UJA1076 datasheet(17/47 Pages) NXP

17 / 47 page

UJA1076_2

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Product data sheet

Rev. 02 — 27 May 2010

17 of 47

NXP Semiconductors

UJA1076

High-speed CAN core system basis chip

6.5.1 RSTN pin

A system reset is triggered if the bidirectional RSTN pin is forced LOW for at least tfltr by

the microcontroller (external reset). A reset pulse is output on RSTN by the SBC when a

system reset is triggered internally.

The reset pulse width (tw(rst)) is selectable (short or long) if the system reset was

generated by a V1 undervoltage event (see Section 6.6.2) or by the SBC leaving Off

(VBAT > Vth(det)pon) or Overtemp (temperature < Tth(rel)otp) modes. A short reset pulse is

selected by connecting a 900

Ω ±10 % resistor between pins RSTN and V1. If a resistor is

not connected, the reset pulse will be long (see Table 11).

In all other cases (e.g. watchdog-related reset events) the reset pulse length will be short.

6.5.2 EN output

The EN pin can be used to control external hardware, such as power components, or as a

general-purpose output when the system is running properly.

In Normal and Standby modes, the microcontroller can set the EN control bit (bit ENC in

the Mode_Control register; see Table 5) via the SPI interface. Pin EN will be HIGH when

ENC = 1 and MC = 10 or 11. A reset event will cause pin EN to go LOW. EN pin behavior

is illustrated in Figure 5.

6.5.3 LIMP output

The LIMP pin can be used to enable the so called ‘limp home’ hardware in the event of an

ECU failure. Detectable failure conditions include SBC overtemperature events, loss of

watchdog service, RSTN or V1 clamped LOW and user-initiated or external reset events.

The LIMP pin is a battery-related, active-LOW, open-drain output.

A system reset will cause the limp home warning control bit (bit LHWC in the

Mode_Control register; see Table 5) to be set. If LHWC is already set when the system

reset is generated, bit LHC will be set which will force the LIMP pin LOW. The application

should clear LHWC after each reset event to ensure the LIMP output is not activated

during normal operation.

In Overtemp mode, bit LHC is always set and, consequently, the LIMP output is always

active. If the application manages to recover from the event that activated the LIMP

output, LHC can be cleared to deactivate the LIMP output.

Fig 5.

Behavior of EN pin

RSTN

ENC

mode

STANDBY

NORMAL

STANDBY

015aaa074

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