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HD3-6409-9 Datasheet(PDF) 6 Page - Intersil Corporation |
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HD3-6409-9 Datasheet(HTML) 6 Page - Intersil Corporation |
6 / 12 page 6 FN2951.2 July 29, 2005 Manchester Code Nonreturn-to-Zero (NRZ) code represents the binary values logic-O and Iogic-1 with a static level maintained throughout the data cell. In contrast, Manchester code represents data with a level transition in the middle of the data cell. Manchester has bandwidth, error detection, and synchronization advantages over NRZ code. The Manchester II code Bipolar One and Bipolar Zero shown below are logical complements. The direction of the transition indicates the binary value of data. A logic-0 in Bipolar One is defined as a Low to high transition in the middle of the data cell, and a logic-1 as a high to low mid bit transition, Manchester Il is also known as Biphase-L code. The bandwidth of NRZ is from DC to the clock frequency fc/2, while that of Manchester is from fc/2 to fc. Thus, Manchester can be AC or transformer coupled, which has considerable advantages over DC coupling. Also, the ratio of maximum to minimum frequency of Manchester extends one octave, while the ratio for NRZ is the range of 5-10 octaves. It is much easier to design a narrow band than a wideband amp. Secondly, the mid bit transition in each data cell provides the code with an effective error detection scheme. If noise produces a logic inversion in the data cell such that there is no transition, an error indiction is given, and synchronization must be re-established. This places relatively stringent requirements on the incoming data. The synchronization advantages of using the HD-6409 and Manchester code are several fold. One is that Manchester is a self clocking code. The clock in serial data communication defines the position of each data cell. Non self clocking codes, as NRZ, often require an extra clock wire or clock track (in magnetic recording). Further, there can be a phase variation between the clock and data track. Crosstalk between the two may be a problem. In Manchester, the serial data stream contains both the clock and the data, with the position of the mid bit transition representing the clock, and the direction of the transition representing data. There is no phase variation between the clock and the data. A second synchronization advantage is a result of the number of transitions in the data. The decoder resynchronizes on each transition, or at least once every data cell. In contrast, receivers using NRZ, which does not necessarily have transitions, must resynchronize on frame bit transitions, which occur far less often, usually on a character basis. This more frequent resynchronization eliminates the cumulative effect of errors over successive data cells. A final synchronization advantage concerns the HD-6409’s sync pulse used to initiate synchronization. This three bit wide pattern is sufficiently distinct from Manchester data that a false start by the receiver is unlikely. FIGURE 4. MANCHESTER CODE BIT PERIOD BINARY CODE NONRETURN TO ZERO BIPOLAR ONE BIPOLAR ZERO 12 3 4 5 01 1 0 0 Crystal Oscillator Mode FIGURE 5. CRYSTAL OSCILLATOR MODE LC Oscillator Mode FIGURE 6. LC OSCILLATOR MODE IX OX X1 R1 C0 16MHz C1 C1 CO C1 = 32pF C0 = CRYSTAL + STRAY X1 = AT CUT PARALLEL RESONANCE FUNDAMENTAL MODE RS (TYP) = 30Ω R1 = 15M Ω C1 C1 L C E C1 2C0 – 2 -------------------------- ≈ f O 1 2 π LC e ----------------------- ≈ C1 = 20pF C0 = 5pF IX OX HD-6409 HD-6409 |
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