AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 72

• Manufacturer Part Number: AN1740
• Description: Applications Using the Analog Subsystem on MC68HC05JJ/JP Series Microcontrollers
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
System Noise
72
NOTE:
Current spikes can occur when discharging an external capacitor
through PB0 which can be disruptive to the internal analog V
respect to the external V
Discharging large capacitors or low impedance loads into the PB0 pin
can cause high current spikes which may disrupt the internal analog V
with respect to the V
Repeated high currents can also damage the discharge device and/or
the device’s interconnect metal. Using capacitors that are less than 2 F
and discharge current peaks less than 25 mA is recommended.
These current transients cause a voltage to the analog subsystem V
reference to rise or drop several tenths of volts during switching
transients. Therefore, these recommendations are given:
System noise is inherent to a mixed signal design such as the JJ/JP
series of MCUs. There will always be some level of noise which cannot
be filtered out, bypassed, or avoided by careful PWB layout. The single
slope A/D method ensures some level of averaging of the input signals,
but additional digital averaging may be necessary to reduce the
unwanted noise further. The penalty in the added filtering will be a
reduction in response time to the input signal.
1. Avoid switching any port pins which carry more than a few
2. Sample the output of the comparators between the switching of
3. If the CPF1 and CPF2 static flags are needed, be sure to clear out
4. Try to position output changes within the same time in each cycle
5. Consider making the A/D conversions during wait mode when the
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milliampere of current while doing A/D conversions.
port conditions and use the dynamic outputs on CMP1 and CMP2.
any flags which may have been set during switching spikes by
including a flag clearing sequence just after the code which
initiated the suspected current spike.
of operation and for each measurement. Transients with low
frequency components, therefore, always will affect all readings in
a similar manner.
internal MCU bus activity has been halted.
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