FX929AD2 datasheet(13/44 Pages) CMLMICRO

13 / 44 page

4-Level FSK Modem Data Pump

FX929A

D/929A/4

The 'Header' block is self-contained in that it includes its own checksum (CRC1), and would normally carry

information such as the address of the calling and called parties, the number of following blocks in the frame (if

any) and miscellaneous control information.

The 'Intermediate' block(s) contain only data, the checksum at the end of the 'Last' block (CRC2) also checks

the data in any preceding 'Intermediate' blocks.

Proprietary systems which do not use the RD-LAP format may use the block structures provided by the

FX929A to build alternative frame formats more suited to the particular application. Some examples are

illustrated in Figure 7a below.

Figure 7a Some Alternative Frame Structures

The FX929A performs all of the block formatting and de-formatting, the binary data transferred between the

modem and its

µC being that enclosed by the thick dashed rectangles near the top of Figure 7.

1.5.5

The Programmer's View

The modem appears to the programmer as 4 write only 8-bit registers shadowed by 3 read only registers,

individual registers being selected by the A0 and A1 chip inputs:

Write to Modem

Read from Modem

Data Buffer

Command Register

Status Register

Control Register

Data Quality Register

Mode Register

not used

Note that there is a minimum allowable time between accesses of the modem's registers, see Section 1.7.1 for

details.

1.5.5.1

Data Block Buffer

This is a 12-byte read/write buffer which is used to transfer data (as opposed to command, status, mode, data

quality or control information) between the modem and the host µC.

It appears to the µC as a single 8-bit register; the modem ensuring that sequential µC reads or writes to the

buffer are routed to the correct locations within the buffer.

The µC should only access this buffer when the Status Register BFREE (Buffer Free) bit is '1'.

The buffer should only be written to while in Tx mode and read from while in Rx mode. Note that in receive

mode the modem will function correctly even if the received data is not read from the Data Buffer by the

µC.

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