Hide a field in node edit form and provide its value programatically.

fieldautovalue.module

<?php

/**
 * Implements hook_form_FORM_ID_alter() for node type 'test'.
 *
 * Hide field_texto.
 */
function fieldautovalue_form_test_node_form_alter(&$form) {
  $form['field_texto']['#access'] = FALSE;
}

/**
 * Implements hook_node_Validate().
 *
 * Set value for field_texto for node type 'test'.
 */
function fieldautovalue_node_validate($node, $form, &$form_state) {
  if ($node->type == 'test') {
    $value = array(LANGUAGE_NONE => array(array('value' => 'lorem ipsum')));
    form_set_value($form['field_texto'], $value, $form_state);
  }
}

Thk Jonhattan.
A good trick.

Keep the default value if you only hide it?

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Well, i tested and work fine !