MC68HC908AS32A Datasheet, PDF (249/280 Pages) Freescale Semiconductor, Inc – Microcontrollers

Monitor Module (MON)
18.2.3 Low-Power Modes
The WAIT and STOP instructions put the MCU in low power-consumption standby modes. If enabled, the
break module will remain enabled in wait and stop modes. However, since the internal address bus does
not increment in these modes, a break interrupt will never be triggered.
18.3 Monitor Module (MON)
This subsection describes the monitor module (MON) and the monitor mode entry methods. The monitor
allows debugging and programming of the microcontroller unit (MCU) through a single-wire interface with
a host computer.
Features include:
• Normal user-mode pin functionality
• One pin dedicated to serial communication between monitor ROM and host computer
• Standard mark/space non-return-to-zero (NRZ) communication with host computer
• Up to 28.8 kBaud communication with host computer
• Execution of code in RAM or FLASH
• FLASH security⁽¹⁾
• FLASH programming
18.3.1 Functional Description
Figure 18-5 shows a simplified diagram of the monitor mode entry.
The monitor module receives and executes commands from a host computer.
Figure 18-6 shows an example circuit used to enter monitor mode and communicate with a host computer
via a standard RS-232 interface.
While simple monitor commands can access any memory address, the MC68HC908AS32A has a FLASH
security feature to prevent external viewing of the contents of FLASH. Proper procedures must be
followed to verify FLASH content. Access to the FLASH is denied to unauthorized users of customer
specified software (see 18.3.2 Security).
In monitor mode, the MCU can execute code down loaded into RAM while most MCU pins retain normal
operating mode functions. All communication between the host computer and the MCU is through the
PTA0 pin. A level-shifting and multiplexing interface is required between PTA0 and the host computer.
PTA0 is used in a wired-OR configuration and requires a pullup resistor.
1. No security feature is absolutely secure. However, Freescale’s strategy is to make reading or copying the FLASH difficult for
unauthorized users.
MC68HC908AS32A Data Sheet, Rev. 2.0
Freescale Semiconductor
249