SAB 82532/SAF 82532
Asynchronous Serial Mode
Semiconductor Group
69
07.96
6.2
Data Reception
6.2.1
The ESCC2 offers the flexibility to combine clock modes, data encoding and data
sampling in many different ways. However, only definite combinations make sense and
are recommended for correct operation:
Operating Modes
Asynchronous Mode
Prerequisites:
Bit clock rate 16 selected (CCR1:BCR = ‘1’)
Clock mode 0, 1, 3b, 4, or 7b selected
NRZ data encoding
The receiver which operates with a clock rate equal to 16 times the nominal data bit rate,
synchronizes itself to each character by detecting and verifying the start bit. Since
character length, parity and stop bit length is known, the ensuing valid bits are sampled.
Oversampling (3 samples) around the nominal bit center in conjunction with majority
decision is provided for every received bit (including start bit).
The synchronization lasts for one character, the next incoming character causes a new
synchronization to be performed. As a result, the demand for high clock accuracy is
reduced. Two communication stations using the asynchronous procedure are clocked
independently, their clocks need not be in phase or locked to exactly the same frequency
but, in fact, may differ from one another within a certain range.
Isochronous Mode
Prerequisites:
Bit clock rate 1 selected (CCR1:BCR = ‘0’)
Clock mode 2, 3a, 6, or 7a (DPLL mode) has to be used in conjunction with FM0, FM1
or Manchester encoding.
The isochronous mode uses the asynchronous character format. However, each data bit
is only sampled once (no oversampling).
In clock modes 0 and 1, the input clock has to be externally phase locked to the data
stream. This mode allows much higher transfer rates. Clock modes 3b, 4 and 7b are not
recommended due to difficulties with bit synchronization when using the internal baud
rate generator.
In clock modes 2, 3a, 6, and 7a, clock recovery is provided by the internal DPLL. Correct
synchronization of the DPLL is achieved if there are enough edges within the data
stream, which is generally ensured only if Bi-Phase encoding (FM0, FM1 or Manchester)
is used.