diff --git a/cpu/ezr32wg/periph/uart.c b/cpu/ezr32wg/periph/uart.c
index ffcb34f65d..68ba4a0624 100644
--- a/cpu/ezr32wg/periph/uart.c
+++ b/cpu/ezr32wg/periph/uart.c
@@ -101,7 +101,7 @@ void uart_poweroff(uart_t dev)
 static inline void rx_irq(int dev)
 {
     if (_uart(dev)->IF & USART_IF_RXDATAV) {
-        char data = (char)_uart(dev)->RXDATA;
+        uint8_t data = (uint8_t)_uart(dev)->RXDATA;
         isr_ctx[dev].rx_cb(isr_ctx[dev].arg, data);
     }
     if (sched_context_switch_request) {
diff --git a/cpu/lm4f120/periph/uart.c b/cpu/lm4f120/periph/uart.c
index 93d671247e..67c6e4e13e 100644
--- a/cpu/lm4f120/periph/uart.c
+++ b/cpu/lm4f120/periph/uart.c
@@ -135,11 +135,8 @@ void isr_uart0(void)
     {
         while(ROM_UARTCharsAvail(UART0_BASE))
         {
-            char cChar;
-            long lChar;
-            lChar = ROM_UARTCharGetNonBlocking(UART0_BASE);
-            cChar = (unsigned char)(lChar & 0xFF);
-            config[UART_0].rx_cb(config[UART_0].arg, cChar);
+            long lchar = ROM_UARTCharGetNonBlocking(UART0_BASE);
+            config[UART_0].rx_cb(config[UART_0].arg, (uint8_t)lchar);
         }
     }
     if (sched_context_switch_request) {
diff --git a/cpu/lpc11u34/periph/uart.c b/cpu/lpc11u34/periph/uart.c
index 234f22f64b..39ef755e95 100644
--- a/cpu/lpc11u34/periph/uart.c
+++ b/cpu/lpc11u34/periph/uart.c
@@ -133,7 +133,7 @@ void uart_poweroff(uart_t uart)
 void UART_0_ISR(void)
 {
     if (UART_0_DEV->LSR & (1 << 0)) {       /* is RDR flag set? */
-        char data = (char)UART_0_DEV->RBR;
+        uint8_t data = (uint8_t)UART_0_DEV->RBR;
         config[UART_0].rx_cb(config[UART_0].arg, data);
     }
     if (sched_context_switch_request) {
diff --git a/cpu/lpc1768/periph/uart.c b/cpu/lpc1768/periph/uart.c
index 37d1a4bebd..304717792e 100644
--- a/cpu/lpc1768/periph/uart.c
+++ b/cpu/lpc1768/periph/uart.c
@@ -200,7 +200,7 @@ void uart_poweroff(uart_t uart)
 void UART_0_ISR(void)
 {
     if (UART_0_DEV->LSR & (1 << 0)) {       /* is RDR flag set? */
-        char data = (char)UART_0_DEV->RBR;
+        uint8_t data = (uint8_t)UART_0_DEV->RBR;
         config[UART_0].rx_cb(config[UART_0].arg, data);
     }
     if (sched_context_switch_request) {
@@ -213,7 +213,7 @@ void UART_0_ISR(void)
 void UART_1_ISR(void)
 {
     if (UART_1_DEV->LSR & (1 << 0)) {       /* is RDR flag set? */
-        char data = (char)UART_1_DEV->RBR;
+        uint8_t data = (uint8_t)UART_1_DEV->RBR;
         config[UART_1].rx_cb(config[UART_1].arg, data);
     }
     if (sched_context_switch_request) {
diff --git a/cpu/lpc2387/periph/uart.c b/cpu/lpc2387/periph/uart.c
index dc02f3ae31..be5926cb38 100644
--- a/cpu/lpc2387/periph/uart.c
+++ b/cpu/lpc2387/periph/uart.c
@@ -84,7 +84,7 @@ void UART0_IRQHandler(void)
         case UIIR_CTI_INT:              /* Character Timeout Indicator */
         case UIIR_RDA_INT:              /* Receive Data Available */
                 do {
-                    char c = (char)U0RBR;
+                    uint8_t c = (uint8_t)U0RBR;
                     _rx_cb(_cb_arg, c);
                 } while (U0LSR & ULSR_RDR);
             break;
diff --git a/cpu/msp430fxyz/periph/uart.c b/cpu/msp430fxyz/periph/uart.c
index a5475fcdbc..2010d37a6b 100644
--- a/cpu/msp430fxyz/periph/uart.c
+++ b/cpu/msp430fxyz/periph/uart.c
@@ -204,7 +204,7 @@ ISR(UART_RX_ISR, isr_uart_0_rx)
     __enter_isr();
 
     uint8_t stat = UART_BASE->ASTAT;
-    char data = (char)UART_BASE->ARXBUF;
+    uint8_t data = (uint8_t)UART_BASE->ARXBUF;
 
     if (stat & (USCI_ASTAT_FE | USCI_ASTAT_OE | USCI_ASTAT_PE | USCI_ASTAT_BRK)) {
         /* some error which we do not handle, just do a pseudo read to reset the
diff --git a/cpu/nrf5x_common/periph/uart.c b/cpu/nrf5x_common/periph/uart.c
index 82f1568c32..067c7a6f27 100644
--- a/cpu/nrf5x_common/periph/uart.c
+++ b/cpu/nrf5x_common/periph/uart.c
@@ -167,7 +167,7 @@ void isr_uart0(void)
 {
     if (NRF_UART0->EVENTS_RXDRDY == 1) {
         NRF_UART0->EVENTS_RXDRDY = 0;
-        char byte = (char)(NRF_UART0->RXD & 0xff);
+        uint8_t byte = (uint8_t)(NRF_UART0->RXD & 0xff);
         uart_config.rx_cb(uart_config.arg, byte);
     }
     if (sched_context_switch_request) {
diff --git a/cpu/sam3/periph/uart.c b/cpu/sam3/periph/uart.c
index 90f72984c7..6548071bff 100644
--- a/cpu/sam3/periph/uart.c
+++ b/cpu/sam3/periph/uart.c
@@ -99,7 +99,7 @@ static inline void isr_handler(int num)
     Uart *dev = uart_config[num].dev;
 
     if (dev->UART_SR & UART_SR_RXRDY) {
-        ctx[num].rx_cb(ctx[num].arg, (char)dev->UART_RHR);
+        ctx[num].rx_cb(ctx[num].arg, (uint8_t)dev->UART_RHR);
     }
     if (sched_context_switch_request) {
         thread_yield();
diff --git a/cpu/samd21/periph/uart.c b/cpu/samd21/periph/uart.c
index 195beed659..e2047951d4 100644
--- a/cpu/samd21/periph/uart.c
+++ b/cpu/samd21/periph/uart.c
@@ -135,7 +135,7 @@ static inline void irq_handler(int dev)
 
     if (uart->INTFLAG.reg & SERCOM_USART_INTFLAG_RXC) {
         /* interrupt flag is cleared by reading the data register */
-        uart_ctx[dev].rx_cb(uart_ctx[dev].arg, (char)(uart->DATA.reg));
+        uart_ctx[dev].rx_cb(uart_ctx[dev].arg, (uint8_t)(uart->DATA.reg));
     }
     else if (uart->INTFLAG.reg & SERCOM_USART_INTFLAG_ERROR) {
         /* clear error flag */
diff --git a/cpu/saml21/periph/uart.c b/cpu/saml21/periph/uart.c
index 0e45995595..bf0a05e311 100644
--- a/cpu/saml21/periph/uart.c
+++ b/cpu/saml21/periph/uart.c
@@ -136,7 +136,7 @@ static inline void irq_handler(uint8_t uartnum, SercomUsart *dev)
 {
     if (dev->INTFLAG.bit.RXC) {
         /* cleared by reading DATA regiser */
-        char data = (char)dev->DATA.reg;
+        uint8_t data = (uint8_t)dev->DATA.reg;
         uart_config[uartnum].rx_cb(uart_config[uartnum].arg, data);
     }
     else if (dev->INTFLAG.bit.ERROR) {
diff --git a/cpu/stm32f0/periph/uart.c b/cpu/stm32f0/periph/uart.c
index 01788d05b7..64f9aef80c 100644
--- a/cpu/stm32f0/periph/uart.c
+++ b/cpu/stm32f0/periph/uart.c
@@ -191,7 +191,7 @@ void uart_poweroff(uart_t uart)
 static inline void irq_handler(uint8_t uartnum, USART_TypeDef *dev)
 {
     if (dev->ISR & USART_ISR_RXNE) {
-        char data = (char)dev->RDR;
+        uint8_t data = (uint8_t)dev->RDR;
         uart_config[uartnum].rx_cb(uart_config[uartnum].arg, data);
     }
     else if (dev->ISR & USART_ISR_ORE) {
diff --git a/cpu/stm32f1/periph/uart.c b/cpu/stm32f1/periph/uart.c
index c3169aaf76..3d18eb4d3b 100644
--- a/cpu/stm32f1/periph/uart.c
+++ b/cpu/stm32f1/periph/uart.c
@@ -147,7 +147,7 @@ void uart_write(uart_t uart, const uint8_t *data, size_t len)
 static inline void irq_handler(uart_t uartnum, USART_TypeDef *dev)
 {
     if (dev->SR & USART_SR_RXNE) {
-        char data = (char)dev->DR;
+        uint8_t data = (uint8_t)dev->DR;
         config[uartnum].rx_cb(config[uartnum].arg, data);
     }
     else if (dev->SR & USART_SR_ORE) {
diff --git a/cpu/stm32f3/periph/uart.c b/cpu/stm32f3/periph/uart.c
index 9f69fff26c..cd1a864d8b 100644
--- a/cpu/stm32f3/periph/uart.c
+++ b/cpu/stm32f3/periph/uart.c
@@ -187,7 +187,7 @@ void uart_write(uart_t uart, const uint8_t *data, size_t len)
 static inline void irq_handler(uint8_t uartnum, USART_TypeDef *dev)
 {
     if (dev->ISR & USART_ISR_RXNE) {
-        char data = (char)dev->RDR;
+        uint8_t data = (uint8_t)dev->RDR;
         uart_config[uartnum].rx_cb(uart_config[uartnum].arg, data);
     }
     else if (dev->ISR & USART_ISR_ORE) {
diff --git a/cpu/stm32f4/periph/uart.c b/cpu/stm32f4/periph/uart.c
index d82654224d..aa0261029f 100644
--- a/cpu/stm32f4/periph/uart.c
+++ b/cpu/stm32f4/periph/uart.c
@@ -165,7 +165,7 @@ void uart_poweroff(uart_t uart)
 static inline void irq_handler(int uart, USART_TypeDef *dev)
 {
     if (dev->SR & USART_SR_RXNE) {
-        char data = (char)dev->DR;
+        uint8_t data = (uint8_t)dev->DR;
         uart_ctx[uart].rx_cb(uart_ctx[uart].arg, data);
     }
     if (sched_context_switch_request) {
diff --git a/cpu/stm32l1/periph/uart.c b/cpu/stm32l1/periph/uart.c
index be56ca96f5..22da881ef7 100644
--- a/cpu/stm32l1/periph/uart.c
+++ b/cpu/stm32l1/periph/uart.c
@@ -166,7 +166,7 @@ void uart_write(uart_t uart, const uint8_t *data, size_t len)
 static inline void irq_handler(uint8_t uartnum, USART_TypeDef *dev)
 {
     if (dev->SR & USART_SR_RXNE) {
-        char data = (char)dev->DR;
+        uint8_t data = (uint8_t)dev->DR;
         uart_config[uartnum].rx_cb(uart_config[uartnum].arg, data);
     }
     if (sched_context_switch_request) {