数电数字逻辑2

1、2.3 Codes in binary systemBCD codes: for decimal digit;Gray code: for numbers in binary system;ASCII code: for characters;Codes in binary systemKey point : Use n bits, we can make 2n different words;To make n code-words, you must use bits.BCD codesBinary codes for decimal digits ( Table 2-9 ) BCD co

2、desExpress 10 digit ( 09) : use 4 or more bits, many different way for coding8421 codeNatural code , just lake 4-bit binary numbers;Each digit is weighted;It has 10 valid code words and 6 invalid code words. BCD codesEach digit is weighted; plementing;Use MSB to express higher/lower part;It has 10 v

3、alid codes and 6 invalid codes.2421 codesBCD codesExcess-3 codeIts digit is not weighted; plementing ;8421 code + “0011” .BCD codesExamples: use BCD code for decimal numbers A = 19.468421 code : A = 0001 1001. 0100 01102421 code : A = 0001 1111. 0100 1100Excess-3 code: A = 0100 1100. 0111 1001 Key p

4、oint Each decimal digit is coded by a 4-bits string!BCD codes1-out-of-10 codeOne hot code;It is very useful in control systems.BCD codesBiquinary code 7-bits; two hot code; first 2 bits is one hot code for higher/lower range; last 5 bits is one hot code in the range. error-detecting property ! Gray

5、codesTarget: code for continues changed numbers (in binary system) to prevent wrong code happened in transition time;Property : In each pair of successive code words, only one bit changes.Gray codesFrom binary number to Gray code The width is same, the MSB is same; From left to right, if a bit in bi

6、nary number is same as its left bit, the gray code is 0, if it is different, the gray code is 1. Examples: binary number: 1001 0010 0110 0011 Gray codes: 1101 1011 0101 0010Codes for other thingsCharacter codes : ASCII , 7-bits string, 128 code words; p.54 Table 2-11Example: How are you! 1001000 110

7、1111 1110111 0100000 1100001 1110010 1100101 0100000 1111001 1101111 1110101 0100001 Codes for other thingsCodes for actions , conditions or states: Use n bits, we can make 2n different words; To make n different words, we must use bits. p.55 Table 2-12Codes in digital circuitParallel codes use n-line to transmit an n-bits code words ; transmit an n-bits code words in one time p