/**
 * Implements hook_field_formatter_settings_form().
 */
function {{ machine_name }}_field_formatter_settings_form($field, $instance, $view_mode, $form, &$form_state) {
  $display = $instance['display'][$view_mode];
  $settings = $display['settings'];

  $element = array();

  if ($display['type'] == 'text_trimmed' || $display['type'] == 'text_summary_or_trimmed') {
    $element['trim_length'] = array(
      '#title' => t('Length'),
      '#type' => 'textfield',
      '#size' => 20,
      '#default_value' => $settings['trim_length'],
      '#element_validate' => array('element_validate_integer_positive'),
      '#required' => TRUE,
    );
  }

  return $element;

}