CMX649 Wireless Voice Link Design Guide
©
2004 CML Microcircuits
Page 31 of 42
process of Manchester decoding. The properly aligned data byte is divided by two,
mapped to a Manchester lookup table, and the decoded value is placed in the R6
register. Since Manchester encoding doubles the amount of data needed for a
transaction, an eight-bit Manchester encoded byte actually includes only four bits of
‘real’ data. For this reason, the decoded data byte is effectively split into upper and
lower nibbles and processed accordingly. A flag is used to indicate whether the
upper or lower nibble is needed, and once both nibbles have been decoded, the
FROMRF_CNTR is decremented. Transmit and receive interrupts are disabled
when FROMRF_CNTR becomes negative, as this indicates that the expected
number of data bytes have been received and the incoming data frame is complete.
The RFCHIPRX ISR is illustrated in the following flowchart:
