Elan Digital Systems Ltd.
28
HD717 USER’S
GUIDE
3.2.2 RECEIVE
The HD717 can receive Low Rate Data (or LRD) in various formats
and at a range of baud rates. The HD717 uses a serial to parallel
(s-to-p) converter and a holding register to maintain a continuous
data stream under interrupt control. The word size for the holding
register is 12-bits (ARINC 717). An interrupt is generated each time
the holding register is moved from the s-to-p converter. This means
that the interrupt frequency will be
(baud-rate)/12 interrupts/sec. Software must ensure that the
interrupts are serviced at a sufficient rate to maintain the data flow.
The interrupt is cleared when the high byte (nibble) of the 12-bit
word is read from the holding register (NB: the top 4 bits of the high
byte read as zero).
Data is received Least Significant Bit first.
The encoding format for the RX data is set by a 3 bit code in the
following way:
Encoding
PCR[6:5:4]
NRZ
NOT POSSIBLE TO RX
Bipolar RZ
001
b
FM0
(Biphase space)
010
b
FM1
(Biphase mark)
(Harvard biphase)
011
b
The baud rate is set using a programmable divider in the HD717.
See the LRD TX section for details of the baud rate.
Reception of Bipolar RZ encoded data is straight forward, as there is
sufficient clock information in every bit to ensure that the receiver is
in-sync with the transmitted data. In this mode, the receiver always
generates an interrupt when 12-bits of data have been collected and
placed in the RX holding register.