CMX148 datasheet(30/75 Pages) CMLMICRO

30 / 75 page

PMR Audio and Data Processor

CMX148

D/148/6

The four Control Field bytes have Forward Error Correction (FEC) applied to them in the transmitter, this

adds 4 bits to every byte and the receiver can correct errors in the received bytes. The 4 received bytes

are then checked for a correct CRC, so that corrupted Frame Heads can be rejected. If checksum A

indicates that the Control Field bytes are correct, the Address (byte 1) is compared with that stored in the

MSK Header Address bits of Modem Address register, $B6 (b15:8). If a match occurs, or if the received

address is '40', then an interrupt is raised indicating a valid Frame Head has been received. The Frame

Head is 80 bits long (16 + 16 + {4x12}). The contents of a received Frame Head can be read from Rx

Data registers $C5 and $C9.

7.9.4.

Data Block Coding

The Data Block follows the Frame Head and can be coded with different levels of error correction and

detection. The Data Block format is controlled by the Frame Format selected in Frame Head byte 2, see

also section 8.1.24. Messages can take the following formats:

Format: Description:

Un-Formatted Data. This mode should be used with the En_RAW bit (b10 of the Modem

Control Register) set to 1. The Interleave and Scrambler settings are ignored. This mode can

be useful when interfacing to a system using a different format to those available in the

CMX148.

In transmit, the device will transmit only the data loaded into the TxData 1 register. The host

should provide bit sync, frame sync and any required formatting data as well as the Data Block

through this register.

In receive, the device will search for the programmed 16-bit Frame Sync pattern and then output

all following data 16 bits at a time. The host will have to perform all other data formatting.

Frame Head only. No Data Block will be added. This format can be useful for indicating channel

or user status by using byte 3 and the User Bit of the Frame Head (see section 8.1.24).

Frame Head followed by raw data. User data is appended to the Frame Head in 2-byte units

with no formatting or CRC added by the CMX148. No size information is set in the Frame Head

and the Data Block may contain any even number of bytes per frame.

Frame Head followed by FEC coded data only. Each byte of the user data has 4 bits of FEC

coding added. No size information is set in the Frame Head and Data Block may contain any

even number of bytes per frame. No CRC is added to the data.

Frame Head followed by FEC coded data with an automatic CRC at the end of the Data

Block. The number of user data bytes in the Frame must be set in Frame Head byte 3. The CRC

is automatically checked in the receiver and the result indicated to the host µC. Up to 255 bytes

of user data can be sent in each frame using this format.

As ‘4 above’, with the addition of all Data Block bytes being interleaved. This spreads the

transmitted information over time and helps reduce the effect of errors caused by fading.

Interleaving is performed on blocks of 4 bytes, the CMX148 automatically adds and strips out

pad bytes to ensure multiples of 4 bytes are sent over the radio channel.

Notes:

•

Format 0, 1, 2 and 3 have no size information requirement and do not reserve Frame Head byte

3. This byte may be freely used by the host µC to convey information. In Format 4 and 5 this byte

must be set to the number of user bytes in the message attached to that Frame Head (

≤255) to

allow the receiver to correctly decode and calculate the CRC.

•

Format 0 data transfers do not use frame heads. In Tx the host µC must transfer the bit and frame

sync data before sending the message data. In Rx the host µC must decode all data after the

frame sync.

CMX148 Datasheet(PDF) 30 Page - CML Microcircuits

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