M68HC11
REFERENCE MANUAL
CONFIGURATION AND MODES OF OPERATION
MOTOROLA
3-19
described. This $55 character can only use the E clock/16/16 (7812 baud for E = 2
MHz) rate since it takes the place of the $FF character, which could have changed the
baud rate. This feature allows for testing the MCU for proper single-chip mode opera-
tion when the E-clock frequency is beyond the capability of the multiplexed expansion
bus. Test equipment can reset the MCU in special test mode (at a legal expansion bus
frequency) and parallel load a program into RAM. The tester can then reset the MCU
in bootstrap mode (at a higher clock frequency) and serially send the $55 character to
cause a jump to the start of RAM. This procedure takes significantly less time than us-
ing the normal bootloading procedure to serially load 256 characters. Since the pro-
gram segments are limited in size by the amount of on-chip RAM, the time required to
load enough program segments to fully test the MCU would make such testing too ex-
pensive for all but a very few applications.
3.6 Test and Bootstrap Mode Applications
Most users are familiar with the uses for normal operating modes, but the special test
and special bootstrap modes may be new. In this section, an example is presented to
stimulate the user’s imagination. After examining this example, some users will think
of ways these special mode variations can help in their applications.
Example 3–1: Programming CONFIG (Uses Special Test Mode)
This example demonstrates how the special test mode can be used to program the
EEPROM-based CONFIG register. Current versions of the M68HC11 Family require
the MCU to be in one of two special modes to program the CONFIG register.
There are several reasons why a user might want to change the CONFIG register.
Suppose the user has an MC68HC811E2 and would like to experiment with it in an
expanded system such as an MC68HC11EVB evaluation board. As shipped from Mo-
torola, the MC68HC811E2 part is not compatible with the memory map of the EVB.
The EEPROM must be disabled by programming the EEON bit to zero, or the upper
four bits of CONFIG must be changed to relocate the EEPROM away from EVB re-
sources. Suppose the user is finished with initial debugging and wants to enable the
COP watchdog system by programming the NOCOP bit in CONFIG to zero. Perhaps
the CONFIG registers in some of the user’s parts have been corrupted during initial
experimentation. Some users forget to control reset during power transitions; thus, the
CONFIG register could be corrupted due to program runaway when V
DD
is too low to
allow proper operation. When this runaway happens, the part is not defective; it needs
to have the CONFIG register changed back to the proper value.
The schematic diagram shown in
Figure 3-1
is a relatively simple expanded-mode
system that can be operated in special test mode. By removing the jumper that pulls
MODB low, this board can also be used in normal expanded mode. An interesting fea-
ture of this system is that the external EPROM appears in the memory map at $A000–
$BFFF and again at $E000–$FFFF because address line ADDR14 is left out of the ad-
dress decode. This feature makes reset vectors in the highest locations of the EPROM
appear the same to the MCU whether the MCU is reset in special test mode or normal
expanded mode with the internal ROM disabled. Several subtle benefits to this feature
are evident. First, it means no decode changes are needed to alternate between nor-