ZR36060PQC datasheet(24/46 Pages) ETC

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Integrated JPEG CODEC

6.5 Data Flow Overview

This section provides an overview of the data flow in the

ZR36060 during compression and decompression. For this

purpose it is useful to view the ZR36060 (see Figure 30 through

Figure 33) roughly as a JPEG engine with one dual port data

buffer on each side: the code FIFO buffer on one side and the

video buffer (strip buffer) on the other side.

6.5.1 Data Flow in Compression

The video input, after being processed in the video interface, is

written to the strip buffer in raster format. The JPEG engine

reads out the data in block format, and writes the JPEG code into

the code FIFO on the other side. From the FIFO the data is trans-

ferred out either by the ZR36060 itself, if it is the code bus

master, or by the host controller, if the ZR36060 is in code slave

mode.

When the ZR36060 is the code bus master (see Figure 30) it

writes out the code as long as its CBUSY input is not asserted.

When the code FIFO is empty the ZR36060 does not perform

code write cycles. If the host controller is too slow and it asserts

CBUSY for too long, the code FIFO might fill up. In order to

prevent overflow, the ZR36060 stops reading data from the Strip

buffer. If this situation continues long enough, the Strip buffer

overflows, because video keeps flowing in. A Strip buffer

overflow is a data corruption event. At the system level this event

may be prevented by two means: First, the host should be able

to accept the code at the same rate it is generated by the

ZR36060. Second, some system configurations may have the

capability to halt the video input stream when the Strip buffer is

close to overflow (16 pixels, or less, from overflow). The

ZR36060 indicates this “nearly full” condition with its RTBSY

output. One configuration for implementing this is if the ZR36060

is the master of the video syncs, and the system stops the video

using the PVALID input. This may be useful, for example, when

compressing still pictures.

When the code interface operates in slave mode (see Figure 31)

the scenario is almost identical. The main difference is that

CBUSY is now an output of the ZR36060, and it is used to

indicate that the code FIFO is “nearly empty”, thus the host must

stop reading out the code until CBUSY indicates that the FIFO

occupancy is above its threshold.

6.5.2 Data Flow in Decompression

The JPEG code is transferred on the code bus into the code

FIFO, either by the ZR36060, in Code Master mode, or by the

host, in Code Slave mode. The JPEG codec engine writes the

decoded video into the Strip buffer in block format. The video

interface reads the video out in raster format, executes the post-

processing operations and outputs the video on the digital video

bus.

When the ZR36060 is the code bus master (see Figure 32) it

reads in the code as long as its CBUSY input is not asserted.

Whenever the code FIFO is full the ZR36060 stops reading code

in. If the host controller is too slow and it asserts CBUSY for too

long, the code FIFO may become empty. In order to prevent

underflow the ZR36060 stops writing data into the Strip buffer. If

this situation continues long enough, the strip buffer underflows,

because the video unit keeps reading out video from the Strip

buffer, in order to keep up with the timing of the digital video bus.

A Strip buffer underflow is a data corruption event. At the system

level this event may be prevented by two means: First, the

system should be able to provide the code at the rate it is

required by the ZR36060. Second, some system configurations

may have the capability to stop the video output stream when the

Strip buffer is close to underflow (16 pixels, or less, away from

underflow). The ZR36060 indicates this “nearly empty” condition

with its RTBSY output. One configuration for implementing this

is if the ZR36060 is the master of the video syncs, and the

Figure 29. IDLE to WAIT-ACTIVE state transistion

START

FRAME

END

state

(async)

IDLE

WAIT ACTIVE

System decides to

compress the next field

(pulls START low).

System senses END deasserted,

so deasserts START to compress

next field only, then stop.

Tb = Delay while the ZR36060 completes execution of the Load command.

Figure 30. Data Flow in Compression, Code Master

Video Bus

Strip Memory

JPEG Codec

CODE FIFO

CODE Bus

RTBSY

CBUSY

CCS

Figure 31. Data Flow in Compression, Code Master

Video Bus

Strip Memory

JPEG Codec

CODE FIFO

CODE Bus

RTBSY

CBUSY

CCS

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