45.6.3.4 Manchester Decoder

When US_MR.MAN is ‘1’, the Manchester decoder is enabled. The decoder performs both preamble and start frame delimiter detection. One input line is dedicated to Manchester encoded input data.

An optional preamble sequence can be defined, and its length is user-defined and totally independent of the emitter side. The length of the preamble sequence is configured using US_MAN.RX_PL. If RX_PL is ‘0’, no preamble is detected and the function is disabled. The polarity of the input stream is configured with US_MAN.RX_MPOL. Depending on the desired application, the preamble pattern matching is to be defined via the US_MAN. See Figure 45-8 for available preamble patterns.

Unlike preamble, the start frame delimiter is shared between Manchester Encoder and Decoder. If US_MR.ONEBIT is written to ‘1’, only a zero-encoded Manchester can be detected as a valid start frame delimiter. If US_MR.ONEBIT is written to ‘0’, only a sync pattern is detected as a valid start frame delimiter. Decoder operates by detecting transition on incoming stream. If RXD is sampled during one quarter of a bit time to zero, a start bit is detected. See Figure 45-13. The sample pulse rejection mechanism applies.

The US_MAN.RXIDLEV informs the USART of the receiver line idle state value (receiver line inactive). The user must define RXIDLEV to ensure reliable synchronization. By default, RXIDLEV is set to ‘1’ (receiver line is at level 1 when there is no activity).

Figure 45-13. Asynchronous Start Bit Detection

The receiver is activated and starts preamble and frame delimiter detection, sampling the data at one quarter and then three quarters. If a valid preamble pattern or start frame delimiter is detected, the receiver continues decoding with the same synchronization. If the stream does not match a valid pattern or a valid start frame delimiter, the receiver resynchronizes on the next valid edge.The minimum time threshold to estimate the bit value is three quarters of a bit time.

If a valid preamble (if used) followed with a valid start frame delimiter is detected, the incoming stream is decoded into NRZ data and passed to the USART for processing. Figure 45-14 illustrates Manchester pattern mismatch. When incoming data stream is passed to the USART, the receiver is also able to detect Manchester code violation. A code violation is a lack of transition in the middle of a bit cell. In this case, the US_CSR.MANERR flag is raised. It is cleared by writing a ‘1’ to US_CR.RSTSTA. See Figure 45-15 for an example of Manchester error detection during data phase.

Figure 45-14. Preamble Pattern Mismatch
Figure 45-15. Manchester Error Flag

When the start frame delimiter is a sync pattern (US_MR.ONEBIT = 0), both command and data delimiter are supported. If a valid sync is detected, the received character is written in RXCHR in the Receive Holding register (US_RHR) and RXSYNH is updated. RXSYNH is set to ‘1’ when the received character is a command, and to ‘0’ if the received character is a data. This alleviates and simplifies the direct memory access as the character contains its own sync field in the same register.

As the decoder is setup to be used in Unipolar mode, the first bit of the frame has to be a zero-to-one transition.