ATmega128L datasheet(85/386 Pages) ATMEL

85 / 386 page

2467S–AVR–07/09

ATmega128

Alternate Functions of

Port G

In ATmega103 compatibility mode, only the alternate functions are the defaults for Port G, and

Port G cannot be used as General Digital Port Pins. The alternate pin configuration is as follows:

• TOSC1 – Port G, Bit 4

TOSC1, Timer Oscillator pin 1: When the AS0 bit in ASSR is set (one) to enable asynchronous

clocking of Timer/Counter0, pin PG4 is disconnected from the port, and becomes the input of the

inverting Oscillator amplifier. In this mode, a Crystal Oscillator is connected to this pin, and the

pin can not be used as an I/O pin.

• TOSC2 – Port G, Bit 3

TOSC2, Timer Oscillator pin 2: When the AS0 bit in ASSR is set (one) to enable asynchronous

clocking of Timer/Counter0, pin PG3 is disconnected from the port, and becomes the inverting

output of the Oscillator amplifier. In this mode, a Crystal Oscillator is connected to this pin, and

the pin can not be used as an I/O pin.

• ALE – Port G, Bit 2

ALE is the external data memory Address Latch Enable signal.

•RD – Port G, Bit 1

RD is the external data memory read control strobe.

•WR – Port G, Bit 0

WR is the external data memory write control strobe.

Table 46 and Table 47 relates the alternate functions of Port G to the overriding signals shown in

Figure 33 on page 71.

Table 45. Port G Pins Alternate Functions

Port Pin

Alternate Function

PG4

TOSC1 (RTC Oscillator Timer/Counter0)

PG3

TOSC2 (RTC Oscillator Timer/Counter0)

PG2

ALE (Address Latch Enable to external memory)

PG1

RD (Read strobe to external memory)

PG0

WR (Write strobe to external memory)

Table 46. Overriding Signals for Alternate Functions in PG4..PG1

Signal Name

PG4/TOSC1

PG3/TOSC2

PG2/ALE

PG1/RD

PUOE

AS0

SRE

PUOV

DDOE

AS0

SRE

DDOV

PVOE

SRE

PVOV

ALE

DIEOE

AS0

DIEOV

–

AIO

T/C0 OSC INPUT

T/C0 OSC OUTPUT

–

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