2002 Microchip Technology Inc.
Preliminary
DS21667C-page 1
M
MCP2551
Features
· Supports 1 Mb/s operation
· Implements ISO-11898 standard physical layer
requirements
· Suitable for 12V and 24V systems
· Externally-controlled slope for reduced RFI
emissions
· Detection of ground fault (permanent dominant)
on TXD input
· Power-on reset and voltage brown-out protection
· An unpowered node or brown-out event will not
disturb the CAN bus
· Low current standby operation
· Protection against damage due to short-circuit
conditions (positive or negative battery voltage)
· Protection against high-voltage transients
· Automatic thermal shutdown protection
· Up to 112 nodes can be connected
· High noise immunity due to differential bus
implementation
· Temperature ranges:
- Industrial (I): -40°C to +85°C
- Extended (E): -40°C to +125°C
Package Types
Block Diagram
R
S
CANH
CANL
V
REF
TXD
V
SS
V
DD
RXD
1
2
3
4
8
7
6
5
PDIP/SOIC
M
C
P
2551
Thermal
Shutdown
V
DD
V
SS
CANH
CANL
TXD
R
S
RXD
V
REF
V
DD
Slope
Control
Power-On
Reset
Reference
Voltage
Receiver
GND
0.5 V
DD
TXD
Dominant
Detect
Driver
Control
High-Speed CAN Transceiver
2002 Microchip Technology Inc.
Preliminary
DS21667C-page 3
MCP2551
1.0
DEVICE OVERVIEW
The MCP2551 is a high-speed CAN, fault-tolerant
device that serves as the interface between a CAN pro-
tocol controller and the physical bus. The MCP2551
provides differential transmit and receive capability for
the CAN protocol controller and is fully compatible with
the ISO-11898 standard, including 24V requirements. It
will operate at speeds of up to 1 Mb/s.
Typically, each node in a CAN system must have a
device to convert the digital signals generated by a CAN
controller to signals suitable for transmission over the
bus cabling (differential output). It also provides a buffer
between the CAN controller and the high-voltage spikes
that can be generated on the CAN bus by outside
sources (EMI, ESD, electrical transients, etc.).
1.1
Transmitter Function
The CAN bus has two states: Dominant and Reces-
sive. A dominant state occurs when the differential volt-
age between CANH and CANL is greater than a
defined voltage (e.g.,1.2V). A recessive state occurs
when the differential voltage is less than a defined volt-
age (typically 0V). The dominant and recessive states
correspond to the low and high state of the TXD input
pin, respectively. However, a dominant state initiated
by another CAN node will override a recessive state on
the CAN bus.
1.1.1
MAXIMUM NUMBER OF NODES
The MCP2551 CAN outputs will drive a minimum load
of 45
,
allowing a maximum of 112 nodes to be con-
nected (given a minimum differential input resistance of
20 k
and a nominal termination resistor value of
120
).
1.2
Receiver Function
The RXD output pin reflects the differential bus voltage
between CANH and CANL. The low and high states of
the RXD output pin correspond to the Dominant and
Recessive states of the CAN bus, respectively.
1.3
Internal Protection
CANH and CANL are protected against battery short-
circuits and electrical transients that can occur on the
CAN bus. This feature prevents destruction of the
transmitter output stage during such a fault condition.
The device is further protected from excessive current
loading by thermal shutdown circuitry that disables the
output drivers when the junction temperature exceeds
a nominal limit of 165°C. All other parts of the chip
remain operational and the chip temperature is lowered
due to the decreased power dissipation in the transmit-
ter outputs. This protection is essential to protect
against bus line short-circuit induced damage.
1.4
Operating Modes
The R
S
pin allows three modes of operation to be
selected:
· High-Speed
· Slope-Control
· Standby
These modes are summarized in Table 1-1.
When in High-Speed or Slope-Control mode, the driv-
ers for the CANH and CANL signals are internally regu-
lated to provide controlled symmetry in order to
minimize EMI emissions.
Additionally, the slope of the signal transitions on
CANH and CANL can be controlled with a resistor con-
nected from pin 8 (R
S
) to ground, with the slope propor-
tional to the current output at R
S
, further reducing EMI
emissions.
1.4.1
HIGH-SPEED
The High-Speed mode is selected by connecting the
R
S
pin to V
SS
. In this mode, the transmitter output driv-
ers have fast output rise and fall times to support high-
speed CAN bus rates.
1.4.2
SLOPE-CONTROL
Slope-Control mode further reduces EMI by limiting the
rise and fall times of CANH and CANL. The slope, or
slew rate (SR), is controlled by connecting an external
resistor (R
EXT
) between R
S
and V
OL
(usually ground).
The slope is proportional to the current output at the R
S
pin. Since the current is primarily determined by the
slope-control resistance value R
EXT
, a certain slew rate
is achieved by applying a respective resistance.
Figure 1-1 illustrates typical slew rate values as a
function of the slope-control resistance value.
1.4.3
STANDBY MODE
The device may be placed in standby or "SLEEP" mode
by applying a high-level to R
S
. In SLEEP mode, the
transmitter is switched off and the receiver operates at
a lower current. The receive pin on the controller side
(RXD) is still functional but will operate at a slower rate.
The attached microcontroller can monitor RXD for CAN
bus activity and place the transceiver into normal oper-
ation via the R
S
pin (at higher bus rates the first CAN
message may be lost).
2002 Microchip Technology Inc.
Preliminary
DS21667C-page 5
MCP2551
1.5
TXD Permanent Dominant
Detection
If the MCP2551 detects an extended low state on the
TXD input, it will disable the CANH and CANL output
drivers in order to prevent the corruption of data on the
CAN bus. The drivers are disabled if TXD is low for
more than 1.25 ms (minimum). This implies a maxi-
mum bit time of 62.5 µs (16 kb/s bus rate) allowing up
to 20 consecutive transmitted dominant bits during a
multiple bit error and error frame scenario. The drivers
remain disabled as long as TXD remains low. A rising
edge on TXD will reset the timer logic and enable the
CANH and CANL output drivers.
1.6
Power-on Reset
When the device is powered on, CANH and CANL
remain in a high-impedance state until V
DD
reaches the
voltage level V
PORH
. In addition, CANH and CANL will
remain in a high-impedance state if TXD is low when
V
DD
reaches V
PORH
. CANH and CANL will become
active only after TXD is asserted high. Once powered
on, CANH and CANL will enter a high-impedance state
if the voltage level at V
DD
falls below V
PORL
, providing
voltage brown-out protection during normal operation.
1.7
Pin Descriptions
The 8-pin pinout is listed in Table 1-3.
TABLE 1-3:
MCP2551 PINOUT
1.7.1
TRANSMITTER DATA INPUT (TXD)
TXD is a TTL compatible input pin. The data on this pin
is driven out on the CANH and CANL differential output
pins. It is usually connected to the transmitter data out-
put of the CAN controller device. When TXD is low,
CANH and CANL are in the dominant state. When TXD
is high, CANH and CANL are in the recessive state,
provided that another CAN node is not driving the CAN
bus with a dominant state. TXD has an internal pull-up
resistor (nominal 25 k
to V
DD
).
1.7.2
GROUND SUPPLY (V
SS
)
Ground supply pin.
1.7.3
SUPPLY VOLTAGE (V
DD
)
Positive supply voltage pin.
1.7.4
RECEIVER DATA OUTPUT (RXD)
RXD is a CMOS-compatible output that drives high or
low depending upon the differential signals on the
CANH and CANL pins and is usually connected to the
receiver data input of the CAN controller device. RXD
is high when the CAN bus is recessive and low in the
dominant state.
1.7.5
REFERENCE VOLTAGE (V
REF
)
Reference Voltage Output (Defined as V
DD
/2).
1.7.6
CAN LOW (CANL)
The CANL output drives the low side of the CAN differ-
ential bus. This pin is also tied internally to the receive
input comparator.
1.7.7
CAN HIGH (CANH)
The CANH output drives the high side of the CAN dif-
ferential bus. This pin is also tied internally to the
receive input comparator.
1.7.8
SLOPE RESISTOR INPUT (R
S
)
The R
S
pin is used to select High-Speed, Slope-Control
or Standby modes via an external biasing resistor.
Pin
Number
Pin
Name
Pin Function
1
TXD
Transmit Data Input
2
V
SS
Ground
3
V
DD
Supply Voltage
4
RXD
Receive Data Output
5
V
REF
Reference Output Voltage
6
CANL
CAN Low-Level Voltage I/O
7
CANH
CAN High-Level Voltage I/O
8
R
S
Slope-Control Input
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