MC9S08RC8 Datasheet, PDF (24/234 Pages) Freescale Semiconductor, Inc – Microcontrollers

Pins and Connections
2.3.4 Background/Mode Select (PTD0/BKGD/MS)
The background/mode select function is shared with an output-only port function on the PTD0/BKDG/MS
pin. While in reset, the pin functions as a mode select pin. Immediately after reset rises, the pin functions
as the background pin and can be used for background debug communication. While functioning as a
background/mode select pin, this pin has an internal pullup device enabled. To use as an output-only port,
BKGDPE in SOPT must be cleared.
If nothing is connected to this pin, the MCU will enter normal operating mode at the rising edge of reset.
If a debug system is connected to the 6-pin standard background debug header, it can hold BKGD/MS low
during the rising edge of reset, which forces the MCU to active background mode.
The BKGD pin is used primarily for background debug controller (BDC) communications using a custom
protocol that uses 16 clock cycles of the target MCU’s BDC clock per bit time. The target MCU’s BDC
clock could be as fast as the bus clock rate, so there should never be any significant capacitance connected
to the BKGD/MS pin that could interfere with background serial communications.
Although the BKGD pin is a pseudo open-drain pin, the background debug communication protocol
provides brief, actively driven, high speedup pulses to ensure fast rise times. Small capacitances from
cables and the absolute value of the internal pullup device play almost no role in determining rise and fall
times on the BKGD pin.
2.3.5 IRO Pin Description
The IRO pin is the output of the CMT. See the Carrier Modulator Timer (CMT) Module Chapter for a
detailed description of this pin function.
2.3.6 General-Purpose I/O and Peripheral Ports
The remaining pins are shared among general-purpose I/O and on-chip peripheral functions such as timers
and serial I/O systems. (Not all pins are available in all packages. See Table 2-2.) Immediately after reset,
all 37 of these pins are configured as high-impedance general-purpose inputs with internal pullup devices
disabled.
NOTE
To avoid extra current drain from floating input pins, the reset initialization
routine in the application program should either enable on-chip pullup
devices or change the direction of unused pins to outputs so the pins do not
float.
For information about controlling these pins as general-purpose I/O pins, see the Chapter 6, “Parallel
Input/Output." For information about how and when on-chip peripheral systems use these pins, refer to the
appropriate chapter from Table 2-1.
MC9S08RC/RD/RE/RG Data Sheet, Rev. 1.11
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Freescale Semiconductor