COMMUNICATION PROTOCOL

MOTOROLA

MC33592 Technical Data

5

COMMUNICATION PROTOCOL

MANCHESTER CODING DESCRIPTION

Manchester coding is defined as follows: data is sent during the first half-bit, complementary data is sent

during the second half-bit.

Figure 3 : Manchester coding example

The signal average value is constant. This allows clock recovery from the data stream itself. In order to

achieve a correct clock recovery, Manchester coded data must have a duty cycle between 48% and 52%.

PREAMBLE, ID, HEADER WORDS AND MESSAGE DESCRIPTION

The following description applies if the Data Manager is enabled (DME=1).

The ID word is a Manchester coded byte whose content has been previously loaded in the Configuration

Register 2. The complement of the ID word is recognized as an ID word. ID word is sent at the same data rate as

data.

A preamble is required:

- before ID,

- before Header if HE=1,

- before data if HE=0.

It enables AGC to settle, and clock recovery. Figure 4 defines the Preamble word. Preamble content must be

carefully defined in order not to be decoded as an ID or Header word.

Figure 4 : Preamble definition

The Header word is a 4 bit Manchester coded message ‘0110’ or its complement sent at the selected data

rate. This bit sequence and its complement must not be found in the sequence preamble and ID word.

Data must follow the Header without any delay.

Data are completed by a End-of-Message (EOM) word, consisting of 2 NRZ consecutive ones or zeroes. If the

complement of the Header word is received, output data are complemented too.

The following example shows a complete message with Preamble, ID, Header words followed by 2 data bits,

and an EOM. The preamble is placed at the beginning of both ID and Header words.

0

0

0

11

1

0

Original data

Manchester coded

‘0’ Manchester

at data rate

‘1’ NRZ > 200µs

i.e.:

2 ‘1’ NRZ at 9.6kBd,

1 ‘1’ NRZ at 4.8kBd

AGC settling time

Clock recovery

ID

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com