MOTOROLA
4
AN1257/D
In applications where a switched power supply is not available or constant power to the MCU is not
necessary, the same 12-volt supply can be connected to both the V
BATT
pin and the V
IGN
pin. If this is done,
whenever the 12-volt supply is connected, the rising edge occurring at the V
IGN
pin will cause the primary
regulator to exit the standby mode and become fully activated. In these applications, it is recommended
that some type of power-on reset (POR) delay circuit be used to connect the power supply to the V
IGN
pin.
This delay should be timed to allow the power supply at the V
BATT
pin to reach the minimum specified
operating level before the rising edge is detected at the V
IGN
pin. In the future, a V
IGN
POR circuit may be
integrated onto these devices to further reduce the number of external components required. Although the
example circuit in Figure 2 has V
IGN
connected to a switched 12-volt supply, an example POR circuit has
been included.
A rising edge detected at the BUS pin (pin 24) also can cause the primary regulator to exit the standby
mode and become fully active, if this mask option is selected. The BUS pin is a serial multiplex interface
pin which allows the MCU to communicate with other components over a serial multiplex communication
network using the SAE standard J1850 communication protocol. Activity detected at the BUS pin could be
due to another node on the serial network transmitting a message or simply due to noise appearing on the
serial bus.
Figure 2. Automotive Voltage Regulator Circuit Example
V
DD
V
SSD
V
DD
V
SSD
V
BATT
V
CCA
V
SSA
2
V
IGN
V
SSA
1
OSC.
NETWORK
RST
BUS
LOAD
REXT1
REXT2
+
AAAA
PORT A 7:0
8
PORT B 7:0
8
PORT C 7:0
8
PORT D 7:0
8
PORT E 7:0
8
PORT F 3:0
4
PWM
IRQ
V
REFH
V
REFL
AAAA
AAAA
B+
4
3
31
30
41
40
22
29
25
23
24
.1
μ
F
10
μ
F
10
μ
F
.1
μ
F
+
SWITCHED B+
R
LOAD
SAE J1850
BUS
V
DD
V
DD
24.9 k
1%
31.6 k
1%
1N914
.1
μ
F
100 k
1N5822
P6KE30A
P6KE15A
EXAMPLE TRANSIENT
PROTECTION CIRCUIT
FOR V
BATT
NOTE:
REXT1, REXT2 and LOAD pins affect SAE J1850 serial bus
communication. For more information on the function of these
pins, refer to the MC68HC705V8 general release specification,
Motorola document number HC705V8GRS/D.
28
27
7
C
LOAD
- DENOTES CHASSIS GROUND
- DENOTES CIRCUIT GROUND
M
C
6
8
H
C
7
0
5
V
8
OSC1
OSC2
.1
μ
F
.01
μ
F
.1
μ
F
V
DD
100 k
1 M