Mikroelektronika MIKROE-724 データシート

174

mikoBasic PRO for dsPIC30/33 and PIC24

MikroElektronika

Read Modify Write Problem

The Microchip microcontrollers use a sequence known as

Read-Modify-Write (RMW) when changing an output state

(1 or 0) on a pin. This can cause unexpected behavior under certain circumstances.

When your program changes the state on a specific pin, for example RB0 in PORTB, the microcontroller first

READs

all 8 bits of the PORTB register which represents the states of all 8 pins in PORTB (RB7-RB0).

The microcontroller then stores this data in the MCU. The bit associated with RB that you’ve commanded to

MODIFY

is changed, and then the microcontroller

WRITEs all 8 bits (RB7-RB0) back to the PORTB register.

During the first reading of the PORT register, you will be reading the actual state of the physical pin.

The problem arises when an output pin is loaded in such a way that its logic state is affected by the load. Instances of

such loads are LEDs without current-limiting resistors or loads with high capacitance or inductance.

For example, if a capacitor is attached between pin and ground, it will take a short while to charge when the pin is set

to 1.

On the other hand, if the capacitor is discharged, it acts like a short circuit, forcing the pin to ‘0’ state, and, therefore, a

read of the PORT register will return 0, even though we wrote a 1 to it.

Lets analyze the following example:

PORTB.B0 = 1

PORTB.B1 = 1

Assume that the PORTB is initially set to zero, and that all pins are set to output. Let’s say we connect a discharged

capacitor to RB0 pin.

The first line,

PORTB.B0 = 1

will be decoded like in this way:

READ

PORTB is

read:

STORE

Data is stored inside a temporary internal