diff --git a/main/gay-ipod-fw.cpp b/main/gay-ipod-fw.cpp
index 025b68cd..f466bef1 100644
--- a/main/gay-ipod-fw.cpp
+++ b/main/gay-ipod-fw.cpp
@@ -1,18 +1,21 @@
 #include <stdio.h>
 
-#include "gpio-expander.h"
-#include "esp_log.h"
+#include "driver/adc.h"
 #include "driver/gpio.h"
 #include "driver/i2c.h"
 #include "driver/spi_common.h"
 #include "driver/spi_master.h"
+#include "esp_adc_cal.h"
+#include "esp_log.h"
 #include "gpio-expander.h"
+#include "gpio-expander.h"
+#include "hal/adc_types.h"
 #include "hal/gpio_types.h"
 #include "hal/spi_types.h"
 
 #define I2C_SDA_IO (GPIO_NUM_0)
 #define I2C_SCL_IO (GPIO_NUM_4)
-#define I2C_CLOCK_HZ (100000)
+#define I2C_CLOCK_HZ (400000)
 
 #define SPI_SDI_IO (GPIO_NUM_19)
 #define SPI_SDO_IO (GPIO_NUM_23)
@@ -87,7 +90,29 @@ extern "C" void app_main(void)
 
   ESP_LOGI(TAG, "Setting default GPIO state");
   gay_ipod::GpioExpander expander;
+
+  // for debugging usb ic
+  //expander.set_sd_mux(gay_ipod::GpioExpander::USB);
+
   ESP_ERROR_CHECK(expander.Write());
 
-  ESP_LOGI(TAG, "Idling.");
+  ESP_LOGI(TAG, "Init ADC");
+
+  // Attentuate our battery voltage pin to read between 150mV and 2450mV.
+  esp_adc_cal_characteristics_t adc1_chars;
+  esp_adc_cal_characterize(ADC_UNIT_1, ADC_ATTEN_DB_11, ADC_WIDTH_BIT_12, 0, &adc1_chars);
+  adc1_config_width(ADC_WIDTH_BIT_12);
+  adc1_config_channel_atten(ADC1_CHANNEL_6, ADC_ATTEN_DB_11);
+
+  while (1) {
+    int adc_raw = adc1_get_raw(ADC1_CHANNEL_6);
+    uint32_t adc_millivolts = esp_adc_cal_raw_to_voltage(adc_raw, &adc1_chars);
+
+    expander.Read();
+
+    ESP_LOGI(TAG, "raw adc: %d, millivolts: %d, wall power? %d", adc_raw, adc_millivolts, expander.charge_power_ok());
+
+    vTaskDelay(pdMS_TO_TICKS(1000));
+    ESP_ERROR_CHECK(expander.Write());
+  }
 }
diff --git a/main/gpio-expander.cpp b/main/gpio-expander.cpp
index 36132356..9e905734 100644
--- a/main/gpio-expander.cpp
+++ b/main/gpio-expander.cpp
@@ -38,8 +38,8 @@ esp_err_t GpioExpander::Read() {
   // it because that would indicate some really very badly wrong more generally.
   i2c_master_start(handle);
   i2c_master_write_byte(handle, (PCA8575_ADDRESS << 1 | I2C_MASTER_READ), true);
-  i2c_master_read_byte(handle, &port_a_, I2C_MASTER_ACK);
-  i2c_master_read_byte(handle, &port_b_, I2C_MASTER_LAST_NACK);
+  i2c_master_read_byte(handle, &input_a_, I2C_MASTER_ACK);
+  i2c_master_read_byte(handle, &input_b_, I2C_MASTER_LAST_NACK);
   i2c_master_stop(handle);
 
   esp_err_t ret = i2c_master_cmd_begin(I2C_NUM_0, handle, PCA8575_TIMEOUT);
@@ -62,4 +62,16 @@ uint8_t GpioExpander::key_states(void) {
   return input_b_ & 0b00111111;
 }
 
+void GpioExpander::set_sd_mux(SdMuxController controller) {
+  port_a_ &= (1 << 5);
+}
+
+void GpioExpander::set_sd_cs(bool high) {
+  port_a_ &= (1 << 6);
+}
+
+void GpioExpander::set_display_cs(bool high) {
+  port_a_ &= (1 << 7);
+}
+
 } // namespace gay_ipod
diff --git a/main/gpio-expander.h b/main/gpio-expander.h
index a4983705..ec34a0f1 100644
--- a/main/gpio-expander.h
+++ b/main/gpio-expander.h
@@ -27,16 +27,44 @@ class GpioExpander {
     bool headphone_detect(void);
     uint8_t key_states(void);
 
+    enum SdMuxController {
+      ESP,
+      USB
+    };
+
+    void set_sd_mux(SdMuxController controller);
+    void set_sd_cs(bool high);
+    void set_display_cs(bool high);
+    
     // Not copyable or movable.
     // TODO: maybe this could be movable?
     GpioExpander(const GpioExpander&) = delete;
     GpioExpander& operator=(const GpioExpander&) = delete;
 
   private:
-    // All power switches high, PWR_OK low (input), both CS pins high.
-    uint8_t port_a_ = uint8_t{0b00001000};
-    // DAC mute output low, everything else is input and so low.
-    uint8_t port_b_ = uint8_t{0b11111111};
+    // Port A:
+    // 0 - audio power enable
+    // 1 - usb interface power enable
+    // 2 - display power enable
+    // 3 - sd card power enable
+    // 4 - charge power ok (active low)
+    // 5 - sd mux switch
+    // 6 - sd chip select
+    // 7 - display chip select
+    // All power switches low, chip selects high, active-low charge power high
+    uint8_t port_a_ = 0b11010000;
+
+    // Port B:
+    // 0 - 3.5mm jack detect (active low)
+    // 1 - dac soft mute switch
+    // 2 - GPIO
+    // 3 - GPIO
+    // 4 - GPIO
+    // 5 - GPIO
+    // 6 - GPIO
+    // 7 - GPIO
+    // DAC mute output low, everything else is active-low inputs.
+    uint8_t port_b_ = 0b11111101;
 
     uint8_t input_a_ = 0;
     uint8_t input_b_ = 0;