+ DAC_TC_CMR = TC_TIMER_CLOCK1 | BV(TC_CMR_WAVE) | TC_CMR_ACPA_SET | TC_CMR_ACPC_CLEAR | BV(TC_CMR_CPCTRG);
+
+ /*
+ * Compute the sample frequency
+ * the RC counter will update every MCK/2 (see above)
+ * so to convert one sample at the user freq we generate
+ * the trigger every TC_CLK / (numer_of_sample * user_freq)
+ * where TC_CLK = MCK / 2.
+ */
+ uint32_t rc = DIV_ROUND((CPU_FREQ / 2), n_sample * freq);
+ DAC_TC_RC = rc;
+ /* generate the square wave with duty = 50% */
+ DAC_TC_RA = DIV_ROUND(50 * rc, 100);