DS730 Datasheet, PDF (35/46 Pages) Xilinx, Inc – LogiCORE IP Video Direct

LogiCORE IP Video Direct Memory Access v1.1
Slave Gen-Lock
When used in Slave Gen-Lock mode, the Start Address register that the Video DMA uses when writing commands
to the VFBC is specified by the Gen-Lock Master. See the "Gen-Lock Operation" section for details about the Gen-
Lock mechanism.
Figure 10 shows the Video DMA operating as a Gen-Lock Slave for multiple read transfers. In this example, the
rd_frame_ptr_in bus is driven by a single master. The core was configured to use 16 frames stores, so the first
series of transfers follows the grey code sequence of 0, 1, 3, 2 and 6. Internal to the Video DMA, this is decoded to
mean that Read Start Address registers 0, 1, 2, 3 and 4 should be used for each successive command.
X-Ref Target - Figure 10
Figure 10: Gen-Lock Slave Register Read Command Mode
Gen-Lock Operation
In many video applications, a producer of data will run at a different rate than the consumer of that data. To avoid
the potential ill effects that such a rate mismatch can cause, frame buffering is often used. Frame buffering allocates
multiple frames worth of memory to be used to hold the data. The data producer writes to one buffer while the
consumer reads from another. The two are kept in sync by not allowing both to use the same buffer at the same time.
Typically one of the two will be forced to either skip or repeat frames as necessary. This type of synchronization is
called Gen-Lock. The Video DMA has been designed to operate as either a Gen-Lock Master or Slave. Both of these
modes are discussed in the following sections.
The Gen-Lock mechanism that the Video DMA implements is based around the Start Address registers. The
number of Start Address registers can be configured to be between 1 and 16. The “Frame Stores” parameter in the
CORE Generator is used to specify this value. The Gen-Lock Master uses the index of the Start Address register to
specify which Start Address register the Gen-Lock Slave should use. This Start Address Register index is encoded
as a Grey code value. The Grey code that is used depends upon the number of Frame Stores that was specified.
Table 11 lists the Grey Codes that are used for each of the 16 possible Frame Store sizes. The Grey code cycles
through all of the codes on the first line first and then cycles through all of the codes on the second line before
repeating the first line.
DS730 September 21, 2010
www.xilinx.com
35
Product Specification