MCP2551
1.7.1
TRANSMITTER DATA INPUT (TXD)
1.5
TXD Permanent Dominant
Detection
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
output 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 kΩ to VDD).
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
maximum 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.
1.7.2
GROUND SUPPLY (VSS)
Ground supply pin.
1.7.3
SUPPLY VOLTAGE (VDD)
1.6
Power-on Reset
Positive supply voltage pin.
When the device is powered on, CANH and CANL
remain in a high-impedance state until VDD reaches the
voltage-level VPORH. In addition, CANH and CANL will
remain in a high-impedance state if TXD is low when
VDD reaches VPORH. 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 VDD falls below VPORL, providing
voltage brown-out protection during normal operation.
1.7.4
RECEIVER DATA OUTPUT (RXD)
RXD is a CMOS-compatible output that drives high or
low depending on 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 (VREF)
1.7
Pin Descriptions
Reference Voltage Output (Defined as VDD/2).
The 8-pin pinout is listed in Table 1-3.
1.7.6
CAN LOW (CANL)
TABLE 1-3:
MCP2551 PINOUT
The CANL output drives the low side of the CAN
differential bus. This pin is also tied internally to the
receive input comparator.
Pin
Number
Pin
Pin Function
Name
1
2
3
4
5
6
7
8
TXD
VSS
Transmit Data Input
Ground
1.7.7
CAN HIGH (CANH)
The CANH output drives the high-side of the CAN
differential bus. This pin is also tied internally to the
receive input comparator.
VDD
RXD
VREF
Supply Voltage
Receive Data Output
Reference Output Voltage
1.7.8
SLOPE RESISTOR INPUT (RS)
CANL CAN Low-Level Voltage I/O
CANH CAN High-Level Voltage I/O
The RS pin is used to select High-speed, Slope-control
or Standby modes via an external biasing resistor.
RS
Slope-Control Input
© 2007 Microchip Technology Inc.
DS21667E-page 5
MICROCHIP [ MICROCHIP ]
