mc68hc912bc32 Freescale Semiconductor, Inc, mc68hc912bc32 Datasheet - Page 316

• Manufacturer Part Number: mc68hc912bc32
• Description: M68hc12 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Electrical Specifications
19.12.2 Example V
Figure 19-2
voltage levels on the V
intended to do, and some design considerations when designing V
The general idea of this circuit implementation is to supply V
converter, like most, provides a shutdown feature which allows the converter’s output to be shut off. When
the SHDN pin on the converter is pulled high, as the 10-kΩ pullup resistor (R1) does, the output V
shorted to the V
the MCU to turn on the converter and supply the 12-volt programming voltage during the programming or
erasing cycle. Simple programming and erasing routines, such as those shown earlier in this application
note, will no longer suffice.
By implementing this solution, V
and fall together. Capacitors C5 and C6 are the normal decoupling capacitors on the V
C3 is used to reduce electromagnetic interference (EMI) in the circuit. If C3 is too large, V
allowed to fall with V
C4 is where the dc-dc converter stores charge to supply V
able to source approximately 30 mA of current for at least 20 µs (based on programming cycle
requirements) and 4 mA of current for at least 10 ms (based on erase cycle requirements).
A certain degree of experimentation might be required when selecting C4 and C3. When trying different
capacitor values, always monitor the effects on V
V
R1 must be no larger than 10 kΩ, to make certain that the SHDN pin on the dc-dc converter is never
allowed to fall below V
ensures this behavior, no matter what port pin is used on the microcontroller or what the internal structure
of that pin looks like. Without a strong enough pullup resistor on R1, the voltage on the SHDN pin might
drop during a reset event, causing the dc-dc converter to activate and begin driving the voltage on V
to begin to rise to 12 volts. This would result in data corruption in the FLASH.
316
FP
pin.
shows an example of a circuit which, if properly implemented, can maintain the appropriate
220 nF
220 nF
DD
supply. This requires that the programming and erasing routines assert a port pin on
DD
FP
DD
5
2
1
4
3
FP
, potentially causing data corruption in the FLASH array. (Refer to
V
C1+
C1 –
C2+
C2–
Protection Circuitry
unless the output pin of the microcontroller is driven low. The external pullup
pin. This section outlines the design for this circuit, what each component is
DD
ST662A
GND
7
FP
SHDN
V
OUT
is tied to V
M68HC12B Family Data Sheet, Rev. 9.1
Figure 19-2. V
6
8
C4
C3
10 µF
1–100 nF
DD
FP
on power-up and power-down, ensuring that they rise
FP
decay during power-down and current supplied to the
Supply Circuit
V
DD
R1 10 kΩ
Out
C5
FP
to the target device. The supply must be
V
FP
from a dc-dc converter. This dc-dc
10 µF
SHUTDOWN
FP
pin protection.
C6
100 nF
V
V
I/O
V
Freescale Semiconductor
DD
FP
SS
DD
FP
Figure
supply lines.
will not be
19-3.)
Out
Out
is