1200/2400bps MSK Modem for Trunked Radio Systems
5 Application
11
MX429A
5.1 Checksum Generation and Checking
Generation – The checksum generator takes the 48 bits from the 6 bytes loaded into the Tx Data Buffer and
divides them modulo–2, by the generating polynomial;-
X15 + X14 + X13 + X11 + X4 + X2 + 1
It then takes the 15-bit remainder from the polynomial divider, inverts the last bit and appends an EVEN parity
bit generated from the initial 48 bits and the 15 bit remainder (with the last bit inverted). This 16–bit word is
used as the "Checksum."
Checking – The checksum checker does two things:
It takes the first 63 bits of a received message, inverts bit 63, and divides them modulo–2, by the generating
polynomial;-
X15 + X14 + X13 + X11 + X4 + X2 + 1
The 15 bits remaining in the polynomial divider are checked for all zero. Secondly, it generates an EVEN
parity bit from the first 63 bits of a received message and compares this bit with the received parity bit (bit 64).
If the 15 bits in the polynomial divider are all zero, and the two parity bits are equal, then the Rx Checksum
True bit (SR D1) bit is set.
5.2 Receive Operation
MODE 1 Where SYNC/SYNT is required after every message
Rx ENABLE
Rx INPUT
SYNC
SYNC
A1
A2
A3
A4
A5
A6
C1
C2
SYNC
SYNC
A1
A2
A3
A4
A5
A6
C1
C2
SYNC
IRQ OUTPUT
READ STATUS
REGISTER
Rx DATA
READY
SYNC (SYNT)
DETECT
Rx CHECKSUM
TRUE
READ Rx
DATA BUFFER
A1
A2
A3
A4
A5
A6
C1
C2
A1
A2
A3
A4
A5
A6
C1
C2
Rx MESSAGE
FORMAT
MODE 2
Where additional data will follow the initial address data, indicated by the state of the Rx Message Format bit
{additional Data Words}
Rx INPUT
SYNC
SYNC
A1
A2
A3
A4
A5
A6
C1
C2
D1
D2
D3
D4
D5
D6
C1
C2
D1
IRQ OUTPUT
READ STATUS
REGISTER
Rx DATA
READY
SYNC (SYNT)
DETECT
Rx CHECKSUM
TRUE
READ Rx
DATA BUFFER
A1
A2
A3
A4
A5
A6
C1
C2
D1
D2
D2
D4
D5
D6
C1
C2
Rx MESSAGE
FORMAT
Rx Message Format is sampled at this point to decide,
(a) whether the message is complete or (b), more data follows
Note: A - Address, C - Checksum, D - Datacode
Figure 5: Receive Operation
© 1998 MX-COM Inc.
www.mxcom.com Tel: 800 638-5577 336 744-5050 Fax: 336 744-5054
Doc. # 20480128.007
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
All trademarks and service marks are held by their respective companies.
CMLMICRO [ CML MICROCIRCUITS ]
