lm9831 National Semiconductor Corporation, lm9831 Datasheet - Page 37

• Manufacturer Part Number: lm9831
• Description: 42-bit Color, 1200dpi Usb Image Scanner
• Manufacturer: National Semiconductor Corporation

tion time for line n-1 (t
leakage may be as much as 2 to 6 times the sensor specified
image lag. This is usually not a problem. If it is, use a sensor with
a low image lag specification, or reduce the brightness of the
CCFL light source.
The second consideration is the stepsize calculation. Using the
ITA’s dual timebases affects the stepsize required to produce an
image with the correct vertical resolution. The solution is to calcu-
late the stepsize using the traditional formula, then multiply it by
the factor (ITA+1)/ITA:
15.0 KNOWN ISSUES AND SOLUTIONS
15.1 14-Bit Data Mode
The 14 Bit Data Mode (register 09, bit 5) has two uses. The first is
for calibration, where very accurate pixel data must be acquired
to allow calculation of the offset and shading coefficients. The
second is for scanning images where the pixel data transmitted to
the PC is greater than 8 bits/channel.
The current silicon has two problems in 14 bit mode:
1) When attempting to read more image data than the DRAM
buffer has available (underflowing the DRAM), the LM9831 may
insert an extra, random byte of data. This is not predictable or
detectable. The fix is simple: poll register 01 before reading regis-
ter 00, and only read the amount of data register 01 says is avail-
able.
For 1M x 16 DRAMs, read (<reg 01> - 1) * 8kbytes of data.
For 256k x 16 DRAMs, read (<reg 01> - 1) * 2kbytes of data.
Note: The information in register 01 (number of bytes of image
data in DRAM buffer) is not valid until the first line of image data
has been digitized. The solution is to poll register 01 until it starts
incrementing. When register 01 has incremented, the data it con-
tains is valid.
2) When the DRAM buffer’s address counter “wraps around”, it
may or may not insert 2 extra bytes of bad data. There is no way
to predict when or if this will happen. This limits the size of a con-
tinuous scan to the size of the DRAM line buffer (148k pixels or
916k pixels). This means that it is impossible to scan large
images with more than 8 bits/channel being transmitted to the
PC. Calibration can be done at 14 bits (since calibration is only a
few lines of data), but you can not reliably transmit 10, 12, or 14
bit images to the PC.
15.2 USB Interrupts
When configured to do so, changes on the Paper Sense and
MISC I/O pins were supposed to generate USB Interrupts. This
functionality is not working at the time of this datasheet’s publica-
tion. The solution (as demonstrated in our Twain Driver software)
is to poll register 02 every 200 to 500ms. This uses very little
additional bandwidth compared to the USB interrupt solution.
15.3 USB Remote Wakeup
When configured to do so, a high-to-low transition on the MISC
stepsize_ITA
INT2
) is 2 to 6 times longer than t
=
stepsize
ITA+1
----------- -
ITA
INT1
, the
37
I/O 1, MISC I/O 2, or MISC I/O 3 inputs was supposed to gener-
ate a USB Remote Wakeup. While the LM9831 passes the Chap-
ter 9 test for Remote Wakeup, we have not been able to get this
function to work with a PC that is in Suspend Mode. At the time of
release we are still trying to verify functionality of this feature.
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