r5200 - trunk/gta02-core/docs/ecn

werner at docs.openmoko.org werner at docs.openmoko.org
Mon Jun 22 21:29:29 CEST 2009

Author: werner
Date: 2009-06-22 21:29:29 +0200 (Mon, 22 Jun 2009)
New Revision: 5200

Completed ECN0009.

Modified: trunk/gta02-core/docs/ecn/STATUS
--- trunk/gta02-core/docs/ecn/STATUS	2009-06-22 19:24:42 UTC (rev 5199)
+++ trunk/gta02-core/docs/ecn/STATUS	2009-06-22 19:29:29 UTC (rev 5200)
@@ -8,7 +8,7 @@
 0006	Discuss	Either remove or populate R1701 (PMU.ADAPTSNS pull-down)
 0007	Discuss	Remove KEEPACT net and connect PMU.KEEPACT directly to IO_3V3
 0008	Edit	Remove external GPS antenna connector and circuit (CON7602)
-0009	Edit	Remove LED transistors (Q1501 and Q1502)
+0009	Discuss	Remove LED transistors (Q1501 and Q1502)
 0010	Edit	Remove audio amplifier (U4101)
 0011	Edit	Reduce number of varistors on USB_VBUS (C4902, C4909)
 0012	Defer	Add fuse to USB_VBUS

Modified: trunk/gta02-core/docs/ecn/ecn0009.txt
--- trunk/gta02-core/docs/ecn/ecn0009.txt	2009-06-22 19:24:42 UTC (rev 5199)
+++ trunk/gta02-core/docs/ecn/ecn0009.txt	2009-06-22 19:29:29 UTC (rev 5200)
@@ -1 +1,49 @@
 Remove LED transistors (Q1501 and Q1502)
+LED1501 is a Harvatek HT-210UD/NB [1]. To determine the forward current
+If through each LED, we need to calculate If = (IO_3V3-Vf)/220R, where
+Vf is a function of If as specified by the data sheet.
+Graphical approximation suggests that If(amber) = 7 mA and
+If(blue) = 2 mA.
+LED1502 is a Harvatek HT-110USD5 [2]. Using the same approximation, we
+obtain also If(red) = 7 mA.
+The I/O pads GPB0 through GPB2 are all of type t8, capable of sinking
+or sourcing at least 8 mA, with Ron <= 50 Ohm. The GPIOs are known to
+withstand permanent shorting to ground with a current well in excess
+of 50 mA.
+We could therefore drive the LEDs directly from the CPU, with the
+following component changes:
+- change R1562 and R1563 to 150 Ohm (requires a new component). With a
+  total series resistance of Ron+150 Ohm, we still don't exceed 8 mA.
+- change R1561 to 0 Ohm or small nominal value (e.g., 33 Ohm, like the
+  now obsolete R1812). Even with just Ron, Vf should still be around
+  3.0 V and the current should thus not exceed 6 mA.
+- remove Q1501, Q1502, and R1564
+- connect R1561 directly to GPB1, R1562 to GPB2, and R1562 to GPB2
+Note that this change reverses the logic of the LEDs: to activate
+them, they have to be driven low. To deactivate, they have to be
+driven high or Z. This also means that the LEDs will be dimly lit
+immediately after reset, when the weak pull-downs are enabled.
+Software changes are required in Qi and the kernel to accommodate
+this change.
+The original GTA02 logic could be preserved for LED1502 by reversing
+it and connecting it to GND instead of IO_3V#, and for LED1501 by
+replacing it with a common-cathode type (if such a component exists).
+[1] http://www.harvatek-intl.com/web/content/download.asp?fx=73
+[2] http://www.harvatek-intl.com/web/content/download.asp?fx=54
+Author: Werner Almesberger <werner at openmoko.org>

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