DSPIC33EP64GS708 datasheet(142/484 Pages) MICROCHIP

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dsPIC33EPXXXGS70X/80X FAMILY

DS70005258C-page 142

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11.7

I/O Helpful Tips

In some cases, certain pins, as defined in

Table 30-11 under “Injection Current”, have internal

protection diodes to VDD and VSS. The term,

“Injection Current”, is also referred to as “Clamp

Current”. On designated pins, with sufficient external

current-limiting precautions by the user, I/O pin

input voltages are allowed to be greater or less

than the data sheet absolute maximum ratings,

with respect to the VSS and VDD supplies. Note

that when the user application forward biases

either of the high or low-side internal input clamp

diodes, that the resulting current being injected

into the device, that is clamped internally by the

VDD and VSS power rails, may affect the ADC

accuracy by four to six counts.

I/O pins that are shared with any analog input pin

(i.e., ANx) are always analog pins, by default, after

any Reset. Consequently, configuring a pin as an

analog input pin automatically disables the digital

input pin buffer and any attempt to read the digital

input level by reading PORTx or LATx will always

return a ‘0’, regardless of the digital logic level on

the pin. To use a pin as a digital I/O pin on a shared

ANx pin, the user application needs to configure the

Analog Pin Configuration registers (i.e., ANSELx) in

the I/O ports module by setting the appropriate bit

that corresponds to that I/O port pin to a ‘0’.

Most I/O pins have multiple functions. Referring to

the device pin diagrams in this data sheet, the prior-

ities of the functions allocated to any pins are

indicated by reading the pin name from left-to-right.

The left most function name takes precedence over

any function to its right in the naming convention.

For example: AN16/T2CK/T7CK/RC1; this indi-

cates that AN16 is the highest priority in this

example and will supersede all other functions to its

right in the list. Those other functions to its right,

even if enabled, would not work as long as any

other function to its left was enabled. This rule

applies to all of the functions listed for a given pin.

Each pin has an internal weak pull-up resistor and

pull-down resistor that can be configured using the

CNPUx and CNPDx registers, respectively. These

resistors eliminate the need for external resistors

in certain applications. The internal pull-up is up to

~(VDD – 0.8), not VDD. This value is still above the

minimum VIH of CMOS and TTL devices.

When driving LEDs directly, the I/O pin can source

or sink more current than what is specified in the

VOH/IOH and VOL/IOL DC characteristics specifica-

tion. The respective IOH and IOL current rating only

applies to maintaining the corresponding output at

or above the VOH, and at or below the VOL levels.

However, for LEDs, unlike digital inputs of an exter-

nally connected device, they are not governed by

the same minimum VIH/VIL levels. An I/O pin output

can safely sink or source any current less than that

listed in the Absolute Maximum Ratings in

Section 30.0 “Electrical Characteristics”of this

data sheet. For example:

VOH = 2.4V @ IOH = -8 mA and VDD = 3.3V

The maximum output current sourced by any 8 mA

I/O pin = 12 mA.

LED source current < 12 mA is technically permitted.

Refer to the VOH/IOH graphs in Section 31.0 “DC

and AC Device Characteristics Graphs” for

additional information.

Note:

Although it is not possible to use a digital

input pin when its analog function is

enabled, it is possible to use the digital I/O

output function, TRISx = 0x0, while the

analog function is also enabled. However,

this is not recommended, particularly if the

analog input is connected to an external

analog voltage source, which would

create signal contention between the

analog signal and the output pin driver.

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