AT89C55WD datasheet(9/35 Pages) ATMEL

9 / 35 page

1921C–MICRO–3/05

AT89C55WD

Memory Organization

The MCS-51 devices have a separate address space for Program and Data Memory. Up to

64 Kbytes each of external Program and Data Memory can be addressed.

6.1

Program Memory

If the EA pin is connected to GND, all program fetches are directed to external memory.

On the AT89C55WD, if EA is connected to V

CC, program fetches to addresses 0000H through

4FFFH are directed to internal memory and fetches to addresses 5000H through FFFFH are to

external memory.

6.2

Data Memory

The AT89C55WD implements 256 bytes of on-chip RAM. The upper 128 bytes occupy a parallel

address space to the Special Function Registers. That means the upper 128 bytes have the

same addresses as the SFR space but are physically separate from SFR space.

When an instruction accesses an internal location above address 7FH, the address mode used

in the instruction specifies whether the CPU accesses the upper 128 bytes of RAM or the SFR

space. Instructions that use direct addressing access SFR space.

For example, the following direct addressing instruction accesses the SFR at location 0A0H

(which is P2).

MOV 0A0H, #data

Instructions that use indirect addressing access the upper 128 bytes of RAM. For example, the

following indirect addressing instruction, where R0 contains 0A0H, accesses the data byte at

address 0A0H, rather than P2 (whose address is 0A0H).

MOV @R0, #data

Note that stack operations are examples of indirect addressing, so the upper 128 bytes of data

RAM are available as stack space.

Hardware Watchdog Timer (One-time Enabled with Reset-out)

The WDT is intended as a recovery method in situations where the CPU may be subjected to

software upsets. The WDT consists of a 13-bit counter and the WatchDog Timer Reset

(WDTRST) SFR. The WDT is defaulted to disable from exiting reset. To enable the WDT, a user

must write 01EH and 0E1H in sequence to the WDTRST register (SFR location 0A6H). When

the WDT is enabled, it will increment every machine cycle while the oscillator is running. The

WDT time-out period is dependent on the external clock frequency. There is no way to disable

the WDT except through reset (either hardware reset or WDT overflow reset). When WDT over-

flows, it will drive an output RESET HIGH pulse at the RST pin.

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