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dsPIC33EPXXXGP50X, dsPIC33EPXXXMC20X/50X, AND PIC24EPXXXGP/MC20X
DS70657E-page 178
Preliminary
2011-2012 Microchip Technology Inc.
11.5
I/O Helpful Tips
1.
In some cases, certain pins as defined in
Table 30-10 under “Injection Current”, have internal protec-
tion 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 volt-
ages 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.
2.
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 in the I/O
Ports module, (i.e., ANSELx), by setting the appro-
priate bit that corresponds to that I/O port pin to a
‘0’.
3.
Most I/O pins have multiple functions. Referring to
the device pin diagrams in the data sheet, the pri-
orities of the functions allocated to any pins are
indicated by reading the pin name from left-to-
right. The left most function name takes prece-
dence over any function to its right in the naming
convention. For example: AN16/T2CK/T7CK/RC1.
This indicates 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.
4.
Each pin has an internal weak pull-up resistor
and pull-down resistor that can be configured
using the CNPUx and CNPDx registers, respec-
tively. 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.
5.
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 characteristic 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 out-
put can safely sink or source any current less than
that listed in the absolute maximum rating section
of the 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
additional information.
6.
The Peripheral Pin Select (PPS) pin mapping rules
are as follows:
a) Only one “output” function can be active on a
given pin at any time regardless if it is a dedi-
cated or remappable function (one pin, one
output).
b) It is possible to assign a “remappable output”
function to multiple pins and externally short
or tie them together for increased current
drive.
c)
If any “dedicated output” function is enabled
on a pin it will take precedence over any
remappable “output” function.
d) If any “dedicated digital”, (input or output),
function is enabled on a pin, any number of
“input” remappable functions can be mapped
to the same pin.
e) If any “dedicated analog” function(s) are
enabled on a given pin, “digital input(s)” of any
kind will all be disabled, although a single “dig-
ital output” at the user cautionary discretion
can be enabled and active as long as there is
no signal contention with an external analog
input signal. For example it is possible for the
ADC to convert the digital output logic level or
to toggle a digital output on a comparator or
ADC input provided there is no external
analog input like for a built-in self test.
f)
Any number of “input” remappable functions
can be mapped to the same pin(s) at the
same time, including to any pin with single
output from either a dedicated or remappable
“output”.
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 cre-
ate signal contention between the analog
signal and the output pin driver.