diff --git a/boards/firefly/include/periph_conf.h b/boards/firefly/include/periph_conf.h
index 56e984d7c2..23a35adaef 100644
--- a/boards/firefly/include/periph_conf.h
+++ b/boards/firefly/include/periph_conf.h
@@ -39,7 +39,7 @@ static const spi_conf_t spi_config[] = {
         .mosi_pin = GPIO_PIN(PORT_B, 1),
         .miso_pin = GPIO_PIN(PORT_B, 3),
         .sck_pin  = GPIO_PIN(PORT_B, 2),
-        .cs_pin   = GPIO_UNDEF
+        .cs_pin   = SPI_CS_UNDEF,
     },
     {
         .num      = 1,
diff --git a/boards/remote-pa/include/periph_conf.h b/boards/remote-pa/include/periph_conf.h
index 8fd0e54014..f352349840 100644
--- a/boards/remote-pa/include/periph_conf.h
+++ b/boards/remote-pa/include/periph_conf.h
@@ -65,7 +65,7 @@ static const spi_conf_t spi_config[] = {
         .mosi_pin = GPIO_PIN(2, 7),
         .miso_pin = GPIO_PIN(0, 4),
         .sck_pin  = GPIO_PIN(1 ,5),
-        .cs_pin   = GPIO_UNDEF
+        .cs_pin   = SPI_CS_UNDEF,
     }
 };
 
diff --git a/cpu/cc2538/include/periph_cpu.h b/cpu/cc2538/include/periph_cpu.h
index dc33a056b0..b0254c6f2f 100644
--- a/cpu/cc2538/include/periph_cpu.h
+++ b/cpu/cc2538/include/periph_cpu.h
@@ -220,6 +220,21 @@ typedef enum {
 #define UART_TXBUF_SIZE    (64)
 #endif
 
+/**
+ * @brief   Define value for unused CS line
+ */
+#define SPI_CS_UNDEF        (GPIO_UNDEF)
+
+#ifndef DOXYGEN
+/**
+ * @brief   Overwrite the default spi_cs_t type definition
+ * @{
+ */
+#define HAVE_SPI_CS_T
+typedef uint32_t spi_cs_t;
+/** @} */
+#endif
+
 /**
  * @name   Override SPI mode settings
  * @{
@@ -282,7 +297,7 @@ typedef struct {
     gpio_t mosi_pin;        /**< pin used for MOSI */
     gpio_t miso_pin;        /**< pin used for MISO */
     gpio_t sck_pin;         /**< pin used for SCK */
-    gpio_t cs_pin;          /**< pin used for CS */
+    spi_cs_t cs_pin;        /**< pin used for CS */
 } spi_conf_t;
 /** @} */