MC68HC908AS32A Datasheet, PDF (89/280 Pages) Freescale Semiconductor, Inc – Microcontrollers

BDLC CPU Interface
NBFS — Normalization Bit Format Select Bit
This bit controls the format of the normalization bit (NB) (see Figure 4-19). SAE J1850 strongly
encourages using an active long (logic 0) for in-frame responses containing cyclical redundancy check
(CRC) and an active short (logic 1) for in-frame responses without CRC.
1 = NB that is received or transmitted is a 0 when the response part of an in-frame response (IFR)
ends with a CRC byte. NB that is received or transmitted is a 1 when the response part of an
in-frame response (IFR) does not end with a CRC byte.
0 = NB that is received or transmitted is a 1 when the response part of an in-frame response (IFR)
ends with a CRC byte. NB that is received or transmitted is a 0 when the response part of an
in-frame response (IFR) does not end with a CRC byte.
TEOD — Transmit End of Data Bit
This bit is set by the programmer to indicate the end of a message is being sent by the BDLC. It will
append an 8-bit CRC after completing transmission of the current byte. This bit also is used to end an
in-frame response (IFR). If the transmit shadow register is full when TEOD is set, the CRC byte will be
transmitted after the current byte in the Tx shift register and the byte in the Tx shadow register have
been transmitted. (See 4.5.3 Rx and Tx Shadow Registers for a description of the transmit shadow
register.) Once TEOD is set, the transmit data register empty flag (TDRE) in the BDLC state vector
register (BSVR) is cleared to allow lower priority interrupts to occur. See 4.6.4 BDLC State Vector
Register.
1 = Transmit end-of-data (EOD) symbol
0 = The TEOD bit will be cleared automatically at the rising edge of the first CRC bit that is sent or
if an error is detected. When TEOD is used to end an IFR transmission, TEOD is cleared when
the BDLC receives back a valid EOD symbol or an error condition occurs.
TSIFR, TMIFR1, and TMIFR0 — Transmit In-Frame Response Control Bits
These three bits control the type of in-frame response being sent. The programmer should not set
more than one of these control bits to a 1 at any given time. However, if more than one of these three
control bits are set to 1, the priority encoding logic will force these register bits to a known value as
shown in Table 4-4. For example, if 011 is written to TSIFR, TMIFR1, and TMIFR0, then internally they
will be encoded as 010. However, when these bits are read back, they will read 011.
Table 4-4. BDLC Transmit In-Frame Response
Control Bit Priority Encoding
Write/Read
TSIFR
0
1
0
0
Write/Read
TMIFR1
0
X
1
0
Write/Read
TMIFR0
0
X
X
1
Actual
TSIFR
0
1
0
0
Actual
TMIFR1
0
0
1
0
Actual
TMIFR0
0
0
0
1
The BDLC supports the in-frame response (IFR) feature of J1850 by setting these bits correctly. The
four types of J1850 IFR are shown below. The purpose of the in-frame response modes is to allow
multiple nodes to acknowledge receipt of the data by responding with their personal ID or physical
address in a concatenated manner after they have seen the EOD symbol. If transmission arbitration is
lost by a node while sending its response, it continues to transmit its ID/address until observing its
unique byte in the response stream. For VPW modulation, because the first bit of the IFR is always
MC68HC908AS32A Data Sheet, Rev. 2.0
Freescale Semiconductor
89