Freescale Semiconductor MPC5200B - Manual (Page 698)

Functional Description
MPC5200B Users Guide, Rev. 1
Freescale Semiconductor 20-19
The SOF symbol is defined as passive to active transition followed by an active period 200 µs in length ( Figure 20-5 (c)). This allows
the data bytes which follow the SOF symbol to begin with a passive bit, regardless of whether it is a logic one or a logic zero.
• EOD - End of Data Symbol
The EOD symbol is defined as an active to passive transition followed by a passive period 200 µs in length ( Figure 20-5 (d)).
• EOF - End of Frame Symbol
The EOF symbol is defined as an active to passive transition followed by a passive period 280 µs in length ( Figure 20-5 (e)). If there
is no IFR byte transmitted after an EOD symbol is transmitted, after another 80 µs the EOD becomes an EOF, indicating the
completion of the message.
• IFS - Inter-Frame Separation Symbol
The IFS symbol is defined as a passive period 300 µs in length. The IFS symbol contains no transition, since when used it always
follows an EOF symbol.( Figure 20-5 (g))
• BREAK - Break Signal
The BREAK signal is defined as a passive to active transition followed by an active period of at least 240 µs ( Figure 20-5 (f)).
• IDLE
An IDLE is defined as a passive period greater than 300 µs in length.
20.8.1.3 J1850 VPW Valid/Invalid Bits & Symbols
The timing tolerances for receiving data bits and symbols from the J1850 bus have been defined to allow for variations in oscillator
frequencies. In many cases the maximum time allowed to define a data bit or symbol is equal to the minimum time allowed to define another
data bit or symbol.
Since the minimum resolution of the BDLC module for determining what symbol is being received is equal to a single period of the MUX
Interface clock, (t
bdlc
). i.e. the receiver symbol timing boundaries are subject to an uncertainty of 1 t
bdlc
due to sampling considerations.
This clock resolution of 1 t
bdlc
allows the BDLC module to properly differentiate between the different bits and symbols, without reducing
the valid window for receiving bits and symbols from transmitters onto the J1850 bus having varying oscillator frequencies.
• Transmit and Receive Symbol Timing Specifications
Table 20-13 through Table 20-18 contain the SAE J1850 transmit and receive symbol timing specifications for the BDLC module.
The units used in these tables are mux interface clock periods (t
bdlc
). The mux interface clock is a divided down version of the bus
clock input to the module (see Section 20.7.3.6, BDLC Rate Select Register (DLCBRSR) - MBAR + 0x1309 ). The mux interface
clock drives the transmit and receive counters which control symbol generation and identification. The symbol timing in effect
during J1850 operations is dependent the state of two control bits: the CLKS bit BDLC Control Register 1, which indicates whether
the bus clock is an integer frequency or a binary frequency; the 4XE bit in BDLC Control Register 2, which is used to select 4X
operation.
Table 20-13 and Table 20-15 indicate the transmit and receive timing for integer bus frequencies (CLKS = 0) and 4X operation disabled (4XE
= 0). It is assumed that for integer bus frequencies the divided down mux interface clock frequency will be 1MHz (t
bdlc
= 1 µs).
Table 20-14 and Table 20-16 indicated the transmit and receive timing for binary bus frequencies (CLKS = 1) and 4X operation disabled (4XE
= 0). It is assumed that for binary bus frequencies the divided down mux interface clock frequency will be 1.048576 MHz (t
bdlc
= 0.953674
µs). The symbol timing values are adjusted to compensate for the shortening of the mux interface clock period.
Table 20-17 and Table 20-18 show how the receive symbol timing values are adjusted when 4X operation is enabled (4XE = 1) for both integer
bus frequencies (CLKS = 0) and binary bus frequencies (CLKS = 1), respectively.
The values specified in the tables are for the symbols appearing on the SAE J1850 bus. These values assume the BDLC module is
communicating on the SAE J1850 bus using an external analog transceiver, and that the BDLC module analog roundtrip delay value
programed into the BDLC Analog Round Trip Delay Register register is the appropriate value for the transceiver being used. If these
conditions are not met, the symbol timings being measured on the SAE J1850 bus will be significantly affected. For a detailed description of
how symbol timings are measured on the SAE J1850 bus, refer to the appropriate SAE documents.
Table 20-13. BDLC Transmitter VPW Symbol Timing for Integer Frequencies
Number Characteristic Symbol Min Typ Max Unit
1 Passive Logic 0 T
tvp1
62 64 66 t
bdlc
2 Passive Logic 1 T
tvp2
126 128 130 t
bdlc
3 Active Logic 0 T
tva1
126 128 130 t
bdlc
4 Active Logic 1 T
tva2
62 64 66 t
bdlc