This mode is set by RESP_CTRL = 0. Figure 42 shows a block diagram of the internal respiration circuitry. The internal modulation and demodulator circuitry can be selectively used. The modulation block is controlled by the RESP_MOD_EN bit and the demodulation block is controlled by the RESP_DEMOD_EN bit. The modulation signal is a square wave of the magnitude VREFP – AVSS. When the internal modulation circuitry is used, the output of the modulation circuitry is available at the RESP_MODP and RESP_MODM pins of the device. This availability allows custom filtering to be added to the square wave modulation signal. In this mode, GPIO1 and GPIO2 can be used for other purposes. The modulation frequency of the respiration circuit is set by the RESP_FREQ bits.