System Considerations
MC1321x Reference Manual, Rev. 1.6
3-34
Freescale Semiconductor
3.12.6.2
MCU Active Current
The MCU active (run) current is dominated by the system clock frequency.
supply current under several conditions. The Wait Mode is not running code but has the clocks active and
has a baseline of about 0.56 mA. As the bus clock gets faster the supply current increases accordingly with
a value of 1.1 mA at 1 MHz bus clock and 6.5 mA at 8 MHz bus frequency. A simple formula for
estimating typical current vs. bus clock is:
Typical current = 0.56 mA + (bus clock frequency (MHz) x 0.77 mA/MHz)
A 16 MHz bus clock example would be:
Typical current @ 16 MHz = 0.56 mA + (16 MHz x 0.77 mA/MHz) = 12.9 mA
Another significant factor can be the ATD. The ATD has a typical enabled current of 0.7 mA. To get lowest
power only enable the ATD as required to sample inputs.
3.12.7
Configuration of Interconnected GPIO for Low Power Operation
A stated elsewhere, device GPIO must be configured properly for low power operation. The GPIO on the
MC1321x are part of either the transceiver or the MCU, and their behavior is somewhat different. Also,
some I/O are interconnected between chips onboard the device (see
Section 3.4, “Internal Functional
Interconnects and System Reset”
), and this affects their use during low power. There are generally the
following low power scenarios:
•
Hardware reset (RESET pin to the MCU is asserted low). NOT RECOMMENDED - this holds the
MCU in reset, but not the transceiver because the transceiver reset is controlled by the MCU. This
mode is not normally recommended as a long-term, “held” state.
— The MCU GPIO default to inputs with no pullups. The user should provide hardware-based
known states for all the inputs if this condition is held and low power is desired.
— The transceiver reset connection (Pad 71) should have a pulldown added to hold the chip in
reset.
— The transceiver GPIO default to inputs with no pullups. The user should provide
hardware-based known states for all the inputs if this condition is held and low power is
desired.
Table 3-12. MCU Active Mode Supply Current
Condition
Current (typ @ 3.0V)
Run supply current
1
at CPU clock = 2 MHZ, f
Bus
= 1 MHz
1
All MCU modules except ATD active, ICG configured for FBE, and no dc loads on port
pins.
1.1 mA
Run supply current at CPU clock = 16 MHZ, f
Bus
= 8 MHz
6.5 mA
Run supply current at CPU clock = 32 MHZ, f
Bus
= 16 MHz
12.9 mA (calculated)
Wait supply current with f
Bus
= 1 MHz
560
μ
A
ATD supply current when enabled
700
μ
A
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