ST92F124 datasheet(339/429 Pages) STMICROELECTRONICS

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CONTROLLER AREA NETWORK (bxCAN)

CONTROLLER AREA NETWORK (Cont’d)

10.10.5.7 Bit Timing

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 the following edges.

Its operation may be explained simply by splitting

nominal bit time into three segments as follows:

– Synchronization segment (SYNC_SEG): a bit

change is expected to occur within this time seg-

ment. It has a fixed length of one time quantum

(1 x tCAN).

– Bit segment 1 (BS1): defines the location of the

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.

– Bit segment 2 (BS2): defines the location of the

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

automatically shortened to compensate for neg-

ative phase drifts.

The resynchronization jump width (RJW) defines

an upper bound to the amount of lengthening or

shortening of the bit segments. It is programmable

between 1 and 4 time quanta.

A valid edge is defined as the first transition in a bit

time from dominant to recessive bus level provid-

ed the controller itself does not send a recessive

bit.

If a valid edge is detected in BS1 instead of

SYNC_SEG, BS1 is extended by up to RJW so

that the sample point is delayed.

Conversely, if a valid edge is detected in BS2 in-

stead of SYNC_SEG, BS2 is shortened by up to

RJW so that the transmit point is moved earlier.

As a safeguard against programming errors, the

configuration of the Bit Timing Register (BTR) is

only possible while the device is in STANDBY

mode.

Note: for a detailed description of the CAN bit tim-

ing and resynchronization mechanism, please re-

fer to the ISO 11898 standard.

Figure 152. Bit Timing

SYNC_SEG

BIT SEGMENT 1 (BS1)

BIT SEGMENT 2 (BS2)

NOMINAL BIT TIME

1 x tCAN

tBS1

tBS2

SAMPLE POINT

TRANSMIT POINT

NominalBitTime

CAN

BS1

BS2

with:

tBS1 = tCAN x (TS1[3:0] + 1) ,

tBS2 = tCAN x (TS2[2:0] + 1),

tCAN = tCPU x BRP,

tCPU = time period of the CPU clock,

BRP = BRP[5:0] + 1 = Baud Rate Prescaler

BRP[5:0] is defined in the CBTR0 Register,

TS1[3:0] and TS2[2:0] are defined in the CBTR1 Register.

audRate

NominalBitTime

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