Datasheet

18.3.7. 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:
Table 18-21 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)
• 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.
The tables below relate the alternate functions of Port G to the overriding signals shown in the figure in
section Alternate Port Functions on page 96.
Table 18-22 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
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