PCA82C252T Philips Semiconductors, PCA82C252T Datasheet - Page 5

• Manufacturer Part Number: PCA82C252T
• Description: Fault-tolerant CAN transceiver
• Manufacturer: Philips Semiconductors

Philips Semiconductors
FUNCTIONAL DESCRIPTION
The PCA82C252 is the interface between the CAN
protocol controller and the physical bus. It is primarily
intended for low speed applications, up to 125 kbaud, in
passenger cars. The device provides differential transmit
capability to the bus and differential receive capability to
the CAN controller.
To reduce RF interference the rise and fall slope are
limited. This allows the use of an unshielded twisted pair or
a parallel pair of wires for the bus. Moreover, it supports
transmission capability on either bus wire if one of the bus
wires is corrupted. The failure detection logic automatically
selects a suitable transmission mode.
In normal operation (no wiring failures) the differential
receiver is output to RXD. The differential receiver inputs
are connected to CANH and CANL through integrated
filters. The filtered input signals are also used for the single
wire receivers. The CANH and CANL receivers have
threshold voltages that ensure a maximum noise margin in
single-wire modes.
Failure detector
The failure detector is active in the normal operation mode
and detects the following single bus failures and switches
to an appropriate mode:
1. CANH wire interrupted
2. CANL wire interrupted
3. CANH short-circuited to battery
4. CANL short-circuited to ground
5. CANH short-circuited to ground
6. CANL short-circuited to battery
7. CANL mutually shorted to CANH.
The differential receiver threshold is set at 2.9 V.
This ensures correct reception in the normal operating
modes and, in the event of failures 1, 2 and 5 with a noise
margin as high as possible. These failures, or recovery
from them, do not destroy ongoing transmissions.
To ensure speed requirements the differential receiver has
an acceleration function.
Failures 3 and 6 are detected by comparators connected
to CANH and CANL, respectively. If the comparator
threshold is exceeded for a certain period of time, the
reception is switched to the single-wire mode. This time is
needed to avoid false triggering by external RF fields.
Recovery from these failures is detected automatically
after a certain time-out (filtering) and no transmission is
lost.
1997 Oct 28
Fault-tolerant CAN transceiver
5
Failures 4 and 7 initially result in a permanent dominant
level at RXD. After a time-out, the CANL driver and the
RTL pin are switched off. Only a weak pull-up at RTL
remains. Reception continues by switching to the
single-wire mode via CANH. When failures 4 or 7 are
removed, the recessive bus levels are restored. If the
differential voltage remains below the recessive threshold
level for a certain period of time, reception and
transmission switch back to the differential mode.
If any of the seven wiring failures occur, the output NERR
will be made LOW. On error recovery, NERR will be made
HIGH again.
During all single-wire transmissions, the EMC
performance (both immunity and emission) is worse than
in the differential mode. Integrated receiver filters
suppress any HF noise induced into the bus wires.
The cut-off frequency of these filters is a compromise
between propagation delay and HF suppression. In the
single-wire mode, low frequency noise cannot be
distinguished from the required signal.
Low power modes
The transceiver provides 3 low power modes which can be
entered and exited via pins STB and EN.
The sleep mode is the mode with the lowest power
consumption. The INH pin is switched to high-impedance
for deactivation of external voltage regulators. CANL is
biased to the battery voltage via the RTL output. If the
supply voltage is provided the RXD and NERR will signal
the wake-up interrupt
The V
mode with an active INH output.
The V
switched to the V
signals the V
show the wake-up interrupt.
Wake-up requests are recognized by the transceiver when
a dominant signal is detected on either bus line or if the
WAKE pin is connected to ground. On a wake-up request
the transceiver will set the INH output which can be used
to activate the external V
provided the wake-up request can be read on the NERR or
RXD outputs, on which the external microcontroller can
wake up the transceiver (switch to normal operating mode)
via STB and EN.
BAT
CC
standby mode is the V
standby mode will react the same as the sleep
BAT
CC
power-on flag and the RXD output will
voltage. In this mode the NERR output
CC
voltage regulator. If V
BAT
standby with RTL
PCA82C252
Product specification
CC
is