PIC17C752-08SLASHCL datasheet(92/328 Pages) MICROCHIP

92 / 328 page

PIC17C7XX

DS30289A-page 92

10.10.2 SUCCESSIVE OPERATIONS ON I/O PORTS

The actual write to an I/O port happens at the end of an

instruction cycle, whereas for reading, the data must be

valid at the beginning of the instruction cycle

(Figure 10-20). Therefore, care must be exercised if a

write followed by a read operation is carried out on the

same I/O port. The sequence of instructions should be

such to allow the pin voltage to stabilize (load depen-

dent) before executing the instruction that reads the

values on that I/O port. Otherwise, the previous state

of that pin may be read into the CPU rather than the

“new” state. When in doubt, it is better to separate

these instructions with a NOP or another instruction not

accessing this I/O port.

Figure 10-21 shows the I/O model which causes this

situation. As the effective capacitance (C) becomes

larger, the rise/fall time of the I/O pin increases. As the

device frequency increases or the effective capaci-

tance increases, the possibility of this subsequent

PORTx read-modify-write instruction issue increases.

This effective capacitance includes the effects of the

board traces.

The best way to address this is to add an series resistor

at the I/O pin. This resistor allows the I/O pin to get to

the desired level before the next instruction.

The use of NOP instructions between the subsequent

PORTx read-modify-write instructions, is a lower cost

solution, but has the issue that the number of NOP

instructions is dependent on the effective capacitance

C and the frequency of the device.

FIGURE 10-20: SUCCESSIVE I/O OPERATION

FIGURE 10-21: I/O CONNECTION ISSUES

PC + 1

PC + 2

PC + 3

Q1 Q2 Q3 Q4

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

Q1 Q2 Q3 Q4

Instruction

fetched

RB7:RB0

MOVWF PORTB

write to

PORTB

NOP

Port pin

sampled here

NOP

MOVF PORTB,W

Instruction

executed

MOVWF PORTB

write to

PORTB

NOP

MOVF PORTB,W

Note:

This example shows a write to PORTB

followed by a read from PORTB.

Note that:

data setup time = (0.25TCY - TPD)

where TCY = instruction cycle

TPD = propagation delay

Therefore, at higher clock frequencies,

a write followed by a read may be prob-

lematic.

PIC17CXXX

I/O

C(1)

VIL

BSF PORTx, PINy

BSF PORTx, PINz

PORTx, PINy

Read PORTx, PINy as low

BSF PORTx, PINz clears the value

to be driven on the PORTx, PINy pin.

Note 1: This is not a capacitor to ground, but the effective

capacitive loading on the trace.

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