2.2.4 Background/mode select (BKGD/MS)
During a power-on-reset (POR) or background debug force reset, the PTA4/ACMPO/
BKGD/MS pin functions as a mode select pin. Immediately after internal reset rises the
pin functions as the background pin and can be used for background debug
communication. While the pin functions as a background/mode selection pin, it includes
an internal pullup device and a standard output driver.
The background debug communication function is enabled when SOPT1[BKGDPE] bit
is set. SOPT1[BKGDPE] is set following any reset of the MCU and must be cleared to
use the PTA4/ACMPO/BKGD/MS pin's alternative pin functions.
If this pin is floating, 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 POR or immediately after issuing a background
debug force reset, which will force the MCU into 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 can run as fast as the bus clock, so
there should never be any significant capacitance connected to the BKGD/MS pin that
interferes with background serial communications. When the pin performs output only
PTA4, it can drive only capacitance-limited MOSFET. Driving a bipolar transistor
directly by PTA4 is prohibited because this can cause mode entry fault and BKGD errors.
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 time. Small capacitances from cables and the absolute value of the internal
pullup device play almost no role in determining rise and fall time on the BKGD pin.
Optional Manual Reset
BKGD/MS
V
DD
V
SS
PTA5/IRQ/FTM1CH0/RESET
Figure 2-7. Typical debug circuit
Chapter 2 Pins and connections
MC9S08PA4 Reference Manual, Rev. 5, 08/2017
NXP Semiconductors
33
Summary of Contents for MC9S08PA4
Page 1: ...MC9S08PA4 Reference Manual Supports MC9S08PA4 Document Number MC9S08PA4RM Rev 5 08 2017 ...
Page 2: ...MC9S08PA4 Reference Manual Rev 5 08 2017 2 NXP Semiconductors ...
Page 22: ...MC9S08PA4 Reference Manual Rev 5 08 2017 22 NXP Semiconductors ...
Page 28: ...System clock distribution MC9S08PA4 Reference Manual Rev 5 08 2017 28 NXP Semiconductors ...
Page 150: ...Port data registers MC9S08PA4 Reference Manual Rev 5 08 2017 150 NXP Semiconductors ...
Page 196: ...Human machine interfaces HMI MC9S08PA4 Reference Manual Rev 5 08 2017 196 NXP Semiconductors ...
Page 224: ...Instruction Set Summary MC9S08PA4 Reference Manual Rev 5 08 2017 224 NXP Semiconductors ...
Page 232: ...Functional Description MC9S08PA4 Reference Manual Rev 5 08 2017 232 NXP Semiconductors ...
Page 258: ...FTM Interrupts MC9S08PA4 Reference Manual Rev 5 08 2017 258 NXP Semiconductors ...
Page 294: ...Functional description MC9S08PA4 Reference Manual Rev 5 08 2017 294 NXP Semiconductors ...
Page 398: ...Resets MC9S08PA4 Reference Manual Rev 5 08 2017 398 NXP Semiconductors ...
Page 400: ...MC9S08PA4 Reference Manual Rev 5 08 2017 400 NXP Semiconductors ...