AT89LP428 Datasheet, PDF (113/149 Pages) ATMEL Corporation – 8-bit Microcontroller with 4K/8K Bytes In-System Programmable Flash

AT89LP428/828
22. On-chip Debug System
The AT89LP428/828 On-chip Debug (OCD) System uses a 2-wire serial interface to control pro-
gram flow; read, modify, and write the system state; and program the nonvolatile memory. The
OCD System has the following features:
• Complete program flow control
• Read-modify-write access to all internal SFRs and data memories
• Four hardware program address breakpoints
• Unlimited program software breakpoints using BREAK instruction
• Break on change in program memory flow
• Break on stack overflow/underflow
• Break on Watchdog overflow
• Break on reset
• Non-intrusive operation
• Programming of nonvolatile memory
22.1
Physical Interface
The On-chip Debug System uses a 2-wire synchronous serial interface to establish communica-
tion between the target device and the controlling emulator system. The OCD interface is
controlled by two User Fuses. OCD is enabled by clearing the OCD Enable Fuse. The OCD
device connections are shown in Figure 22-1. When OCD is enabled, the RST port pin is config-
ured as an input for the Debug Clock (DCL). Either the XTAL1, XTAL2 or P3.7 pin is configured
as a bi-directional data line for the Debug Data (DDA) depending on the clock source selected. If
the Internal RC Oscillator is selected, XTAL1 is configured as DDA (A). If the External Clock is
selected, XTAL2 is configured as DDA (B). If the Crystal Oscillator is selected, P3.7 is config-
ured as DDA (C). When OCD is enabled, the type of interface used depends on the OCD
Interface Select User Fuse. This fuse selects between a normal Two-wire Interface (TWI) and a
fast Two-wire Interface (FTWI). It is the duty of the user to program this fuse to the correct set-
ting for their debug system at the same time they enable OCD (see “User Configuration Fuses”
on page 121).
When designing a system where On-chip Debug will be used, the following observations must
be considered for correct operation:
• P3.6/RST cannot be connected directly to V_CC and any external capacitors connected to RST
must be removed.
• All external reset sources must be removed.
• If P3.7 needs to be debugged in-system using the crystal oscillator, the external clock option
should be selected. The quartz crystal and any capacitors on XTAL1 or XTAL2 must be
removed and an external clock signal must be driven on XTAL1. Some emulator systems may
provide a user-configurable clock for this purpose.
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