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+USB NKRO MEMO
+=============
+2010/12/09
+
+
+References
+----------
+USB - boot mode, NKRO, compatibility, etc...
+ http://geekhack.org/showthread.php?t=13162
+NKey Rollover - Overview, Testing Methodology, and Results
+ http://geekhack.org/showwiki.php?title=NKey+Rollover+-+Overview+Testing+Methodology+and+Results
+dfj's NKRO(2010/06)
+ http://geekhack.org/showpost.php?p=191195&postcount=251
+ http://geekhack.org/showthread.php?p=204389#post204389
+
+
+Terminology
+---------
+NKRO
+ghost
+matrix
+mechanical with diodes
+membrane
+
+
+OS Support Status
+-----------------
+USB NKRO is possible *without* a custom driver.
+At least following OS's supports.
+ Windows7 64bit
+ WindowsXP
+ Windows2000 SP4
+ Ubuntu10.4(Linux 2.6)
+ MacOSX(To be tested)
+
+
+Custom Driver for USB NKRO
+--------------------------
+NOT NEEDED
+at least when using following report formats on Windows, Linux or MacOSX.
+
+
+USB NKRO methods
+----------------
+1. Virtual keyboards
+ Keyboard can increase its KRO by using virtual keyboards with Standard or Extended report.
+ If the keyboard has 2 virtual keyboard with Standard report(6KRO), it gets 12KRO.
+ Using this method means the keyboard is a composite device.
+
+2. Extended report
+ It needs large report size for this method to achieve NKRO.
+ If a keyboard has 101keys, it needs 103byte report. It seems to be inefficient.
+
+3. Bitmap report
+ If the keyboard has less than 128keys, 16byte report will be enough for NKRO.
+ The 16byte report seems to be reasonable cost to get NKRO.
+
+
+Report Format
+-------------
+Other report formats than followings are possible, though these format are typical one.
+
+1. Standard 8bytes
+ modifiers(bitmap) 1byte
+ reserved 1byte(not used)
+ keys(array) 1byte*6
+Standard report can send 6keys plus 8modifiers simultaneously.
+Standard report is used by most keyboards in the marketplace.
+Standard report is identical to boot protocol report.
+Standard report is hard to suffer from compatibility problems.
+
+2. Extended standard 16,32,64bytes
+ modifiers(bitmap) 1byte
+ reserved 1byte(not used)
+ keys(array) 1byte*(14,32,62)
+Extended report can send N-keys by using N+2bytes.
+Extended report is expected to be compatible with boot protocol.
+
+3. Bitmap 16,32,64bytes
+ keys(bitmap) (16,32)bytes
+Bitmap report can send at most 128keys by 16bytes and 256keys by 32bytes.
+Bitmap report can achieve USB NKRO efficiently in terms of report size.
+Bitmap report needs a deliberation for boot protocol implementation.
+Bitmap report descriptor sample:
+ 0x05, 0x01, // Usage Page (Generic Desktop),
+ 0x09, 0x06, // Usage (Keyboard),
+ 0xA1, 0x01, // Collection (Application),
+ // bitmap of modifiers
+ 0x75, 0x01, // Report Size (1),
+ 0x95, 0x08, // Report Count (8),
+ 0x05, 0x07, // Usage Page (Key Codes),
+ 0x19, 0xE0, // Usage Minimum (224),
+ 0x29, 0xE7, // Usage Maximum (231),
+ 0x15, 0x00, // Logical Minimum (0),
+ 0x25, 0x01, // Logical Maximum (1),
+ 0x81, 0x02, // Input (Data, Variable, Absolute), ;Modifier byte
+ // LED output report
+ 0x95, 0x05, // Report Count (5),
+ 0x75, 0x01, // Report Size (1),
+ 0x05, 0x08, // Usage Page (LEDs),
+ 0x19, 0x01, // Usage Minimum (1),
+ 0x29, 0x05, // Usage Maximum (5),
+ 0x91, 0x02, // Output (Data, Variable, Absolute),
+ 0x95, 0x01, // Report Count (1),
+ 0x75, 0x03, // Report Size (3),
+ 0x91, 0x03, // Output (Constant),
+ // bitmap of keys
+ 0x95, (REPORT_BYTES-1)*8, // Report Count (),
+ 0x75, 0x01, // Report Size (1),
+ 0x15, 0x00, // Logical Minimum (0),
+ 0x25, 0x01, // Logical Maximum(1),
+ 0x05, 0x07, // Usage Page (Key Codes),
+ 0x19, 0x00, // Usage Minimum (0),
+ 0x29, (REPORT_BYTES-1)*8-1, // Usage Maximum (),
+ 0x81, 0x02, // Input (Data, Variable, Absolute),
+ 0xc0 // End Collection
+where REPORT_BYTES is a report size in bytes.
+
+
+Considerations
+--------------
+Compatibility
+ boot protocol
+ minor/old system
+ Some BIOS doesn't send SET_PROTOCOL request, a keyboard can't switch to boot protocol mode.
+ This may cause a problem on a keyboard which uses other report than Standard.
+Reactivity
+ USB polling time
+ OS/Driver processing time
+
+
+Windows Problem
+---------------
+1. Windows accepts only 6keys in case of Standard report.
+ It should be able to send 6keys plus 8modifiers.
+2. Windows accepts only 10keys in case of 16bytes Extended report.
+ It should be able to send 14keys plus 8modifiers.
+3. Windows accepts only 18keys in case of 32bytes Extended report.
+ It should be able to send 30keys plus 8modifiers.
+If keys are pressed in excess of the number, wrong keys are registered on Windows.
+
+This problem will be reportedly fixed soon.(2010/12/05)
+ http://forums.anandtech.com/showpost.php?p=30873364&postcount=17
+
+
+Tools for testing NKRO
+----------------------
+Browser App:
+http://www.microsoft.com/appliedsciences/content/projects/KeyboardGhostingDemo.aspx
+http://random.xem.us/rollover.html
+
+Windows:
+AquaKeyTest.exe http://geekhack.org/showthread.php?t=6643
+
+Linux:
+xkeycaps
+xev
+showkeys
+
+EOF