ST90135 datasheet(91/190 Pages) STMICROELECTRONICS

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ST90158 - I/O PORTS

PORT CONTROL REGISTERS (Cont’d)

During Reset, ports with weak pull-ups are set in

bidirectional/weak pull-up mode and the output

Data Register is set to FFh. This condition is also

held after Reset, except for Ports 0 and 1 in ROM-

less devices, and can be redefined under software

control.

Bidirectional ports without weak pull-ups are set in

high impedance during reset. To ensure proper

levels during reset, these ports must be externally

connected to either VDD or VSS through external

pull-up or pull-down resistors.

Other reset conditions may apply in specific ST9

devices.

8.4 INPUT/OUTPUT BIT CONFIGURATION

By programming the control bits PxC0.n and

PxC1.n (see Figure 49) it is possible to configure

bit Px.n as Input, Output, Bidirectional or Alternate

Function Output, where X is the number of the I/O

port, and n the bit within the port (n = 0 to 7).

When programmed as input, it is possible to select

the input level as TTL or CMOS compatible by pro-

gramming the relevant PxC2.n control bit, except

where the Schmitt trigger option is assigned to the

pin.

The output buffer can be programmed as push-

pull or open-drain.

A weak pull-up configuration can be used to avoid

external pull-ups when programmed as bidirec-

tional (except where the weak pull-up option has

been permanently disabled in the pin hardware as-

signment).

Each pin of an I/O port may assume software pro-

grammable Alternate Functions (refer to the de-

vice Pin Description and to Section 8.5 ALTER-

NATE FUNCTION ARCHITECTURE). To output

signals from the ST9 peripherals, the port must be

configured as AF OUT. On ST9 devices with A/D

Converter(s), configure the ports used for analog

inputs as AF IN.

The basic structure of the bit Px.n of a general pur-

pose port Px is shown in Figure 50.

Independently of the chosen configuration, when

the user addresses the port as the destination reg-

ister of an instruction, the port is written to and the

data is transferred from the internal Data Bus to

the Output Master Latches. When the port is ad-

dressed as the source register of an instruction,

the port is read and the data (stored in the Input

Latch) is transferred to the internal Data Bus.

When Px.n is programmed as an Input:

(See Figure 51).

– The Output Buffer is forced tristate.

– The data present on the I/O pin is sampled into

the Input Latch at the beginning of each instruc-

tion execution.

– The data stored in the Output Master Latch is

copied into the Output Slave Latch at the end of

the execution of each instruction. Thus, if bit Px.n

is reconfigured as an Output or Bidirectional, the

data stored in the Output Slave Latch will be re-

flected on the I/O pin.

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