ST72521 STMICROELECTRONICS [STMicroelectronics], ST72521 Datasheet - Page 133

• Manufacturer Part Number: ST72521
• Description: 8-BIT MCU WITH NESTED INTERRUPTS, FLASH, 10-BIT ADC, FIVE TIMERS, SPI, SCI, I2C, CAN INTERFACE
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

CONTROLLER AREA NETWORK (Cont’d)
10.8.3.4 Bit Timing Logic
The bit timing logic monitors the serial bus-line and
performs sampling and adjustment of the sample
point by synchronizing on the start-bit edge and re-
synchronizing on following edges.
Its operation may be explained simply when the
nominal bit time is divided into three segments as
follows:
– Synchronisation segment (SYNC_SEG): a bit
– Bit segment 1 (BS1): defines the location of the
– Bit segment 2 (BS2): defines the location of the
Figure 71. Bit Timing
change is expected to lie within this time seg-
ment. It has a fixed length of one time quanta (1
x t
sample point. It includes the PROP_SEG and
PHASE_SEG1 of the CAN standard. Its duration
is programmable between 1 and 16 time quanta
but may be automatically lengthened to compen-
sate for positive phase drifts due to differences in
the frequency of the various nodes of the net-
work.
transmit point. It represents the PHASE_SEG2
of the CAN standard. Its duration is programma-
ble between 1 and 8 time quanta but may also be
CAN
).
SYNC_SEG
1 x t
CAN
BIT SEGMENT 1 (BS1)
t
BS1
NOMINAL BIT TIME
– Resynchronization Jump Width (RJW): de-
To guarantee the correct behaviour of the CAN
controller, SYNC_SEG + BS1 + BS2 must be
greater than or equal to 5 time quanta.
For a detailed description of the CAN resynchroni-
zation mechanism and other bit timing configura-
tion constraints, please refer to the Bosch CAN
standard 2.0.
As a safeguard against programming errors, the
configuration of the Bit Timing Register (BTR) is
only possible while the device is in STANDBY
mode.
automatically shortened to compensate for neg-
ative phase drifts.
fines an upper bound to the amount of lengthen-
ing or shortening of the bit segments. It is
programmable between 1 and 4 time quanta.
SAMPLE POINT
BIT SEGMENT 2 (BS2)
t
BS2
TRANSMIT POINT
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