EXT_ATTR_RO
static EXT_ATTR_RO(signal, duty, 0);
static EXT_ATTR_RO(signal, duty, 1);
static EXT_ATTR_RO(signal, duty, 2);
static EXT_ATTR_RO(signal, duty, 3);
static EXT_ATTR_RO(signal, period, 0);
static EXT_ATTR_RO(signal, period, 1);
static EXT_ATTR_RO(signal, period, 2);
static EXT_ATTR_RO(signal, period, 3);
static EXT_ATTR_RO(signal, phase, 0);
static EXT_ATTR_RO(signal, phase, 1);
static EXT_ATTR_RO(signal, phase, 2);
static EXT_ATTR_RO(signal, phase, 3);
static EXT_ATTR_RO(signal, polarity, 0);
static EXT_ATTR_RO(signal, polarity, 1);
static EXT_ATTR_RO(signal, polarity, 2);
static EXT_ATTR_RO(signal, polarity, 3);
static EXT_ATTR_RO(signal, running, 0);
static EXT_ATTR_RO(signal, running, 1);
static EXT_ATTR_RO(signal, running, 2);
static EXT_ATTR_RO(signal, running, 3);
static EXT_ATTR_RO(signal, start, 0);
static EXT_ATTR_RO(signal, start, 1);
static EXT_ATTR_RO(signal, start, 2);
static EXT_ATTR_RO(signal, start, 3);
static EXT_ATTR_RO(freq, frequency, 0);
static EXT_ATTR_RO(freq, frequency, 1);
static EXT_ATTR_RO(freq, frequency, 2);
static EXT_ATTR_RO(freq, frequency, 3);