Analog Devices manual ADuC812 Hardware Design Considerations, Clock Oscillator

ADuC812

ADuC812 HARDWARE DESIGN CONSIDERATIONS

This section outlines some of the key hardware design consider- ations that must be addressed when integrating the ADuC812 into any hardware system.

Clock Oscillator

The clock source for the ADuC812 can come either from an external source or from the internal clock oscillator. To use the internal clock oscillator connect a parallel resonant crystal between Pins 32 and 33, and connect a capacitor from each pin to ground as shown below.

ADuC812

Note that a second very important function of the EA pin is described in the Single Pin Emulation Mode section of this data sheet.

External program memory (if used) must be connected to the ADuC812 as illustrated in Figure 37. Note that 16 I/O lines (Ports 0 and 2) are dedicated to bus functions during external program memory fetches. Port 0 (P0) serves as a multiplexed address/data bus. It emits the low byte of the program counter (PCL) as an address, and then goes into a float state awaiting the arrival of the code byte from the program memory. During the time that the low byte of the program counter is valid on P0, the signal ALE (Address Latch Enable) clocks this byte into an address latch. Meanwhile, Port 2 (P2) emits the high byte of the

XTAL1

XTAL2

TO INTERNAL TIMING CIRCUITS

program counter (PCH), then PSEN strobes the EPROM and the code byte is read into the ADuC812.

ADuC812		EPROM
	P0	D0–D7
		(INSTRUCTION)

Figure 35. External Parallel Resonant Crystal Connections

ADuC812

A0–A7

LATCH

ALE

EXTERNAL

CLOCK

SOURCE

XTAL1

XTAL2

TO INTERNAL TIMING CIRCUITS

P2	A8–A15
PSEN	OE

Figure 37. External Program Memory Interface

Figure 36. Connecting an External Clock Source

Whether using the internal oscillator or an external clock source, the ADuC812’s specified operational clock speed range is 300 kHz to 16 MHz. The core itself is static, and will function all the way down to dc. But at clock speeds slower that 400 kHz the ADC will no longer function correctly. Therefore to ensure specified operation, use a clock frequency of at least 400 kHz and no more than 16 MHz.

External Memory Interface

In addition to its internal program and data memories, the ADuC812 can access up to 64K bytes of external program memory (ROM/PROM/etc.) and up to 16M bytes of external data memory (SRAM).

To select from which code space (internal or external program memory) to begin executing instructions, tie the EA (external access) pin high or low, respectively. When EA is high (pulled up to V ), user program execution will start at address 0 of the

Note that program memory addresses are always 16 bits wide, even in cases where the actual amount of program memory used is less than 64K bytes. External program execution sacrifices two of the 8-bit ports (P0 and P2) to the function of addressing the program memory. While executing from external program memory, Ports 0 and 2 can be used simultaneously for read/write access to exter- nal data memory, but not for general-purpose I/O.

Though both external program memory and external data memory are accessed by some of the same pins, the two are completely independent of each other from a software point of view. For example, the chip can read/write external data memory while executing from external program memory.

Figure 38 shows a hardware configuration for accessing up to 64K bytes of external RAM. This interface is standard to any 8051-compatible MCU.

DD

internal 8K bytes Flash/EE code space. When EA is low (tied to ground) user program execution will start at address 0 of the external code space. In either case, addresses above 1FFF hex (8K) are mapped to the external space.

ADuC812

P0

ALE

P2

RD

WR

LATCH

SRAM

D0–D7 (DATA)

A0–A7

A8–A15

OE

WE

Figure 38. External Data Memory Interface (64 K Address Space)

–40– REV. B