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