SMSC COM20020 manual Description of PIN Functions, A1,A2/ALE

DESCRIPTION OF PIN FUNCTIONS

DIP PIN	PLCC PIN	NAME
NO.	NO.		SYMBOL		DESCRIPTION
		MICROCONTROLLER INTERFACE
1-3	1-3	Address	A0/nMUX,	Input.	On a non-multiplexed bus, these
		0-2	A1,A2/ALE	signals are directly connected to the low bits
				of the host address bus. On a multiplexed
				address/data bus, A0/nMUX is tied low, A1
				is left open, and A2 is tied to the Address
				Latch Enable signal of the host. A1 is
				connected to an internal pull-up resistor.
4-11	4-6,8-12	Data 0-7	AD0-AD2,	Input/Output. On a non-multiplexed bus,
			D3-D7	these signals are used as the data lines for
				the device. On a multiplexed address/data
				bus, AD0-AD2 act as the address lines
				(latched by ALE) and as the low data lines
				for the device. D3-D7 are always used for
				data only. These signals are connected to
				internal pull-up resistors.
23	27	nRead/nData	nRD/nDS	Input.	On a 68XX-like bus, this active low
		nStrobe		signal is issued by the microcontroller as the
				data strobe signal to strobe the data onto
				the bus. On a 80XX-like bus, this active low
				signal is issued by the microcontroller to
				indicate a read operation. In this case, a
				logic "0" on this pin, when the COM20020 is
				accessed, enables data from the device to
				the data bus to be read by the
				microcontroller.
22	26	nWrite/	nWR/DIR	Input.	On a 68XX-like bus, this signal is
		Direction		issued by the microcontroller as the
				Read/nWrite signal to determine the
				direction of data transfer. In this case, a
				logic "1" selects a read operation, while a
				logic "0" selects a write operation. In this
				case, data is actually strobed by the nDS
				signal. On an 80XX-like bus, this active low
				signal is issued by the microcontroller to
				indicate a write operation. In this case, a
				logic "0" on this pin, when the COM20020 is
				accessed, enables data from the data bus to
				be written to the device.

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