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3.4 数据移位寄存器数据移位寄存器如图13所示.由2个4bit移位寄存器SRR24组成.数据移位寄存器用来储存输入数据,直至收到一个完整的字符.通用商品码是7位码元中的2个码条,因而每收到7bit后,便收到一个完整的字符.版本E码和版本A码分别有6个和12个字符长.因而需要用2个计数器,以便跟踪位计数和字符计数.虽说需要用2个CBU34计数器并非直接是数据移位寄存器的组成部分,但它们跟踪已经接收到的位数和字符数.既然CBU34是4bit计数器,它们分别预置9和10作为位计数和字符计数.在阅读版本A的情况下,字符计数器在收到第6个字符后清零,以适应条形码中的12个字符.位计数器在计数到7位时产生LASTBIT(末位)信号,它告诉其它逻辑电路已经收到了一个完整的字符.另一方面,宇符计数器在收到6个字符时发出LASTCHAR(末字符)信号.控制状态机用LASTCHAR信号来定义各种机器状态.
3.4 Data Shift Register The data shift register is shown in Figure 13. It consists of two 4-bit shift registers, SRR24, which are used to store the incoming data until a complete character is received. The common product code is a 7-bit code Element in the two bar, so every received 7bit, they receive a complete character version E code and version A code, respectively, 6 and 12 characters long.Therefore, the need for two counters to track the bit Counting and Character Counting Although 2 CBU34 counters are not directly part of the data shift register, they track the number of bits and characters that have been received. Since CBU 34 is a 4-bit counter, they prepend 9 and 10, respectively, as Bit count, and character count. The character counter is cleared after the sixth character is received to fit the 12 characters in the barcode, as of version A. The bit counter generates a LASTBIT (last bit) when counting to 7 bits, Signal that tells other logic that it has received a complete character, and on the other hand, the UFO counter issues a LASTCHAR (last character) signal upon receipt of 6. The control state machine uses the LASTCHAR signal to define various machine states.