
8
A6155
during an open window,or until a fresh power-up se-
quence. The system enable output, EN , can be used to
prevent critical control functions being activated in the
event of the system going into this failure mode (see
section “Enable-EN Output”).
The RES output must be pulled up to V
even if that
output is not used by the system (see Fig 9).
Combined Voltage and Timer Action
The combination of voltage and timer actions is illustrat-
ed by the sequence of events shown in Fig. 7. On pow-
er-up, when the voltage at V
reaches V
, the power-
on-reset, POR, delay is initialized and holds RES active
for the time of the POR delay. A TCL pulse will have
no effect until this power-on-reset delay is completed.
When the risk exists that TCL temporarily floats, e.g.
during T
, a pull-up to V
is required on that pin. After
the POR delay has elapsed, RES goes inactive and the
watchdog timer starts acting. If no TCL pulse occurs,
RES goes active low for a short time T
after each
closed and open window period. A TCL pulse coming
during the open window clears the watchdog timer.
When the TCL pulse occurs too early (during the closed
window), RES goes active and a new timeout sequence
starts. A voltage drop below the V
level for longer than
typically 5
μ
s overrides the timer and immediately forces
RES active and EN inactive. Any further TCL pulse has
no effect until the next power-up sequence has
completed.
Enable - EN Output
The system enable output, EN , is inactive always when
RES is active and remains inactive after a RES pulse
until the watchdog is serviced correctly 3 consecutive
times (ie. the TCL pulse must come in the open
window). After three consecutive services of the
watchdog with TCL during the open window, the EN
goes active low.
A malfunctioning system would be repeatedly reset by
the watchdog. In a conventional system critical motor
controls could be energized each time reset goes inac-
tive (time allowed for the system to restart) and in this
way the electrical motors driven by the system could
function out of control. The A6155 prevents the above
failure mode by using the EN output to disable the motor
controls until software has successfully cleared the
watchdog three times (ie. the system has correctly re-
started after a reset condition).
CAN -Bus Sleep Mode Detector
If the microcontroller is in standby mode that means it
does not have any pulses on the TCL input. After 3 reset
pulse periods (T
+ T
+ T
) on the RES output, the
A6155 switches on an internal resistor of 1 M
, and it
will have a reset pulse of typically 3 ms every 1 second
on the RES output. When a TCL edge (rising or falling)
appears on the TCL input or the power supply goes
down and up, the A6155 switches to the R input.