AT76C101

6

Before loading, the quantization tables have to be con-

verted from the JPEG interchange format (JPEG_Q_Table)

to the AT76C101 specific format (AT76C101_Q_Table).

This format varies, based on the mode of operation of the

chip (compression or decompression). The steps to convert

to the AT76C101 specific format are given below:

• The Quantization Tables are stored in the zigzag form in

the JPEG interchange format. Convert these tables to

the normal (un-zigzagged) form.

• Convert

to

AT76C101

format

using

the

following

algorithm where, M_Factor[i][j] = 16,384, if (i=1 and j=1)

32,768/sqrt(2),

if (i=1 and j=2.8) or (i=2.8 and j=1)

32,768 otherwise:

for (i = 1..8) begin

for (j = 1..8) begin

if (mode = COMPRESS)

AT76C101_Q_Table[i][j] =

(M_Factor[i][j]/

JPEG_Q_Table[i][j]) + 0.5

else if (mode = DECOMPRESS)

AT76C101_Q_Table[i][j] =

JPEG_Q_Table[i][j]

end

end

The Quantization tables are 16 bits wide and are loaded

using the host bus. The host bus accesses the quantization

table values in two 8-bit halves, with the lower address cor-

responding to the lower order byte and the higher address

corresponding to the higher order byte (Figure 4).

Huffman Table Loading

Like the Quantization tables, the AT76C101 Huffman

tables are specific to the chip. Before normal operation of

the chip, the Huffman tables must be loaded in the

AT76C101 specific format into the on-chip RAM. The

INT_Table_Load_Enable register has to be set to load the

Huffman tables. When the Huffman tables are loaded into

the internal RAM, they remain valid until the power is

switched off or until they are reprogrammed (they are not

affected by RESET). The Huffman_Table_Load_Enable

register has to be reset after the tables are loaded and

before normal operation of the chip can begin.

The AT76C101 can store up to four Huffman tables: 2 Huff-

man AC tables and 2 Huffman DC tables. The memory

organization and address mapping of the Huffman tables

are given in Figure 5. The Huffman tables are 28 bits wide

and consist of three fields, the size of the Huffman code

(stored as 1 less than actual size), the Huffman code and

the 8-bit symbol corresponding to the Huffman code. Each

Huffman table value is accessed from the host bus in four

8-bit slices, with the lower order 8 bits corresponding to the

lowest address (for e.g. 2000H) and the highest 8-bit slice

(bit 24 through bit 31) corresponding to the highest address

(2003H).

Figure 5. Internal Memory Organization (Huffman Tables)

Y

1000h

1004h

1040h

UV

DC HUFFMAN TABLE

2000h

2004h

2400h

AC HUFFMAN TABLE

8-BIT SYMBOL

CODE (BITS 3-0)

1000h

1001h

28 BITS

Y

UV

SIZE -1

CODE (BITS 11-4)

0000

CODE (BITS 15-12)

1002h

1003h

0000

CODE

SIZE - 1

8-BIT SYMBOL

16 BITS

4 BITS

8 BITS