diff --git a/src/board/system76/common/include/board/parallel.h b/src/board/system76/common/include/board/parallel.h
index 530def7..3b65d1b 100644
--- a/src/board/system76/common/include/board/parallel.h
+++ b/src/board/system76/common/include/board/parallel.h
@@ -8,6 +8,6 @@
 
 extern bool parallel_debug;
 bool parallel_init(void);
-int16_t parallel_write(uint8_t * data, int16_t length);
+int16_t parallel_write(uint8_t * data, uint16_t length);
 
 #endif // _BOARD_PARALLEL_H
diff --git a/src/board/system76/common/parallel.c b/src/board/system76/common/parallel.c
index fd460a3..435bb36 100644
--- a/src/board/system76/common/parallel.c
+++ b/src/board/system76/common/parallel.c
@@ -82,14 +82,14 @@ bool parallel_init(void) {
     return parallel_wait_peripheral(STS_WAIT, 0);
 }
 
-int16_t parallel_write(uint8_t * data, int16_t length) {
+int16_t parallel_write(uint8_t * data, uint16_t length) {
     // Assert nWRITE
     KSIGDAT &= ~CTL_WRITE;
 
     // Set data lines as outputs
     KSOLGOEN = 0xFF;
 
-    int16_t i;
+    uint16_t i;
     for (i = 0; i < length; i++) {
         // Wait for peripheral to indicate it's ready for next cycle
         if (!parallel_wait_peripheral(STS_WAIT, 0)) {