User manual

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 specic pin, for example RB0 in PORTB, the microcontroller rst 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 rst 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 rst line, PORTB.B0 = 1 will be decoded like in this way:

READ PORTB is read:

STORE Data is stored inside a temporary internal