ST92F120JR9T datasheet(255/320 Pages) STMICROELECTRONICS

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J1850 Byte Level Protocol Decoder (JBLPD)

J1850 BYTE LEVEL PROTOCOL DECODER (Cont’d)

Figure 117. Local Loopback structure

10.8.3.8 Peripheral clock management

To work correctly, the encoder and decoder sec-

tions of the peripheral need an internal clock at

1MHz. This clock is used to evaluate the protocol

symbols timings in transmission and in reception.

The prescaled clock is obtained by dividing the

MCU internal clock frequency. The CLKSEL regis-

ter allows the selection of the right prescaling fac-

tor. The six least significant bits of the register

(FREQ[5:0]) must be programmed with a value us-

ing the following formula:

MCU Internal Freq. = 1MHz * (FREQ[5:0] + 1).

Note: If the MCU internal clock frequency is lower

than 1MHz, the JBLPD is not able to work correct-

ly. If a frequency lower than 1MHz is used, the

user program must disable the JBLPD.

Note: When the MCU internal clock frequency or

the clock prescaler factor are changed, the JBLPD

could lose synchronization with the J1850 bus.

Passive state

VPWO from the

peripheral logic

VPWI toward the

J1850 decoder

Polarity

manager

OPTI ONS.INPOL

OPTIO NS.LOOPB

MCU VPWO

MCU VPWI

JBLPD peripheral

MCU

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