AMD Am188TMER, Am186TMER user manual Memory Interface, Serial Communications Port

1.3.2Memory Interface

The integrated memory controller logic of the Am186ER and Am188ER microcontrollers provides a direct address bus interface to memory devices. The use of an external address latch controlled by the address latch enable (ALE) signal is not required.

Individual byte write-enable signals are provided to eliminate the need for external high/ low-byte, write-enable circuitry. The maximum bank size programmable for the memory chip-select signals is increased to 512 Kbyte to facilitate the use of high-density memory devices.

Improved memory timing specifications enable the use of zero-wait-state memories with 70-ns access times at 40-MHz CPU operation. This reduces overall system cost significantly by allowing the use of commonly available memory devices. The integrated 32-Kbyte RAM operates at the same speed as zero-wait-state external memory.

Figure 1-3illustrates an Am186ER microcontroller-based configuration with 512 Kbyte of external Flash EPROM in addition to the internal 32-Kbyte memory. Additional external RAM can also be added. The external memory interface requires the following:

νThe processor A19–A0 bus connects to the memory address inputs.

νThe AD bus connects directly to the data inputs/outputs.

νThe UCS chip select connects to the memory chip-select input.

External read operations require that the RD output connects to the SRAM Output Enable (OE) input pin. External write operations require that the byte write enables connect to the SRAM Write Enable (WE) input pin.

The example design shown in Figure 1-3uses a 4-Mbit (256-K x 16) external Flash EPROM for application memory, mapped into the upper region of the microcontroller’s 1-Mbyte address space at 80000h–FFFFFh. After a valid reset, the Am186ER or Am188ER microcontroller will fetch the first instruction from address FFFF0h. The user application can then enable and configure the location of the integrated 32-Kbyte RAM within the remaining address space; in this example, it would be at address 00000h to accommodate the interrupt vector table.

1.3.3Serial Communications Port

The integrated universal asynchronous receiver/transmitter (UART) controller in the Am186ER and Am188ER microcontrollers eliminates the need for external logic to implement a communications interface. The integrated UART generates the serial clock from the CPU clock so that no external time-base oscillator is required.

Figure 1-3shows a minimal implementation of an RS-232 console or modem communications port. The RS-232 to CMOS voltage-level converter is required for the proper electrical interface with the external device.

The Am186ER and Am188ER microcontrollers also include a synchronous serial interface. For more information, see Chapter 11.

Features and Performance 1-7