DS2792 Programmable Fuel Gauge with UART Interface

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the program and data EEPROM contents over the UART interface. Updates to the program and data EEPROM are

protected against unauthorized writes by a 256-bit user password. A read protection bit is provided to prevent

reading either EEPROM.

MAXQ20 Core Architecture

The DS2792 employs a MAXQ20 low-cost, high-performance, CMOS, fully static, 16-bit RISC microcontroller with

EEPROM memory. Fetch and execution operations are completed in one cycle without pipelining, since the

instruction contains both the op code and data. The highly efficient core is supported by 16 accumulators and a 16-

level hardware stack, enabling fast subroutine calling and task switching. Data can be quickly and efficiently

manipulated with three internal data pointers. Multiple data pointers allow more than one function to access data

memory without having to save and restore data pointers each time. The data pointers can automatically increment

or decrement following an operation, eliminating the need for software intervention.

Instruction Set

The instruction set is composed of fixed-length, 16-bit instructions that operate on registers and memory locations.

The instruction set is highly orthogonal, allowing arithmetic and logical operations to use any register along with the

accumulator. Special-function registers control the peripherals and are subdivided into register modules. The family

architecture is modular, so that new devices and modules can reuse code developed for existing products.

The architecture is transport-triggered. This means that writes or reads from certain register locations can also

cause side effects to occur. These side effects form the basis for higher level op codes, such as ADDC, OR, JUMP,

etc. The op codes are implemented as MOVE instructions between register locations, while the assembler handles

the encoding, which need not be a concern to the programmer. The 16-bit instruction word is designed for efficient

execution.

Bit 15 indicates the format for the source field of the instruction. Bits 0 to 7 of the instruction represent the source

for the transfer. Depending on the value of the format field, this can either be an immediate value or a source

register. If this field represents a register, the lower four bits contain the module specifier and the upper four bits

contain the register index in that module.

Bits 8 to 14 represent the destination for the transfer. This value always represents a destination register, with the

lower four bits containing the module specifier and the upper three bits containing the register subindex within that

module. Any time that it is necessary to directly select one of the upper 24 registers as a destination, the prefix

register PFX is needed to supply the extra destination bits. This prefix register write is inserted automatically by the

assembler and requires only one additional execution cycle. Refer to the MAXQ Family User's Guide for complete

instruction set information.

Memory Organization

The DS2792 incorporates several memory areas:

The memory is implemented using the Harvard architecture, with separate address spaces for program and data

memory. A pseudo-Von Neumann memory map is also utilized placing ROM, application code, and data memory

into a single contiguous memory map. The pseudo-Von Neuman memory map allows data memory to be mapped

into program space, permitting code execution from data memory. In addition, program memory can be mapped

into data space, permitting code constants to be accessed as data memory. Figure 3 shows the DS2792’s memory

map when executing from program memory space. Refer to the MAXQ Family User's Guide: DS2792 Supplement

for memory map information when executing from data or ROM space.