full 23 i/o pins on atmega328

This is a question of curiousity I know there are a total 23 I/o on the atmega328 20 are normally accessible to an arduino board, but on a breadboard what would it take to access the others? I know 1 is the reset and is not a good idea due to the high voltage reset needed afterwards, the other two I guess are the xtal? If its running 8Mhz internal oscillator are those pins free to be I/o? How would I access those without direct port manipulation? Does the ide support it?

Reset = PC6 The xtal pins are PB6 and 7

You can use them just fine subject to the things you mentioned, ie HV programming and internal oscillator.

Does the ide support it?

No, AFAIK anyway.

How would I access those without direct port manipulation?

You don't, just use direct port manipulation.


Rob

It wouldn’t be difficult to make those extra pins accessible to the normal Arduino libraries. Some minor tweaking of pins_arduino.h is all that’s needed. For example (defining PB6 & PB7 as pins 14 & 15):

/*
  pins_arduino.h - Pin definition functions for Arduino
  Part of Arduino - http://www.arduino.cc/

  Copyright (c) 2007 David A. Mellis

  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General
  Public License along with this library; if not, write to the
  Free Software Foundation, Inc., 59 Temple Place, Suite 330,
  Boston, MA  02111-1307  USA

  $Id: wiring.h 249 2007-02-03 16:52:51Z mellis $
*/

#ifndef Pins_Arduino_h
#define Pins_Arduino_h

#include <avr/pgmspace.h>

#define NUM_DIGITAL_PINS            20
#define NUM_ANALOG_INPUTS           6
#define analogInputToDigitalPin(p)  ((p < 6) ? (p) + 14 : -1)

#if defined(__AVR_ATmega8__)
#define digitalPinHasPWM(p)         ((p) == 9 || (p) == 10 || (p) == 11)
#else
#define digitalPinHasPWM(p)         ((p) == 3 || (p) == 5 || (p) == 6 || (p) == 9 || (p) == 10 || (p) == 11)
#endif

const static uint8_t SS   = 10;
const static uint8_t MOSI = 11;
const static uint8_t MISO = 12;
const static uint8_t SCK  = 13;

const static uint8_t SDA = 18;
const static uint8_t SCL = 19;
const static uint8_t LED_BUILTIN = 13;

const static uint8_t A0 = 16;
const static uint8_t A1 = 17;
const static uint8_t A2 = 18;
const static uint8_t A3 = 19;
const static uint8_t A4 = 20;
const static uint8_t A5 = 21;
const static uint8_t A6 = 22;
const static uint8_t A7 = 23;

#define digitalPinToPCICR(p)    (((p) >= 0 && (p) <= 21) ? (&PCICR) : ((uint8_t *)0))
#define digitalPinToPCICRbit(p) (((p) <= 7) ? 2 : (((p) <= 13) ? 0 : 1))
#define digitalPinToPCMSK(p)    (((p) <= 7) ? (&PCMSK2) : (((p) <= 13) ? (&PCMSK0) : (((p) <= 21) ? (&PCMSK1) : ((uint8_t *)0))))
#define digitalPinToPCMSKbit(p) (((p) <= 7) ? (p) : (((p) <= 13) ? ((p) - 8) : ((p) - 14)))

#ifdef ARDUINO_MAIN

// On the Arduino board, digital pins are also used
// for the analog output (software PWM).  Analog input
// pins are a separate set.

// ATMEL ATMEGA8 & 168 / ARDUINO
//
//                  +-\/-+
//            PC6  1|    |28  PC5 (AI 5)
//      (D 0) PD0  2|    |27  PC4 (AI 4)
//      (D 1) PD1  3|    |26  PC3 (AI 3)
//      (D 2) PD2  4|    |25  PC2 (AI 2)
// PWM+ (D 3) PD3  5|    |24  PC1 (AI 1)
//      (D 4) PD4  6|    |23  PC0 (AI 0)
//            VCC  7|    |22  GND
//            GND  8|    |21  AREF
//            PB6  9|    |20  AVCC
//            PB7 10|    |19  PB5 (D 13)
// PWM+ (D 5) PD5 11|    |18  PB4 (D 12)
// PWM+ (D 6) PD6 12|    |17  PB3 (D 11) PWM
//      (D 7) PD7 13|    |16  PB2 (D 10) PWM
//      (D 8) PB0 14|    |15  PB1 (D 9) PWM
//                  +----+
//
// (PWM+ indicates the additional PWM pins on the ATmega168.)

// ATMEL ATMEGA1280 / ARDUINO
//
// 0-7 PE0-PE7   works
// 8-13 PB0-PB5  works
// 14-21 PA0-PA7 works 
// 22-29 PH0-PH7 works
// 30-35 PG5-PG0 works
// 36-43 PC7-PC0 works
// 44-51 PJ7-PJ0 works
// 52-59 PL7-PL0 works
// 60-67 PD7-PD0 works
// A0-A7 PF0-PF7
// A8-A15 PK0-PK7


// these arrays map port names (e.g. port B) to the
// appropriate addresses for various functions (e.g. reading
// and writing)
const uint16_t PROGMEM port_to_mode_PGM[] = {
	NOT_A_PORT,
	NOT_A_PORT,
	(uint16_t) &DDRB,
	(uint16_t) &DDRC,
	(uint16_t) &DDRD,
};

const uint16_t PROGMEM port_to_output_PGM[] = {
	NOT_A_PORT,
	NOT_A_PORT,
	(uint16_t) &PORTB,
	(uint16_t) &PORTC,
	(uint16_t) &PORTD,
};

const uint16_t PROGMEM port_to_input_PGM[] = {
	NOT_A_PORT,
	NOT_A_PORT,
	(uint16_t) &PINB,
	(uint16_t) &PINC,
	(uint16_t) &PIND,
};

const uint8_t PROGMEM digital_pin_to_port_PGM[] = {
	PD, /* 0 */
	PD,
	PD,
	PD,
	PD,
	PD,
	PD,
	PD,
	PB, /* 8 */
	PB,
	PB,
	PB,
	PB,
	PB,
	PB,
	PB,
	PC, /* 16 */
	PC,
	PC,
	PC,
	PC,
	PC,
};

const uint8_t PROGMEM digital_pin_to_bit_mask_PGM[] = {
	_BV(0), /* 0, port D */
	_BV(1),
	_BV(2),
	_BV(3),
	_BV(4),
	_BV(5),
	_BV(6),
	_BV(7),
	_BV(0), /* 8, port B */
	_BV(1),
	_BV(2),
	_BV(3),
	_BV(4),
	_BV(5),
	_BV(6),
	_BV(7),
	_BV(0), /* 16, port C */
	_BV(1),
	_BV(2),
	_BV(3),
	_BV(4),
	_BV(5),
};

const uint8_t PROGMEM digital_pin_to_timer_PGM[] = {
	NOT_ON_TIMER, /* 0 - port D */
	NOT_ON_TIMER,
	NOT_ON_TIMER,
	// on the ATmega168, digital pin 3 has hardware pwm
#if defined(__AVR_ATmega8__)
	NOT_ON_TIMER,
#else
	TIMER2B,
#endif
	NOT_ON_TIMER,
	// on the ATmega168, digital pins 5 and 6 have hardware pwm
#if defined(__AVR_ATmega8__)
	NOT_ON_TIMER,
	NOT_ON_TIMER,
#else
	TIMER0B,
	TIMER0A,
#endif
	NOT_ON_TIMER,
	NOT_ON_TIMER, /* 8 - port B */
	TIMER1A,
	TIMER1B,
#if defined(__AVR_ATmega8__)
	TIMER2,
#else
	TIMER2A,
#endif
	NOT_ON_TIMER,
	NOT_ON_TIMER,
	NOT_ON_TIMER,
	NOT_ON_TIMER,
	NOT_ON_TIMER,
	NOT_ON_TIMER, /* 16 - port C */
	NOT_ON_TIMER,
	NOT_ON_TIMER,
	NOT_ON_TIMER,
	NOT_ON_TIMER,
};

#endif

#endif

The symbols for the analog pins, A0-A5 actually have values of 14-19, but maybe PB6 and PB7 could be 20 and 21.

A6 and A7 are usable. The IDE recognizes them even though they aren't broken out to pins on the Arduino board

I think those are for the smd version which has 2 extra analog pins

Details... ;)

Jack, it is possible to reassign the numbers associated with the anlogue pins, also in pins_arduino.h. In the listing above I moved A0-A7 to numbers 16-23 to make room for the new pins.

Just remember that A6, A7 are analog input only.