85
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 AS0 SRE SRE
PUOV 0 0 0 0
DDOE AS0 AS0 SRE SRE
DDOV 0 0 1 1
PVOE 0 0 SRE SRE
PVOV 0 0 ALE RD
DIEOE AS0 AS0 0 0
DIEOV 0 0 0 0
DI – – – –
AIO T/C0 OSC INPUT T/C0 OSC OUTPUT – –
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