Texas-instruments TMS320C64x DSP Manuel d'utilisateur télécharger pdf (Page 101)

TSI Capture Mode

3-39Video Capture PortSPRU629

Figure 3–23. Program Clock Reference (PCR) Header Format

47 15 14 9 8 0

PCR

Reserved PCR extension

The video port, in conjunction with the VCXO interpolated control (VIC), allows

a combined hardware and software solution to synchronize the local system

time clock (STC) with the encoder time clock reference transmitted in the bit

stream.

The video port maintains a hardware counter that counts the system time. The

counter is driven by a system time clock (STCLK) input driven by an external

VCXO. The counter is split into two fields: a 33-bit field (PCR base) that counts

at 90 kHz and a 9-bit field (PCR extension) that counts at 27 MHz. The 9-bit

counter counts from 0 to 299 at 27 MHz. Each time the 9-bit counter rolls over

to 0, the 33-bit counter is incremented by 1. This is equivalent to the PCR time-

stamp transmitted in the bit-stream. The 33-bit field can also be programmed

to count at 27 MHz for compatibility with the MPEG-1 32-bit PCR, by setting

the CTMODE bit in VCCTL to 1; in which case, the PCR extension portion of

the counter is not used. Figure 3–24 shows the system time clock counter

operation.

Figure 3–24. System Time Clock Counter Operation

27 MHz

Modulo 300

Counter 233

PCR Extension

PCR Base

CTMODE

STCLK

90 kHz

External VCXO

On reception of a packet (during the sync byte), a snapshot of the counter is

captured. This snapshot, or timestamp, is inserted in the receiving FIFO at the

end of each data packet. Software uses this timestamp, to determine the devi-

ation of the local system time clock from the encoder time clock. Any time a

packet with a PCR header is received, the timestamp for that packet is

compared with the PCR value by software. A PLL is implemented in software

to synchronize the STCLK with the encoder time clock value in the PCR. This

algorithm then drives the VIC, which drives the VDAC output to the external

VCXO, which supplies STCLK.

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Texas-instruments TMS320C64x DSP Manuel d'utilisateur Page 101