AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 73

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

Analog Subsystem Low-Power Modes
Use During Wait
Mode
AN1740
MOTOROLA
Refer to the application note In-Circuit and Emulation Consideration for
MC68HC05JJ/JP Series Microcontrollers , Motorola document order
number AN1741/D, for more information on good PWB layout
techniques and limiting these noise sources.
The JJ/JP series of MCUs has the low-power modes common to other
Motorola MC68HC05 devices. CMOS digital logic consumes very little
power when no switching is occurring. Wait and stop modes specifically
reduce the number of digital functions which can switch. However,
analog circuits continue to consume power as long as they are
connected to a power source. Therefore, the amount of supply current
required in wait and stop modes highly depends on the use of the analog
subsystem. Similar to the 16-bit timer, the analog functions will remain
active during wait mode. Therefore, these comparators can be used to
wake up the device from wait mode by the analog interrupt.
The analog functions can also be left active during stop mode, but the
user must realize that a higher stop I
comparator inputs will not restart the device from stop mode, but the
output of comparator 1 can be routed out through the PB4 pin by setting
the COE1 bit in the ASR. In this case, PB4 can be connected to the IRQ
or RESET pin to initiate a restart if the output of comparator 1 goes low.
In wait mode, the CPU bus activity is stopped, but the clocks to the two
timer functions are kept active. The analog functions do not require
clocks to operate, so the comparators and the current source will still be
active in wait mode if they are not powered down prior to executing the
wait instruction. An advantage of wait mode is the reduction of digital
switching noise as previously discussed. This makes wait mode a good
candidate for taking “low noise” readings with the analog subsystem.
Freescale Semiconductor, Inc.
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DD
Analog Subsystem Low-Power Modes
current will exist. Tripping the
Application Note
73