DB829 Datasheet, PDF (9/29 Pages) CML Microcircuits – Low Voltage Operation

Baseband Signal Processor
9
MX829 PRELIMINARY INFORMATION
5.2.3 CONTROL 2 Register (Hex address $11)
This register is used to control the functions of the device as described below:
CHKSUM
(Bit 7)
In the Tx mode, when this bit is "1", the checksum generator is enabled. All complete bytes that are
transmitted after this time are used in the checksum calculation.
When this bit goes from "1" to "0", the checksum generator will complete its calculations on the
current byte and the result will be sent as the next two bytes of transmitted data.
In the Rx mode, the "0" to "1" transition of the CHKSUM bit is used at the start of the next byte
received at DEMODIN to manually reset the Rx checksum calculation, see Figure 4. The calculation
can also be reset automatically by a SYNC, SYNT, or RX SYNC WORD detection - see section 5.2.5.
In this case, the Rx checksum calculation starts with the first data byte after the 2-byte sync word has
been detected. The CHKSUM bit can be reset to "0" at any time. The result of the checksum is made
available in the STATUS Register after the reception of every complete byte (See section 5.3.1).
Note that the device is designed to work with any message length, and as a consequence it is not
aware of the position of the checksum within the incoming data message. It thus performs a
checksum assessment after every received byte. The controlling software should use its knowledge
of the system message length in order to determine which RXSUMF reading is valid, i.e. after the
second of the two checksum bytes has been received.
The timing of data bytes relative to the checksum bit is shown in Figure 3 and Figure 4.
TXDATAF (bit 3)
STATUS REGISTER
READ STATUS
REGISTER
WRITE TO
Tx DATA BUFFER
CHKSUM (bit 7)
CONTROL 2 REGISTER
Tx OUTPUT
1
2
3
4
5
6
7
1
2
3
4
5
2 - BYTE
CHECKSUM
CHECKSUM based on bytes 3, 4, 5.
Figure 3: Checksum Generation in Tx Mode
6
7
DEMODIN
(Rx Data)
RXDATAF (bit 4)
STATUS REGISTER
READ STATUS
REGISTER
READ Rx
DATA BUFFER
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
CHKSUM (bit 7)
CONTROL 2 REGISTER
RXSUMF (bit 6)
STATUS REGISTER
CHECK CHECK CHECK CHECK
4
4+5 4+5+6 4+5 +6+7
Figure 4: Checksum Calculation in Rx Mode
© 1998 MX•COM Inc.
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