diff --git a/tests/periph/flashpage/main.c b/tests/periph/flashpage/main.c
index d152b305a4..bc633ff475 100644
--- a/tests/periph/flashpage/main.c
+++ b/tests/periph/flashpage/main.c
@@ -23,11 +23,13 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
+#include <ctype.h>
 
 #include "od.h"
 #include "shell.h"
 #include "periph/flashpage.h"
 #include "unaligned.h"
+#include "fmt.h"
 
 #define LINE_LEN            (16)
 
@@ -44,7 +46,7 @@
 /*
  * @brief   Allocate an aligned buffer for raw writings
  */
-static char raw_buf[RAW_BUF_SIZE] ALIGNMENT_ATTR;
+static uint8_t raw_buf[RAW_BUF_SIZE] ALIGNMENT_ATTR;
 
 #ifdef MODULE_PERIPH_FLASHPAGE_PAGEWISE
 /**
@@ -219,7 +221,7 @@ static int cmd_write_raw(int argc, char **argv)
 #endif
 
     if (argc < 3) {
-        printf("usage: %s <addr> <data>\n", argv[0]);
+        printf("usage: %s <addr> <hexadecimal data>\n", argv[0]);
         return 1;
     }
 
@@ -229,15 +231,27 @@ static int cmd_write_raw(int argc, char **argv)
     addr = getaddr(argv[1]);
 #endif
     /* try to align */
-    memcpy(raw_buf, argv[2], strlen(argv[2]));
+    int len;
+    if ((len = strlen(argv[2])) % 2 || (unsigned)len > sizeof(raw_buf) * 2) {
+        printf("error: data must have an even length and must be <= %u\n",
+               sizeof(raw_buf) * 2);
+        return 1;
+    }
+    for (int i = 0; i < len; i++) {
+        if (!isxdigit((int)(argv[2][i]))) {
+            printf("error: data must be hexadecimal\n");
+            return 1;
+        }
+    }
+    len = fmt_hex_bytes(raw_buf, argv[2]);
 
-    flashpage_write((void*)addr, raw_buf, strlen(raw_buf));
+    flashpage_write((void*)addr, raw_buf, len);
 #if (__SIZEOF_POINTER__ == 2)
     printf("wrote local data to flash address %#" PRIx16 " of len %u\n",
-           addr, strlen(raw_buf));
+           addr, len);
 #else
     printf("wrote local data to flash address %#" PRIx32 " of len %u\n",
-           addr, strlen(raw_buf));
+           addr, len);
 #endif
     return 0;
 }
@@ -423,7 +437,7 @@ static int cmd_test_last_raw(int argc, char **argv)
     (void) argv;
     unsigned last_free = flashpage_last_free();
 
-    memset(raw_buf, 0, sizeof(raw_buf));
+    memset(raw_buf, 0xff, sizeof(raw_buf));
 
     /* try to align */
     memcpy(raw_buf, "test12344321tset", 16);
@@ -437,7 +451,7 @@ static int cmd_test_last_raw(int argc, char **argv)
     flashpage_write(flashpage_addr(last_free), raw_buf, sizeof(raw_buf));
 
     /* verify that previous write_raw effectively wrote the desired data */
-    if (memcmp(flashpage_addr(last_free), raw_buf, strlen(raw_buf)) != 0) {
+    if (memcmp(flashpage_addr(last_free), raw_buf, 16) != 0) {
         puts("error verifying the content of last page");
         return 1;
     }
@@ -584,10 +598,10 @@ static int cmd_test_last_rwwee_raw(int argc, char **argv)
     /* erase the page first */
     flashpage_rwwee_write_page(((int)FLASHPAGE_RWWEE_NUMOF - 1), NULL);
 
-    flashpage_rwwee_write(flashpage_rwwee_addr((int)FLASHPAGE_RWWEE_NUMOF - 1), raw_buf, strlen(raw_buf));
+    flashpage_rwwee_write(flashpage_rwwee_addr((int)FLASHPAGE_RWWEE_NUMOF - 1), raw_buf, 16);
 
     /* verify that previous write_raw effectively wrote the desired data */
-    if (memcmp(flashpage_rwwee_addr((int)FLASHPAGE_RWWEE_NUMOF - 1), raw_buf, strlen(raw_buf)) != 0) {
+    if (memcmp(flashpage_rwwee_addr((int)FLASHPAGE_RWWEE_NUMOF - 1), raw_buf, 16) != 0) {
         puts("error verifying the content of last RWWEE page");
         return 1;
     }
@@ -682,7 +696,7 @@ static const shell_command_t shell_commands[] = {
     { "read", "Copy the given page to the local page buffer and dump to STDOUT", cmd_read },
     { "write", "Write the local page buffer to the given page", cmd_write },
 #endif
-    { "write_raw", "Write (ASCII, max 64B) data to the given address", cmd_write_raw },
+    { "write_raw", "Write raw bytes (max 64B) to the given address", cmd_write_raw },
     { "erase", "Erase the given page buffer", cmd_erase },
 #ifdef MODULE_PERIPH_FLASHPAGE_PAGEWISE
     { "edit", "Write bytes to the local page buffer", cmd_edit },