DS89C420_05 Datasheet, PDF (16/47 Pages) Maxim Integrated Products – Ultra-High-Speed Microcontroller

DS89C420 Ultra-High-Speed Microcontroller
All standard SFR locations from the 8051 are duplicated in the DS89C420 and several SFRs have been added for
the unique features of the DS89C420. Most of these features are controlled by bits in SFRs located in unused
locations in the 8051 SFR map. This allows for increased functionality while maintaining complete instruction set
compatibility. Table 1 summarizes the SFRs and their locations. Table 2 specifies the default reset condition for all
SFR bits.
DATA POINTERS
The data pointers (DPTR and DPTR1) are used to assign a memory address for the MOVX instructions. This
address can point to a MOVX RAM location (on-chip or off-chip), or a memory-mapped peripheral. Two pointers
are useful when moving data from one memory area to another, or when using a memory-mapped peripheral for
both source and destination addresses. The user selects the active pointer through a dedicated SFR bit (Sel =
DPS.0), or activates an automatic toggling feature for altering the pointer selection (TSL = DPS.5). An additional
feature, if selected, provides automatic incrementing or decrementing of the current DPTR.
STACK POINTER
The stack pointer denotes the register location at the top of the stack, which is the last used value. The user can
place the stack anywhere in the scratchpad RAM by setting the stack pointer to the desired location, although the
lower bytes are normally used for working registers.
I/O PORTS
The DS89C420 offers four 8-bit I/O ports. Each I/O port is represented by an SFR location, and can be written or
read. The I/O port has a latch that contains the value written by software.
COUNTER/TIMERS
Three 16-bit timer/counters are available in the DS89C420. Each timer is contained in two SFR locations that can
be read or written by software. The timers are controlled by other SFRs described in the “SFR Bit Description”
section of the Ultra-High-Speed Flash Microcontroller User’s Guide.
SERIAL PORTS
The DS89C420 provides two UARTs that are controlled and accessed by SFRs. Each UART has an address that
is used to read and write the UART. The same address is used for read and write operations, which are
distinguished by the instruction. Its own SFR control register controls each UART.
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