NM95HS01 Datasheet, PDF (8/20 Pages) National Semiconductor (TI) – HiSeC-TM High Security Rolling Code Generator

Data Frame Fields (Continued)
The preamble has a fixed format of two bit times at system
logic high then one-bit time at system logic low then eight
zeroes using the user-selected bit coding format This ar-
rangement is clearly shown in Figure 6 for several bit coding
formats
If desired a preamble may be isolated from the frame by
eight-bit times at logic low during a frame transmission This
can be achieved by enabling the sync field in NRZ mode
with the byte 0h
SYNC FIELD
If enabled the sync field is transmitted in every normal data
frame or sync frame to provide a bit timing reference for the
rest of the frame This allows the decoder to determine the
proper bit coding format the generator is using and to syn-
chronize to it
The sync field option is set with the SyncPresent bit in the
EEPROM array If SyncPresent e 0 no sync field is sent If
SyncPresent e 1 an 8-bit sync field is included in the data
transmission This 8-bit field is transmitted Most Significant
Bit first
The sync field data is programmable and can be encoded
with any user-selected bit coding format or with an NRZ
(unencoded binary) bit format The option to select between
a user bit coding format and NRZ format is set by configur-
ing the SyncType bit in the EEPROM array If SyncType e
0 sync field data is sent according to the user-selected IR
or RF bit coding format If SyncType e 1 the information is
sent in NRZ format with the bit length determined by the
chosen IR or RF bit coding format
For NRZ bit coding both high and low bit times are the
same as the IR or RF bit coding time For bit coding modes
where the ‘‘1’’s and ‘‘0’’s have different bit lengths all IR
modes for example the length of the NRZ ‘‘1’’ and ‘‘0’’
bits have correspondingly different bit lengths
RF bit coding format 7 is a special case As in the other
formats if SyncType e 0 information is sent according to
the user-set IR or RF bit coding format However if Sync-
Type e 1 a ‘‘0’’ is sent using the bit coding determined by
the IR or RF coding format and a ‘‘1’’ is sent as an NRZ
zero This is to maintain the ‘‘spirit’’ of the low duty cycle
arrangement for RF format 7
Figure 7 shows sync field examples for several bit coding
formats
FIGURE 6 Preamble Format Examples
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FIGURE 7 Sync Field ExampIes for Data Byte 03h
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