EM78P809N

8-Bit Microcontroller

4.13 I/O Ports

The I/O registers, Port 6, Port 7, Port 8, and Port 9 are bi-directional tri-state I/O ports. Each I/O pin can be defined as “input” or “output” pin by the I/O control register (IOC6 ~ IOC9). The I/O registers and I/O control registers are both readable and writable. The I/O interface circuits for Port 6, Port 7, Port 8, and Port 9 are shown in Fig. 27.

PCRD

PORT

0 M

U

1 X

Q

P

D

 

 

R

 

 

CLK

PCW R

Q

C

 

 

L

 

 

 

 

 

Q

P

D

 

R

 

 

 

 

 

 

CCLK PDW R

Q L

PDRD

IOD

Fig. 28.The I/O Port and I/O Control Register Circuit

4.14RESET and Wake-up

4.14.1RESET

A RESET is initiated by one of the following events:

(1)Power-on reset

(2)/RESET pin input “low”

(3)WDT timeout. (if enabled)

The device is kept in a RESET condition for a period of approx. 18ms1 (one oscillator start-up timer period) after the reset is detected. Once a RESET occurs, the following functions are performed.

„ The oscillator starts or is running

„ The Program Counter (R2) is reset to all “0”.

„ When power is switched on, the upper 2 bits of R3, the upper 2 bits of R4 and the bits 6 ~ 4 of R5 are cleared.

„ All I/O port pins are configured as input mode (high-impedance state).

1 NOTE: VDD = 5V, set up time period = 16.2ms ± 30% VDD = 3V, set up time period = 19.6ms ± 30%

Product Specification (V1.0) 07.26.2005

47

(This specification is subject to change without further notice)

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IBM EM78P809N manual 13 I/O Ports, Reset and Wake-up