Motorola USB08 Instrucciones de operaciones descargar pdf (Pagina 13)

3 MON08 MONITOR FIRMWARE

3.1 Introduction

The HC908 series of micro controllers come with a small monitor program (MON08) already

programmed into ROM. This program has a software bit-bang emulation of the standard RS232 serial

protocol and six low-level commands which together allow the user to read and write RAM memory,

read the stack pointer (SP) and run a program. It is the function of Xplor8 to interface with this

monitor program and use its low level commands to perform realistic tasks such as programming

FLASH memory.

To be able to enter this monitor program after resetting the MCU requires that the reset vector points

to its start. As the normal reset vector (at $FFFE/F) may have been programmed by the user to point

to the start of the user’s own program, the HC908 MCUs have a special mechanism to force the CPU

to use different addresses for the reset and SWI vectors. This mechanism involves having certain

voltages on various pins at the moment of reset - see any HC908 data sheet for details.

The monitor uses only a single pin on the MCU to communicate with the outside world. This means

that full duplex communication is not possible, and also that every byte sent to the MCU from the host

is inevitably also returned to the host as an unwanted echo. Xplor8 automatically disregards these

unwanted echos.

The monitor program on the MCU provides six basic communication functions:

1. Read MCU memory

2. Write MCU memory

3. Indexed read of MCU memory

4. Indexed write of MCU memory

5. Read the Stack Pointer (SP)

6. Run a user program

All Xplor8's features are provided by using one or more of these six functions. When the CPU enters

the monitor program it executes a software interrupt (SWI) instruction. This automatically stacks the

program counter, A, X and CCR registers, and the monitor is so written that it also stacks the H

command which unstacks H and then executes a Return from Interrupt (RTI).

Once a user program is running on the MCU there is no way for Xplor8 to seize back control, unless:

• The user program arrives at a pre-set breakpoint

• The user program arrives at an SWI instruction

• The MCU is reset

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Motorola USB08 Instrucciones de operaciones Pagina 13