Microcontrollers in Practice
Packed with hundreds of practical examples and exercises to foster mastery of concepts and details, this practical guide provides readers with hands-on knowledge of how to implement three families of microcontrollers.
TABLE OF CONTENTS Microcontrollers in Practice
A.8: AT90S8515 Operating with External RAM
A.8 AT90S8515 Operating with External RAM
Some AVR microcontrollers can allocate some of the I/O lines to extend the bus of the data memory for connecting external memory or other RAM-like external devices. Figure A8.1 presents an example of implementation of the external bus for the microcontroller AT90S8515.
Figure A8.1: AT90S8515 operating with external bus for data memory
In this example, the external memory is a 32K 862256 circuit, selected directly with the address line A15, which makes it visible in the address range $0000-$7FFF in the address space of the data memory.
The control signals for the external bus are generated by the MCU. ALE (Address Latch Enable) is active HIGH, and strobes the lower half of the address into the external address latch ICI (74LS573), while RD\ (Read) and WR\ (Write) are active LOW and indicate the direction of the data transfer on the external bus. They are directly connected to the control inputs OE\ and WR\ of the external RAM circuit IC2.
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