Handle I2C multiplexer channel selection

src/main.c

@@ -3,26 +3,67 @@
 #include "bus/twi.h"
 
 #include <avr/interrupt.h>
-#include <util/delay.h>
-#define Sleep(ms) _delay_ms(ms)
 
-int main(int argc, char **argv)
+int main(void)
 {
-	USART_Init();
-	TWI_Init();
-
-	sei();
-
-	Info("Initializing...");
-	Sleep(500);
+	char crb;
 
 	// ADS1115 (ADC)
 	// PCA9546 (Multiplexer TWI)
 	// ATH20 (3x)
 	// BMP280 (3x)
 
-	UNUSED(argc);
-	UNUSED(argv);
+	USART_Init();
+	TWI_Init();
+
+	sei();
+
+	Info("Initializing...");
+
+	// PCA9546 I2C Multiplexer
+	// =======================
+
+	TWI_Stop();
+
+	// Excerpts taken from the PCA9546A datasheet:
+	// https://www.ti.com/lit/ds/symlink/pca9546a.pdf
+
+	// Following a start condition, the bus master
+	// must output the address of the slave it is
+	// accessing.
+
+	//  7  6  5  4   |   3   2   1   0
+	//  1  1  1  0   |  A2  A1  A0  RW
+	//  FIXED_____   |  SELECTABLE____
+
+	TWI_Start(0x70, 0);
+	TWI_Wait_ACK();
+
+	// Following the successful acknowledgment of
+	// the slave address, the bus master sends a byte
+	// which is stored in the control register.
+
+	//  7  6  5  4   |   3   2   1   0
+	//  X  X  X  X   |  B3  B2  B1  B0
+	//               |  CHANNEL_______
+
+	crb = 0x01; //  Channel 0  |  00000001
+	//  = 0x02  //  Channel 1  |  00000010
+	//  = 0x04  //  Channel 2  |  00000100
+	//  = 0x08  //  Channel 3  |  00001000
+
+	TWI_Write(crb);
+
+	// When a channel is selected, the channel
+	// becomes active after a stop condition has been
+	// placed on the I2C bus. A stop condition always
+	// must occur right after the acknowledge cycle.
+
+	TWI_Wait_ACK();
+	TWI_Stop();
+
+	Info("Switched to TWI channel 0.");
+	Sleep(500);
 
 	return 0;
 }