Altera Serial Digital Interface (SDI) MegaCore Function - Manual (Page 135)

February 2013 Altera Corporation Serial Digital Interface (SDI) MegaCore Function
User Guide
C. Receive and Retransmit
You cannot reuse the HD and 3G recovered clocks for transmitting because the
transmitter jitter is then entirely dependent on the input jitter and jitter transfer
function.
The general recommended approach to system locking with the SDI MegaCore
function is to use a voltage-controlled crystal oscillator (VCXO) external to the device.
The VCXO must be locked to the receiver clock out of the SDI MegaCore function. The
SDI MegaCore function then uses the clean VCXO output as the transmit clock.
Loopback FIFO Buffer
For more efficient transmission, place a FIFO or buffer between the receiver clock
domain logic and the transmit clock domain logic. The decoded receiver data is
connected to the transmitter input through a FIFO buffer. When the receiver is locked,
the logic writes the receiver data to the FIFO buffer. When the FIFO is half full, the
transmitter starts reading, encoding and transmitting the data.
Figure C–1 shows the clocking scheme of the received and retransmitted data.
Figure C–1. Receive and Retransmit
SDI Receiver
refclk
rx_clk wrclk
sdi_in
rx_data data
rdclk
q
SDI Transmitter
refclk
tx_clk gxb_clkout
sdi_out tx_data
VCXO XTAL
FIFO
PFD
CTRL