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DS87C323-QND Datasheet(PDF) 4 Page - Dallas Semiconductor |
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DS87C323-QND Datasheet(HTML) 4 Page - Dallas Semiconductor |
4 / 175 page High-Speed Microcontroller User’s Guide 4 of 175 retains the value which software writes. In general, during a read operation, software reads the state of the external pin. Each port is represented by a SFR location. Timer/Counters Three 16-bit Timer/Counters are available in the High-Speed Microcontroller. Each timer is contained in two SFR locations that can be written or read by software. The timers are controlled by other SFRs described in Section 4. UARTs The High-Speed Microcontroller provides one or two UARTs. These are controlled and accessed as SFRs. Each UART has an address that is used to read or write the UART. The same address is used for both read and write operations. The microcontroller distinguishes between a read and a write by the instruction. Each UART is controlled by its own SFR control register. SCRATCHPAD REGISTERS (RAM) The High-Speed Core provides 256 bytes of Scratchpad RAM for general purpose data and variable storage. The first 128 bytes are directly available to software. The second 128 are available through indirect addressing discussed below. Selected portions of this RAM have other optional functions. Stack The stack is a RAM area that the microcontroller uses to store return address information during Calls and Interrupts. The user can also place variables on the stack when necessary. The Stack Pointer mentioned above designates the RAM location that is the top of the stack. Thus, depending on the value of the Stack Pointer, the stack can be located anywhere in the 256 bytes of RAM. A common location would be in the upper 128 bytes of RAM, as these are accessible through indirect addressing only. Working Registers The first thirty-two bytes of the Scratchpad RAM can be used as four banks of eight Working Registers for high speed data movement. Using four banks, software can quickly change context by simply changing to a different bank. In addition to the Accumulator, the Working Registers are commonly used as data source or destination. Some of the Working Registers can also be used as pointers to other RAM locations (indirect addressing). PROGRAM COUNTER The Program Counter (PC) is a 16-bit value that designates the next program address to be fetched. On- chip hardware automatically increments the PC value to move to the next ROM location. ADDRESS/DATA BUS The High-Speed Microcontroller addresses a 64KB program and 64KB data memory area. In the ROMless versions, all memory is outside. Other versions use a combination of internal and external memory. When external memory is accessed, Ports 0 and 2 are used as a multiplexed address and data bus. Port 2 provides the address MSB. Even versions with internal memory can use the bus on Ports 0 and 2 to access more memory. WATCHDOG TIMER The Watchdog Timer provides a supervisory function for applications that cannot afford to run out of control. The Watchdog Timer is a programmable free running timer. If allowed to reach the termination of its count, if enabled, the Watchdog will reset the CPU. Software must prevent this by cleaning or resetting the Watchdog prior to its time-out. |
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