diff --git a/hw/char/stm32l4x5_usart.c b/hw/char/stm32l4x5_usart.c
index fc5dcac0c4..859fc6236a 100644
--- a/hw/char/stm32l4x5_usart.c
+++ b/hw/char/stm32l4x5_usart.c
@@ -154,6 +154,28 @@ REG32(RDR, 0x24)
 REG32(TDR, 0x28)
     FIELD(TDR, TDR, 0, 9)
 
+#define ISR_RESET_VALUE (0x020000C0)
+
+static void stm32l4x5_update_isr(Stm32l4x5UsartBaseState *s)
+{
+    if (!(s->cr1 & R_CR1_UE_MASK)) {
+        s->isr = ISR_RESET_VALUE;
+        return;
+    }
+
+    if (s->cr1 & R_CR1_TE_MASK) {
+        s->isr |= R_ISR_TEACK_MASK;
+    } else {
+        s->isr &= ~R_ISR_TEACK_MASK;
+    }
+
+    if (s->cr1 & R_CR1_RE_MASK) {
+        s->isr |= R_ISR_REACK_MASK;
+    } else {
+        s->isr &= ~R_ISR_REACK_MASK;
+    }
+}
+
 static void stm32l4x5_update_irq(Stm32l4x5UsartBaseState *s)
 {
     if (((s->isr & R_ISR_WUF_MASK) && (s->cr3 & R_CR3_WUFIE_MASK))        ||
@@ -367,7 +389,7 @@ static void stm32l4x5_usart_base_reset_hold(Object *obj, ResetType type)
     s->brr = 0x00000000;
     s->gtpr = 0x00000000;
     s->rtor = 0x00000000;
-    s->isr = 0x020000C0;
+    s->isr = ISR_RESET_VALUE;
     s->rdr = 0x00000000;
     s->tdr = 0x00000000;
 
@@ -456,6 +478,7 @@ static void stm32l4x5_usart_base_write(void *opaque, hwaddr addr,
     case A_CR1:
         s->cr1 = value;
         stm32l4x5_update_params(s);
+        stm32l4x5_update_isr(s);
         stm32l4x5_update_irq(s);
         return;
     case A_CR2:
@@ -508,12 +531,12 @@ static const MemoryRegionOps stm32l4x5_usart_base_ops = {
     .endianness = DEVICE_NATIVE_ENDIAN,
     .valid = {
         .max_access_size = 4,
-        .min_access_size = 4,
+        .min_access_size = 2,
         .unaligned = false
     },
     .impl = {
         .max_access_size = 4,
-        .min_access_size = 4,
+        .min_access_size = 2,
         .unaligned = false
     },
 };
diff --git a/tests/qtest/stm32l4x5_usart-test.c b/tests/qtest/stm32l4x5_usart-test.c
index 8902518233..ef886074c0 100644
--- a/tests/qtest/stm32l4x5_usart-test.c
+++ b/tests/qtest/stm32l4x5_usart-test.c
@@ -36,6 +36,8 @@ REG32(GTPR, 0x10)
 REG32(RTOR, 0x14)
 REG32(RQR, 0x18)
 REG32(ISR, 0x1C)
+    FIELD(ISR, REACK, 22, 1)
+    FIELD(ISR, TEACK, 21, 1)
     FIELD(ISR, TXE, 7, 1)
     FIELD(ISR, RXNE, 5, 1)
     FIELD(ISR, ORE, 3, 1)
@@ -191,7 +193,7 @@ static void init_uart(QTestState *qts)
 
     /* Enable the transmitter, the receiver and the USART. */
     qtest_writel(qts, (USART1_BASE_ADDR + A_CR1),
-        R_CR1_UE_MASK | R_CR1_RE_MASK | R_CR1_TE_MASK);
+        cr1 | R_CR1_UE_MASK | R_CR1_RE_MASK | R_CR1_TE_MASK);
 }
 
 static void test_write_read(void)
@@ -202,6 +204,11 @@ static void test_write_read(void)
     qtest_writel(qts, USART1_BASE_ADDR + A_TDR, 0xFFFFFFFF);
     const uint32_t tdr = qtest_readl(qts, USART1_BASE_ADDR + A_TDR);
     g_assert_cmpuint(tdr, ==, 0x000001FF);
+
+    /* Official STM HAL uses uint16_t for TDR */
+    qtest_writew(qts, USART1_BASE_ADDR + A_TDR, 0xFFFF);
+    const uint16_t tdr16 = qtest_readw(qts, USART1_BASE_ADDR + A_TDR);
+    g_assert_cmpuint(tdr16, ==, 0x000001FF);
 }
 
 static void test_receive_char(void)
@@ -296,6 +303,35 @@ static void test_send_str(void)
     qtest_quit(qts);
 }
 
+static void test_ack(void)
+{
+    uint32_t cr1;
+    uint32_t isr;
+    QTestState *qts = qtest_init("-M b-l475e-iot01a");
+
+    init_uart(qts);
+
+    cr1 = qtest_readl(qts, (USART1_BASE_ADDR + A_CR1));
+
+    /* Disable the transmitter and receiver. */
+    qtest_writel(qts, (USART1_BASE_ADDR + A_CR1),
+        cr1 & ~(R_CR1_RE_MASK | R_CR1_TE_MASK));
+
+    /* Test ISR ACK for transmitter and receiver disabled */
+    isr = qtest_readl(qts, (USART1_BASE_ADDR + A_ISR));
+    g_assert_false(isr & R_ISR_TEACK_MASK);
+    g_assert_false(isr & R_ISR_REACK_MASK);
+
+    /* Enable the transmitter and receiver. */
+    qtest_writel(qts, (USART1_BASE_ADDR + A_CR1),
+        cr1 | (R_CR1_RE_MASK | R_CR1_TE_MASK));
+
+    /* Test ISR ACK for transmitter and receiver disabled */
+    isr = qtest_readl(qts, (USART1_BASE_ADDR + A_ISR));
+    g_assert_true(isr & R_ISR_TEACK_MASK);
+    g_assert_true(isr & R_ISR_REACK_MASK);
+}
+
 int main(int argc, char **argv)
 {
     int ret;
@@ -308,6 +344,7 @@ int main(int argc, char **argv)
     qtest_add_func("stm32l4x5/usart/send_char", test_send_char);
     qtest_add_func("stm32l4x5/usart/receive_str", test_receive_str);
     qtest_add_func("stm32l4x5/usart/send_str", test_send_str);
+    qtest_add_func("stm32l4x5/usart/ack", test_ack);
     ret = g_test_run();
 
     return ret;
