AN2727 Freescale Semiconductor / Motorola, AN2727 Datasheet - Page 28

• Manufacturer Part Number: AN2727
• Description: Designing Hardware for the HCS12 D-Family
• Manufacturer: Freescale Semiconductor / Motorola

Output Drive Currents
or
In either case, the maximum IDD specification for the pin will be exceeded, and the MCU will be damaged.
To minimize the impact of the ports being reconfigured accidentally as outputs, configure for reduced
drive output, and ensure that the port data registers match the polarity of the pull device(s) / supply rail
connection.
There are several possible strategies:
Output Drive Currents
The D-family has an
on a pin exceeds 25 mA peak at any time, the I/O structure may become may become damaged or suffer
degradation.
limit during switching.
The device maximum I/O current is limited by its power dissipation and will be application dependent. The
P
the I/O current. Ensure that the total I/O power dissipation plus the internal device dissipation combined
does not cause the device junction temperature to exceed the appropriate limit (for C, V or M
specification) in the application environment.
In production, I/O limits are tested by holding each pin at the VOL and VOH limit and measuring that the
I/O current exceeds the IOL and IOH spec limits respectively (actually a measure of RDSON).
From the VOHL specification it can be seen that in the conduction range the I/O driver has a maximum
R
will be lower than this; it will also be reduced at lower temperatures.
28
IO
DSON
1. Minimum risk, highest cost: pull each unused input pin to a supply rail with an individual pull
2. Highest risk, lowest cost: tie each unused input pin directly to a supply rail. This offers the highest
3. High risk, low cost: common up all unused inputs to a single pull resistor to a supply rail. This offers
4. Medium risk, medium cost: a better compromise of cost versus risk is to connect all unused inputs
•
= ΣR
a port is connected directly to a supply rail and might be driven to the opposite polarity from the
supply rail.
resistor. This ensures no possibility of I/O conflict, as described above.
risk of unintentional conflict with a supply rail.
the highest risk of unintentional conflict with another I/O pin.
on each I/O port together and connect them to a separate pull resistor per I/O port.
= 0.8 V / 10 mA = 80 Ω (at the maximum temperature specified for the device). Typically, RDSON
DSON
It is strongly recommended to stay well below the 25 mA limit, to avoid peaks exceeding this
Some external peripherals tristate their outputs when disabled (including
SPI interface lines). Where this is the case, Stop and Wait IDD can be
minimized by enabling the internal pull devices on appropriate inputs while
the external peripheral is disabled.
x I
IO
2
“Instantaneous Maximum current single pin limit for all digital I/O” of
term in the data sheet power dissipation calculation indicates the heating effect of
Designing Hardware for the HCS12 D-Family, Rev. 0
NOTE
±
Freescale Semiconductor
25 mA. If the current