got the output check stuff actually compiling and closer to working
diff --git a/bbb_cape/src/cape/data_struct.h b/bbb_cape/src/cape/data_struct.h
index d60a0e9..6d747bb 100644
--- a/bbb_cape/src/cape/data_struct.h
+++ b/bbb_cape/src/cape/data_struct.h
@@ -82,9 +82,12 @@
       int32_t left_drive;
       int32_t right_drive;
 
-      // The length of the pulse from the ultrasonic sensor in 100kHZ ticks.
+      // The length of the pulse from the ultrasonic sensor in 100KHz ticks.
       uint32_t ultrasonic_pulse_length;
 
+      // The length of the pulse from the sidecar PWM output in 10MHz ticks.
+      uint32_t output_check_pulse_length;
+
       int32_t shooter_position, pusher_distal_posedge_position,
           pusher_proximal_posedge_position;
 
diff --git a/bbb_cape/src/cape/digital.h b/bbb_cape/src/cape/digital.h
index 2f227a5..6071ca8 100644
--- a/bbb_cape/src/cape/digital.h
+++ b/bbb_cape/src/cape/digital.h
@@ -8,7 +8,7 @@
 // For all of the digital functions, a high voltage level on the input reads as
 // 1 (and a low to high transition is a positive edge).
 // Encoder inputs 0-7 A and B are mapped to "digital inputs" 12-27 (12 is 0A,
-13 is B, 14 is 1A, etc).
+// 13 is B, 14 is 1A, etc).
 
 static inline int digital_read(int num) {
   switch (num) {
diff --git a/bbb_cape/src/cape/peripherial_usage.notes b/bbb_cape/src/cape/peripherial_usage.notes
index 757f14a..c8005c3 100644
--- a/bbb_cape/src/cape/peripherial_usage.notes
+++ b/bbb_cape/src/cape/peripherial_usage.notes
@@ -57,4 +57,4 @@
 [robots]
 robot_comp
   TIM11
-  TIM11_IRQ:3 (aka TIM1_TRG_COM)
+  TIM11_IRQ:1 (aka TIM1_TRG_COM)
diff --git a/bbb_cape/src/cape/robot_comp.c b/bbb_cape/src/cape/robot_comp.c
index cab3e28..03e529a 100644
--- a/bbb_cape/src/cape/robot_comp.c
+++ b/bbb_cape/src/cape/robot_comp.c
@@ -6,9 +6,7 @@
 #include "cape/analog.h"
 #include "cape/digital.h"
 #include "cape/util.h"
-
-// TIM11.1 on PB9, aka digital input 6.
-static volatile uint32_t ultrasonic_pulse_length = 0;
+#include "cape/timer.h"
 
 typedef struct {
   uint32_t posedges, negedges;
@@ -91,27 +89,15 @@
 CLAW(10, 11, 9, bottom_claw, 7);
 SHOOTER(7, 5, 4, 8, 0)
 
-void TIM1_TRG_COM_TIM11_IRQHandler(void) {
-  TIM11->SR = ~TIM_SR_CC1IF;
-  if (digital_read(6)) {
-    TIM11->EGR = TIM_EGR_UG;
-  } else {
-    ultrasonic_pulse_length = TIM11->CCR1;
-  }
-}
+// TIM11.1 on PB9, aka digital input 6.
+timer_declare(TIM11, TIM1_TR_GCOM_TIM11_IRQHandler, 1, 1, ultrasonic)
 
 void robot_init(void) {
   gpio_setup_alt(GPIOB, 9, 3);
   RCC->APB2ENR |= RCC_APB2ENR_TIM11EN;
-  TIM11->CR1 = TIM_CR1_URS;
-  TIM11->DIER = TIM_DIER_CC1IE;
   TIM11->CCMR1 = TIM_CCMR1_CC1S_0 /* input pin 1 -> timer input 1 */;
-  TIM11->CCER = TIM_CCER_CC1P | TIM_CCER_CC1NP | TIM_CCER_CC1E;
-  TIM11->EGR = TIM_EGR_UG;
-  TIM11->PSC = 1200 - 1;  // 100kHZ timer
-  TIM11->CR1 |= TIM_CR1_CEN;
-  NVIC_SetPriority(TIM1_TRG_COM_TIM11_IRQn, 3);
-  NVIC_EnableIRQ(TIM1_TRG_COM_TIM11_IRQn);
+  TIM11->PSC = 1200 - 1;  // 100KHz timer
+  timer_setup(TIM11, TIM1_TRG_COM_TIM11_IRQn, 1);
 }
 
 void robot_fill_packet(struct DataStruct *packet) {
@@ -125,7 +111,7 @@
   packet->main.battery_voltage_high = analog_get(5);
   packet->main.battery_voltage_low = analog_get(3);
 
-  packet->main.ultrasonic_pulse_length = ultrasonic_pulse_length;
+  packet->main.ultrasonic_pulse_length = ultrasonic_length;
 
   fill_top_claw_values(packet);
   fill_bottom_claw_values(packet);
diff --git a/bbb_cape/src/cape/timer.h b/bbb_cape/src/cape/timer.h
new file mode 100644
index 0000000..a356de1
--- /dev/null
+++ b/bbb_cape/src/cape/timer.h
@@ -0,0 +1,34 @@
+#ifndef CAPE_TIMER_H_
+#define CAPE_TIMER_H_
+
+#include <STM32F2XX.h>
+
+// inverted is 1 for timing low periods, 0 for high ones.
+#define timer_declare(timer, irq, input, inverted, name)                   \
+  static volatile uint32_t name##_length;                                  \
+  void irq(void) {                                                         \
+    timer->SR = ~TIM_SR_CC##input##IF;                                     \
+    const uint32_t ccer = timer->CCER;                                     \
+    timer->CCER = ccer ^ (TIM_CCER_CC##input##P | TIM_CCER_CC##input##NP); \
+    const uint32_t rising = ccer & TIM_CCER_CC##input##P;                  \
+    if (inverted ? rising : !rising) {                                     \
+      name##_length = timer->CCR##input;                                   \
+    } else {                                                               \
+      timer->EGR = TIM_EGR_UG;                                             \
+    }                                                                      \
+  }
+
+// You need to enable the clock, set up the alt function for the input pin,
+// set the prescaler, and set up the timer input before calling this.
+#define timer_setup(timer, irq, input)                           \
+  do {                                                           \
+    timer->CR1 = TIM_CR1_URS;                                    \
+    timer->DIER = TIM_DIER_CC##input##IE;                        \
+    timer->CCER = TIM_CCER_CC##input##P | TIM_CCER_CC##input##E; \
+    timer->EGR = TIM_EGR_UG;                                     \
+    timer->CR1 |= TIM_CR1_CEN;                                   \
+    NVIC_SetPriority(irq, 1);                                    \
+    NVIC_EnableIRQ(irq);                                         \
+  } while (0)
+
+#endif  // CAPE_TIMER_H_