MC68HC11KW1 Datasheet, PDF (54/238 Pages) Motorola, Inc – High-density complementary metal oxide semiconductor HCMOS) microcontroller unit

4.3
System initialization
Registers and bits that control initialization and the basic operation of the MCU are protected
against writes except under special circumstances. The following table lists registers that can be
written only once after reset, or that must be written within the first 64 cycles after reset.
Table 4-3 Registers with limited write access
4
Register
Register
Must be written in
Write
address
name
first 64 cycles
once only
$x024 Timer interrupt mask register 2 (TMSK2)
(1)
—
$x035 Block protect register (BPROT)
(2)
—
$x037 EEPROM mapping register (INIT2)
No
Yes
$x038 System configuration options register 2 (OPT2)
No
(3)
$x039 System configuration options register (OPTION)
(4)
—
$x03D RAM and I/O map register (INIT)
(5)
—
$x081 Timer control register 4 (Timer 2) TCTL4
No
(6)
$x091 Timer control register 6 (Timer 3) TCTL6
No
(6)
(1) When SMOD = 0, bits 1 and 0 can be written only once, during the first 64 cycles, after which they become read-only.
When SMOD = 1, however, these bits can be written at any time. All other bits can be written at any time.
(2) Bits can be written to zero once and only in the first 64 cycles or in special modes. Bits can be set to one at any time.
(3) Bit 4 (IRVNE) can be written only once.
(4) When SMOD = 0, bits 5, 4, 2, 1, and 0 can be written once and only in the first 64 cycles.When SMOD = 1,
however, bits 5, 4, 2, 1, and 0 can be written at any time. All other bits can be written at any time.
(5) When SMOD = 0, bits can be written only once, during the first 64 cycles, after which the register becomes
read-only. When SMOD = 1, bits can be written at any time.
(6) Bits 5, 4, 3 and 2 can be written only once.
4.3.1 Mode selection
The four mode variations are selected by the logic states of the mode A (MODA) and mode B
(MODB) pins during reset. The MODA and MODB logic levels determine the logic state of the
special mode (SMOD) and mode A (MDA) control bits in the highest priority I-bit interrupt and
miscellaneous (HPRIO) register.
After reset is released, the mode select pins no longer influence the MCU operating mode. In
single chip operating mode, MODA pin is connected to a logic zero. In expanded mode, MODA is
normally connected to V_DD through a pull-up resistor of 4.7 kΩ. The MODA pin also functions as
the load instruction register (LIR) pin when the MCU is not in reset. The open-drain active low LIR
output pin drives low during the first E cycle of each instruction. The MODB pin also functions as
the stand-by power input (VSTBY), which allows the RAM contents to be maintained in the
absence of V_DD.
4-10
OPERATING MODES AND ON-CHIP MEMORY
MC68HC11KW1