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MRF89XA датащи(PDF) 86 Page - Microchip Technology |
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MRF89XA датащи(HTML) 86 Page - Microchip Technology |
86 / 140 page MRF89XA DS70000622D-page 86 Preliminary 2010-2017 Microchip Technology Inc. 3.11.4 DC-FREE DATA MECHANISMS The payload to be transmitted may contain long sequences of ‘1’s and ‘0’s. These continuous sequences introduce a DC bias in the transmitted signal, which causes a non-uniform power distribution spectrum over the occupied channel bandwidth. These sequences also degrade the performance of the demodulation and data, and clock recovery functions in the receiver, which could further introduce data dependencies in the normal operation of the demodulator. System performance can be enhanced if the payload bits are randomized to reduce DC biases and increase the number of bit transitions. Therefore, it is useful if the transmitted data is random and DC-free. To handle such instances, two techniques are available in the packet handler: Manchester encoding and Data Whitening. However, only one of the two methods should be enabled at a time. 3.11.4.1 Manchester Data Encoding Manchester encoding/decoding is enabled by setting the MCHSTREN bit (PLOADREG<7>) and can be used in Packet mode only. The NRZ data is converted to Manchester code by coding ‘1’ as ‘10’ and ‘0’ as ‘01’. Figure 3-29 illustrates Manchester data encoding. NRZ data is converted to Manchester by encoding the 1 bits as 10 chip sequences, and the 0 bits as 01 chip sequences. Manchester encoding guarantees DC- balance and frequent data transitions in the encoded data. The maximum Manchester chip rate corresponds to the maximum bit rate given in the Transmitter Electrical specifications in Table 5-6. In this case, the maximum chip rate is the maximum bit rate given in the specifications section and the actual bit rate is half the chip rate. Manchester encoding and decoding is only applied to the payload and CRC checksum while the preamble and Sync Word are kept NRZ. However, the chip rate from the preamble to CRC is the same and defined by the BRVAL<6:0> bits (BRSREG<6:0>) (Chip Rate = Bit Rate NRZ = 2 x Bit Rate Manchester). Therefore, Manchester encoding or decoding is made transparent to the user who still provides or retrieves NRZ data to or from the FIFO. See the Manchester encoding or decoding bit pattern in Figure 3-30. 3.11.4.2 Data Whitening Another technique called data whitening or scrambling is widely used for randomizing the user data before radio transmission. The data is whitened using a random sequence on the TX side and dewhitened on the RX side using the same sequence. Compared to the Manchester technique, it has the advantage of retaining the NRZ data rate (that is, actual bit rate is not halved). The whitening or dewhitening process is enabled by setting the WHITEN1 bit (PKTCREG<4>). A 9-bit Linear Feedback Shift Register (LFSR) is used to generate a random sequence. The payload and 2-byte CRC checksum is then XORed with this random sequence as illustrated in Figure 3-31. The data is dewhitened on the receiver side by XORing with the same random sequence. Payload whitening or dewhitening is made transparent to the user who still provides or retrieves NRZ data to or from the FIFO. FIGURE 3-29: MANCHESTER DATA ENCODING FIGURE 3-30: MANCHESTER ENCODING/DECODING ...Sync Payload... RF chips @ BR ... 1 1 1 0 1 0 0 1 0 0 1 0 1 1 0 1 0 ... User/NRZ bits Manchester OFF ... 1 1 1 0 1 0 0 1 0 0 1 0 1 1 0 1 0 ... User/NRZ bits Manchester ON ... 1 1 1 0 1 0 0 1 0 0 1 1 ... t 1/BR 1/ BR |
Аналогичный номер детали - MRF89XA |
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Аналогичное описание - MRF89XA |
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