MOTOROLA
3-8
CONFIGURATION AND MODES OF OPERATION
M68HC11
REFERENCE MANUAL
the case of an expanded mode system where ROM is enabled and both RAM and reg-
isters have been remapped to $F000. For accesses from $F000–$F03F, ROM and
RAM are disabled, and registers have highest access priority. From $F040–$F0FF,
ROM is disabled, and RAM has access priority.
Some users have questions about the priority of access for unused register locations
in the 64-byte register space or the priority of registers in an external MC68HC24. In
the previous example, $F035 would correspond to an unused location in the 64-byte
register space (the register block was moved from its usual position of $1000–$103F
such that it overlaps RAM and ROM at $F000). Reads of this address access the un-
driven internal data bus, and any data present on the data bus pins is ignored. Six lo-
cations in the 64-byte register space become external accesses when the
MC68HC11A8 is operating in an expanded mode. This process allows the MC68HC24
to properly emulate the internal parallel I/O functions associated with the 18 MCU pins,
which are dedicated to the multiplexed expansion bus. Again referring to the earlier
example, if any of these six addresses are accessed, the internal ROM and RAM are
disabled so the CPU gets valid data from the external MC68HC24, which is consid-
ered a part of the internal register space. The six locations of interest are $x002–$x007
(PIOC, PORTC, PORTB, PORTCL, one reserved location, and DDRC). Although x is
usually one, it was changed to $F by software in this example.
3.3.2 Protected Control Bits in the TMSK2 Register
The following register diagram and paragraphs describe the time-protected timer pres-
cale select bits (PR[1:0]) in the timer mask register 2 (TMSK2). The upper four bits of
this register, which are related to the timer and pulse accumulator subsystems, will be
discussed in
SECTION 10 MAIN TIMER AND REAL-TIME INTERRUPT
and
SEC-
TION 11 PULSE ACCUMULATOR
. Bits 3 and 2 are not implemented and always read
as zeros.
PR[1:0] — Timer Prescaler Select
These two bits select the prescale rate for the main 16-bit free-running timer system.
The following table shows the relationship between the prescale factor and the value
of these control bits. A prescale factor of one corresponds to a timer count rate of E
clock divided by one; a prescale factor of 16 corresponds to a timer count rate of E
clock divided by 16. In normal modes, this prescale rate can only be changed once
within the first 64 bus cycles after reset, and the resulting count rate stays in effect until
the next reset.
TMSK2 —
Timer Mask Register 2
$1024
BIT 7
TOI
0
6
5
4
3
0
0
2
0
0
1
BIT 0
PR0
0
RTII
0
PAOVI
0
PAII
0
PR1
0
RESET: