MC9S12A128 Datasheet, PDF (45/92 Pages) Motorola, Inc – Microcontroller unit (MCU)

MC9S12A128 Device Guide — V01.01
4.2.2 Special Operating Modes
There are two special operating modes that correspond to normal operating modes. These operating modes
are commonly used in factory testing and system development.
4.2.2.1 Special Single-Chip Mode
When the MCU is reset in this mode, the background debug mode is enabled and active. The MCU does
not fetch the reset vector and execute application code as it would in other modes. Instead the active
background mode is in control of CPU execution and BDM firmware is waiting for additional serial
commands through the BKGD pin. When a serial command instructs the MCU to return to normal
execution, the system will be configured as described below unless the reset states of internal control
registers have been changed through background commands after the MCU was reset.
There is no external expansion bus after reset in this mode. Ports A and B are initially simple bidirectional
I/O pins that are configured as high-impedance inputs with internal pull-ups disabled; however, writing to
the mode select bits in the MODE register (which is allowed in special modes) can change this after reset.
All of the Port E pins (except PE4/ECLK) are initially configured as general purpose high-impedance
inputs with pull-ups enabled. PE4/ECLK is configured as the E clock output in this mode.
The pins associated with Port E bits 6, 5, 3, and 2 cannot be configured for their alternate functions IPIPE1,
IPIPE0, LSTRB, and R/W while the MCU is in single chip modes. In single chip modes, the associated
control bits PIPOE, LSTRE and RDWE are reset to zero. Writing the opposite value into these bits in
single chip mode does not change the operation of the associated Port E pins.
Port E, bit 4 can be configured for a free-running E clock output by clearing NECLK=0. Typically the only
use for an E clock output while the MCU is in single chip modes would be to get a constant speed clock
for use in the external application system.
4.2.2.2 Special Test Mode
In expanded wide modes, Ports A and B are configured as a 16-bit multiplexed address and data bus and
Port E provides bus control and status signals. In special test mode, the write protection of many control
bits is lifted so that they can be thoroughly tested without needing to go through reset.
4.2.3 Test Operating Mode
There is a test operating mode in which an external master, such as an I.C. tester, can control the on-chip
peripherals.
4.2.3.1 Peripheral Mode
This mode is intended for Motorola factory testing of the MCU. In this mode, the CPU is inactive and an
external (tester) bus master drives address, data and bus control signals in through Ports A, B and E. In
effect, the whole MCU acts as if it was a peripheral under control of an external CPU. This allows faster
testing of on-chip memory and peripherals than previous testing methods. Since the mode control register
is not accessible in peripheral mode, the only way to change to another mode is to reset the MCU into a
different mode. Background debugging should not be used while the MCU is in special peripheral mode
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