Baseband Signal Processor

9

MX829 PRELIMINARY INFORMATION

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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.

READ STATUS

REGISTER

Tx OUTPUT

CHECKSUM based on bytes 3, 4, 5.

TXDATAF (bit 3)

STATUS REGISTER

1

2

34

5

2 - BYTE

CHECKSUM

67

CHKSUM (bit 7)

CONTROL 2 REGISTER

WRITE TO

Tx DATA BUFFER

12

5

6

7

3

4

Figure 3: Checksum Generation in Tx Mode

READ STATUS

REGISTER

RXDATAF (bit 4)

STATUS REGISTER

DEMODIN

(Rx Data)

1

READ Rx

DATA BUFFER

0

23

4

56

7

CHKSUM (bit 7)

CONTROL 2 REGISTER

1

2

3

4

4

CHECK

CHECK

CHECK

CHECK

5

4+

5

7

6

++

4 5 6

++ +

4 5

7

6

RXSUMF (bit 6)

STATUS REGISTER

Figure 4: Checksum Calculation in Rx Mode