increasing. One complication is
>the old specification allowed a rather high output-low voltage. At 0.5V
>maximum, it is a significant proportion of the nominal 3.3V supply. The
>current was also rather high when considering the LED's available today.
>The SAS PHY working group makes the following recommendation for
>consideration in the SAS specification for the READY LED electrical
>characteristics. We realize that this may be an aggressive specification at
>this time, but feel that it most likely can be achieved with today's
>commonly used technologies. Check the SAS specification (Section 5) for the
>complete set of READY LED implementation requirements.
>
>Output characteristics of the READY LED signal (tested through the device
>mated connector)
>
>     State              Output current             Output voltage
>
>Drive LED off      -100 µA < IOH < 100 µA         0 < VOH < 3.6 V
>Drive LED on             IOL > 10 mA              0 < VOL < 0.15V
>
>
>Alvin Cox
>Seagate Technology, LLC
>
>*
>* For T10 Reflector information, send a message with
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John Lohmeyer                  Email: lohmeyer at t10.org
LSI Logic Corp.                Voice: +1-719-533-7560
4420 ArrowsWest Dr.              Fax: +1-719-533-7183
Colo Spgs, CO 80907

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