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