L9805 datasheet(65/103 Pages) STMICROELECTRONICS

65 / 103 page

65/103

L9805

CONTROLLER AREA NETWORK (Cont’d)

5.6.3.4 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-

synchronization 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

change is expected to lie within this time seg-

ment. It has a fixed length of one time quanta (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.

Figure 1. Bit Timing

SYNC_SEG

BIT SEGMENT 1 (BS1)

BIT SEGMENT 2 (BS2)

NOMINAL BIT TIME

1 x tCAN

tBS1

tBS2

SAMPLE POINT

TRANSMIT POINT

L9805 Datenblatt(PDF) 65 Page - STMicroelectronics

L9805 Datenblatt(HTML) 65 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datenblatt Download

Link URL