EF6805P2 datasheet(19/31 Pages) STMICROELECTRONICS

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SOFTWARE

BIT MANIPULATION

The EF6805U3 MCU has the ability to set or clear

any single random access memory or input/output

bit (except the data direction register, see Caution

below), with a single instruction (BSET, BCLR). Any

bit in page zero including ROM, except the DDRs,

can be tested, using the BRSET and BRCLR ins-

tructions, and the program branches as a result of

its state. The carry bit equals the value of the bit ref-

erenced by BRSET or BRCLR. A rotate instruction

may then be used to accumulate serial input data in

a RAM location or register. The capability to work

with any bit in RAM, ROM, or I/O allows the user to

have individual flags in RAM or to handle I/O bits as

control lines.

The coding example in figure 21 illustrates the use-

fulness of the bit manipulation and test instructions.

Assume that the MCU is to communicate with an ex-

ternal serial device.

The external device has a data ready signal, a data

output line, and a clock line to clock data one bit at

a time. LSB first, out of the device. The MCU waits

until the data is ready, clocks the external device,

picks up the data in the carry flag (C bit), clears the

clock line, and finally accumulates the data bit in a

RAM location.

Caution

The corresponding DDRs for ports A, B, and C

are write-only registers (registers at $004, $005,

and $006). A read operation on these registers

is undefined. Since BSET and BCLR are read-

modify-write functions, they cannot be used to

set or clear a DDR bit (all ”unaffected” bits would

be set). It is recommended that all DDR bits in a

port be written using a single-store instruction.

Figure 21 : Bit Manipulation Example.

ADDRESSING MODES

The EF6805P2 MCU has 10 addressing modes

which are explained briefly in the following para-

graphs. For additional details and graphical illustra-

tions, refer to the 6805 Family User’s Manual.

The term ”effective address” (EA) is used in descri-

bing the address modes. EA is defined as the ad-

dress from which the argument for an instruction is

fetched or stored.

IMMEDIATE - In the immediate addressing mode,

the operand is contained in the byte immediately fol-

lowing the opcode. The immediate addressing

mode is used to access constants which do not

change during program execution (e.g;, a constant

used to initialize a loop counter).

DIRECT - In the direct addressing mode, the effec-

tive address of the argument is contained in a single

byte following the opcode byte. Direct addresing al-

lows the user to directly address the lowest 256

bytes in memory with a single 2-byte instruction.

This includes the on-chip RAM and I/O registers and

128 bytes of ROM. Direct addressing is an effective

use of both memory and time.

EXTENDED - In the extended addressing mode, the

effective address of the argument is contained in the

two bytes following the opcode. Instructions using

extended addressing are capable of referencing ar-

guments anywhere in memory with a single 3-byte

instruction. When using the Motorola assembler, the

programmer need not specify whether an instruction

uses direct or extended addressing. The assembler

automatically selects the shortest for of the instruc-

tion.

RELATIVE - The relative addressing mode is only

used in branch instructions. In relative addressing,

the contents of the 8-bit signed byte following the op-

code (the offset) is added to the PC if and only if the

EF6805U3

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