AT76C101 Просмотр технического описания (PDF) - Atmel Corporation

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AT76C101

JPEG Image Compression Processor

Atmel Corporation 

AT76C101

The Huffman tables have the same format in both the com-

pression and decompression mode, but the arrangement of

the codes within the tables varies for the two modes. In the

compression mode, the Huffman tables are indexed by the

8-bit symbol corresponding to the code. For example, the

luminance AC Huffman table value corresponding to the 8-

bit symbol value of 17 will be stored in the 17th location of

the table, i.e. in the address locations 4068 to 4071. In the

decompression mode, the Huffman tables are arranged in

consecutive locations in increasing order of Huffman size.

All codes of the same Huffman size are then arranged in

terms of increasing Huffman code value. Appendix A,

annex C of the JPEG International Standard gives the pro-

cedure for generating the Huffman codes and sizes from

the Huffman table information extracted from the com-

pressed JPEG header.

MaxMin Table Loading

The Maxmin tables are an extra set of tables which are

required only for the decompression operation. Similar to

the Huffman tables, these tables can be loaded only after

the INT_Table_Load_Enable register has been set, and

this register has to be reset before normal operation of the

AT76C101 can commence. This table is required to keep

track of the maximum and minimum Huffman code values

for each Huffman code length. Since there is a maximum of

4 Huffman tables allowed and the longest Huffman code is

16 bits wide, the required maxmin table size is 64 words.

The four maxmin tables are arranged in increasing order of

size and each word of the maxmin table is 40 bits wide with

three fields, a pointer to the position of the minimum code

of that Huffman size, the minimum Huffman code and the

maximum Huffman code corresponding to that Huffman

code size. The maxmin tables are accessed by the host

bus in terms of five 8-bit slices with the lowest address cor-

responding to bit 0-7 of the maxmin table value and the

highest address corresponding to bit 32-39 (Figure 4).

These tables are generated from the AT76C101 specific

decompression mode Huffman tables and the algorithm to

develop them is given below:

for (i = 1..16) begin

maxmin_min_code[i] = -1

maxmin_max_code[i] = -1

end

for (i=1..total_num_of_huffman_codes)

if (maxmin_min_code[size[i]] == -1)

begin

maxmin_min_code[size[i]] = code[i]

maxmin_val_ptr[size[i]] = i

end

maxmin_max_code[size[i]] = code[i]

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