From c114fb9c06b3547cd90990610cda98cc6e5be495 Mon Sep 17 00:00:00 2001
From: Linus Nordberg <linus@nordberg.se>
Date: Thu, 19 May 2016 15:55:05 +0200
Subject: Explain SPI dummy write in comments.

---
 tamper.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tamper.c b/tamper.c
index d4db2b9..2893a6c 100644
--- a/tamper.c
+++ b/tamper.c
@@ -111,14 +111,23 @@ spi_setup(int on_flag)
 static inline void
 spi_write(uint8_t val)
 {
+  /* Move the value to be sent to the SPI slave into the SPI
+     register. This starts the SPI clock. */
   SPDR = val;
+
+  /* Wait for the byte to be shifted into the slave. */
   loop_until_bit_is_set(SPSR, SPIF);
 }
 
 static inline uint8_t
 spi_read()
 {
+  /* Start clocking the SPI slave by moving a dummy byte (0) into the
+     SPI register and wait for the byte from the slave to be shifted
+     in. */
   spi_write(0);
+
+  /* Read the SPI register and return the value. */
   return SPDR;
 }
 
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