MC9S08GB60A Datasheet, PDF (30/302 Pages) Freescale Semiconductor, Inc – HCS08 Microcontrollers

Chapter 2 Pins and Connections
debug connector so a development system can directly reset the MCU system. If desired, a manual external
reset can be added by supplying a simple switch to ground (pull reset pin low to force a reset).
Whenever any reset is initiated (whether from an external signal or from an internal system), the reset pin
is driven low for approximately 34 cycles of f_{Self_reset}, released, and sampled again approximately 38
cycles of f_{Self_reset} later. If reset was caused by an internal source such as low-voltage reset or watchdog
timeout, the circuitry expects the reset pin sample to return a logic 1. The reset circuitry decodes the cause
of reset and records it by setting a corresponding bit in the system control reset status register (SRS).
In EMC-sensitive applications, an external RC filter is recommended on the reset pin. See Figure 2-4 for
an example.
2.3.4 Background / Mode Select (PTG0/BKGD/MS)
The background/mode select (BKGD/MS) shares its function with an I/O port 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,
the pin includes an internal pullup device, input hysteresis, a standard output driver, and no output slew
rate control. When used as an I/O port (PTG0) the pin is limited to output only.
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 General-Purpose I/O and Peripheral Ports
The remaining 55 pins are shared among general-purpose I/O and on-chip peripheral functions such as
timers and serial I/O systems. (17 of these pins are not bonded out on the 48-pin package, 20 of these pins
are not bonded out on the 44-pin package and 22 of hese pins are not bonded out on the 42-pin package.)
Immediately after reset, all 55 of these pins are configured as high-impedance general-purpose inputs with
internal pullup devices disabled.
NOTE
To prevent 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.
MC9S08GB60A Data Sheet, Rev. 2
30
Freescale Semiconductor