CX25870/871
3.0 PC Board Considerations
Flicker-Free Video Encoder with Ultrascale Technology
3.6 Applications Information
3.6 Applications Information
3.6.1 Changes Required to Accommodate CX25870/871 in Bt868/869-Designs
3.6.1.1 Software
The CX25870/871 is software backward compatible with Conexant’s first
generation VGA Encoder, the Bt868/869. This means that all register indices for
the Bt868/869 were carried forward to the exact same indices for the
CX25870/871. For Conexant’s second generation encoder, new registers were
added, but the actual addresses used were outside of the 0x6C to 0xD6 range
reserved for the Bt868/869 legacy functionality. Some reserved bits within the
Bt868/869 did take on significance with the CX25870/871 where necessary.
Table 3-2. Relative Register Map for CX25870/871
Shared CX25870/871 &
Bt868/869 registers
Register Addresses 0x00 to 0x04 (Read Only)
(Must be accessed through ‘Legacy’ read procedure with ESTATUS[1:0] bits in Bt868/869)
(‘Standard’ or ‘Legacy’ read-back procedure ok for CX25870/871)
Register Address 0x06(Read Only)
CX25870/871 specific
registers
and
Register Addresses 0x28E to 0x6A (Read/Write)
Register Address 0x6C(Read/Write)
Shared CX25870/871 &
Bt868/869 registers
to
Register Address 0xD6(Read/Write)
Register Address 0xD8(Read/Write)
CX25870/871 specific
register
The most significant difference in software between the two encoders is the
fact that the CX25870/871 can be read from using the Standard serial method as
well as the Legacy serial method. To use the Standard procedure, the master
issues CX25870’s device ID and subaddress in consecutive bytes, and the slave
acknowledges with a pulse after each transaction. Upon completion of these 2
steps, the slave transmits the final byte which contains the 8 bits of data. The
Bt868/869 cannot be read from in this manner and instead relies solely on the
Legacy method. This process is explained step-by-step in the ‘TV Auto-Detection
Procedures’ section of this specification.
Another difference in terms of software between the two encoders is the
power-up video output routing. The CX25870 after power-up or a signal-driven
reset transmits Video0 = composite on DAC_A, Video1 = Luma (Y) on DAC_B,
Video2 = Chroma (C) on DAC_C, and Video3 = Luma Delay on DAC_D.
The Bt868 was different in this respect. On power-up, it sent out
Video0 = composite from DAC_A, DAC_B, and DAC_C. Reprogramming
register 0xCE correctly ensures proper video output routing.
100381B
Conexant
3-11
CONEXANT [ CONEXANT SYSTEMS, INC ]
