33742 datasheet(37/65 Pages) FREESCALE

37 / 65 page

Analog Integrated Circuit Device Data

Freescale Semiconductor

33742

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

Figure 23 illustrates how the wake-up signal is generated.

First the CAN signal is detected by a low consumption

receiver (WU receiver). Then the signal passes through a

pulse width filter, which discards the undesired pulses. The

pulse must have a width bigger than 0.5

µs and smaller than

500

µs to be accepted. When a pulse is discarded, the pulse

counter is reset and no wake-up signal is generated. When a

pulse is accepted, the pulse counter is incremented and, after

three pulses, the internal wake-up signal is asserted.

Each one of the pulses must be spaced by no more than

500

µs. If not, the counter will be reset and no wake-up signal

will be generated. This is accomplished by the wake-up

timeout generator. The wake-up cycle is completed (and the

wake-up flag reset) when the CAN interface is brought to

CAN Normal mode.

Figure 23. Wake-Up Functional Block Diagram

CAN WAKE-UP REPORT

The CAN wake-up reporting depend upon the low power

mode the SBC is in.

If the SBC is placed into Sleep mode (VDD and V2 off), the

CAN wake-up or any wake-up results in the VDD regulator

turning on, leading to turning on the MCU supply and

releasing reset. If the 33742 is in Stop mode (V2 off and VDD

active), the CAN wake-up or any wake-up is signalled by a

pulse on the INT output. In addition the CANWU bit is set in

the CAN register.

If the SBC is in Normal or Standby mode and the CAN

interface is in Sleep mode with wake-up enabled, the CAN

wake-up is reported by the CANWU bit in the CAN register.

In the event the SBC is in Normal mode and CAN Sleep

mode with wake-up enabled, it is recommended that the user

check for the CANWU bit prior to placing the 33742 in Sleep

or Stop mode in case bus traffic has occurred while the CAN

interface was in Sleep mode.

After a CAN wake-up, a flag is set in the CAN register. Bit

CANWU reports the CAN wake-up event while the 33742

was in Sleep or Stop mode. This bit is set until the CAN is in

placed by SPI command into TXRX mode and the CAN

CAN BUS DIAGNOSTIC

The SBC can diagnose CANH or CANL lines short to

GND, shorts to VSUP or VDD.

As illustrated in Figure 24, several single-ended

comparators are implemented on the CANH and CANL bus

lines. These comparators monitor the bus voltage level in the

recessive and dominant states. This information is then

managed by a logic circuit to determine if a failure has

occurred and to report it. Table 11 indicates the state of the

comparators in the event of bus failure and the state of the

drivers; that is, whether they are recessive or dominant.

Internal Wake-Up

Latch

RST

Time-out

RST

Counter

Pulse OK

Narrow

Pulse

Pulse Width

Filter

Time-out

Generator

Standby

WU Receiver

CANH

CANL

Signal

33742 Datasheet(PDF) 37 Page - Freescale Semiconductor, Inc

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