ST10F280_12 datasheet(99/239 Pages) STMICROELECTRONICS

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ST10F280

Parallel ports

Doc ID 8673 Rev. 3

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12.1.4

Alternate port functions

Each port line has one associated programmable alternate input or output function. PORT0

and PORT1 may be used as the address and data lines when accessing external memory.

Port 4 outputs the additional segment address bits A23/A19/A18/A16 in systems where

more than 64 KBytes of memory are to be accessed directly.

Port 6 provides the optional chip select outputs and the bus arbitration lines.

Port 2, Port 7 and Port 8 are associated with the capture inputs or compare outputs of the

CAPCOM units and/or with the outputs of the PWM module.

Port 2 is also used for fast external interrupt inputs and for timer 7 input.

Port 3 includes alternate input/output functions of timers, serial interfaces, the optional bus

control signal BHE/WRH and the system clock output (CLKOUT).

Port 5 is used for the analog input channels to the A/D converter or timer control signals.

If an alternate output function of a pin is to be used, the direction of this pin must be

programmed for output (DPx.y=‘1’), except for some signals that are used directly after reset

and are configured automatically. Otherwise the pin remains in the high-impedance state

and is not effected by the alternate output function. The respective port latch should hold a

‘1’, because its output is ANDed with the alternate output data (except for PWM output

signals).

If an alternate input function of a pin is used, the direction of the pin must be programmed

for input (DPx.y=‘0’) if an external device is driving the pin. The input direction is the default

after reset. If no external device is connected to the pin, however, one can also set the

direction for this pin to output. In this case, the pin reflects the state of the port output latch.

Thus, the alternate input function reads the value stored in the port output latch. This can be

used for testing purposes to allow a software trigger of an alternate input function by writing

to the port output latch.

On most of the port lines, the user software is responsible for setting the proper direction

when using an alternate input or output function of a pin. This is done by setting or clearing

the direction control bit DPx.y of the pin before enabling the alternate function. There are

port lines, however, where the direction of the port line is switched automatically. For

instance, in the multiplexed external bus modes of PORT0, the direction must be switched

several times for an instruction fetch in order to output the addresses and to input the data.

Obviously, this cannot be done through instructions. In these cases, the direction of the port

line is switched automatically by hardware if the alternate function of such a pin is enabled.

PN1EC ALE Edge Characteristic (rise/fall time)

00: Fast edge mode, rise/fall times depend on the driver’s dimensioning.

01: Slow edge mode, rise/fall times ~60 ns

10: Reserved

11: Reserved

PN1DC ALE Driver Characteristic (output current)

00: High Current mode: Driver always operates with maximum strength.

01: Dynamic Current mode: Driver strength is reduced after the target level has been

reached.

10: Low Current mode: Driver always operates with reduced strength.

11: Reserved

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