PCVisionplus
Hardware Reference
Theory of Operation
2–13
Rev 02; February 8, 2002
Clipping should not be enabled when padding is enabled. Clipping is used to transfer 8-bit image data to the primary
(8-bit VGA) display surface, and padding is used to transfer 8-bit or 12-bit image data to the secondary (16-bit
YCrCb) display surface. The two features should never be used together.
2.4.4.5 Bus Master Data Shift
The BMC includes a mode for internally shifting or normalizing 12-bit data to 16 bits. In 12-bit mode each 12-bit
pixel occupies two bytes. If the data is not normalized to 16 bits by the input LUT, the low byte contains pixel bits 7
through 0, and the high byte contains pixel bits 11 through 8 with the upper 4 bits filled with “don’t care” values, as
shown in Figure 2–12. With BMSHIFT enabled, each word read from the image buffer memory is left-shifted four
places. The 12-bit image data was moved from the lower 12 bits of the data word to the upper 12 bits of the data word.
This is also called “normalization to 16 bits”.
xxxx 11 ... 0
xxxx 11 ... 0
xxxx 11 ... 0
xxxx 11 ... 0
0
15
pixel0
pixel1
pixel2
pixel3
Image Buffer Memory
Bus Master Controller Output
11 .... 0 xxxx
11 .... 0 xxxx
pixel0
pixel1
Figure 2–12. Bus Master Data Shift
2.4.5 Bus Master Latency
Transfer speed is mainly dependent on the host system and the target’s capabilities. Bus traffic (other bus masters
arbitrating for and gaining bus ownership) and target capabilities (PCVisionplus can only transfer data as fast as the
target can receive it) have implications on overall transfer speed. Within the configuration register set are several
registers which define the amount of time the PCVisionplus requires for bus master operations. The Latency Timer
register is set with a value (number of PCI-bus clocks) that defines the minimum guaranteed time PCVisionplus will
be allowed to hold bus ownership during a bus master transfer. This register acts as a counter during bus master
transfers, and once it counts down from its programmed value any other bus master request will cause PCVisionplus
to lose bus ownership. This register can be set to prevent multiple bus disconnects during a PCVisionplus transfer.
Two other registers exist in the configuration register set which provide information to the system about the time the
PCVisionplus requires for bus master transfers. Minimum Grant is set to the maximum value, indicating the PCVi-
sionplus desires at least 64 us (microseconds) for each transfer. Maximum Latency is set to the minimum value,
indicating the PCVisionplus desires 250 ns (nanoseconds) in between bus master transfers. The use of these registers
is system dependent, and not always required.
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