diff --git a/firmware/libcommon/source/card_emu.c b/firmware/libcommon/source/card_emu.c
index ab54fc5f..bee03834 100644
--- a/firmware/libcommon/source/card_emu.c
+++ b/firmware/libcommon/source/card_emu.c
@@ -376,7 +376,7 @@ static void emu_update_fidi(struct card_handle *ch)
 	int rc;
 
 	rc = iso7816_3_compute_fd_ratio(ch->F_index, ch->D_index);
-	if (rc > 0 && rc < 0x400) {
+	if (rc > 0 && rc <= (US_FIDI_FI_DI_RATIO_Msk >> US_FIDI_FI_DI_RATIO_Pos)) {
 		TRACE_INFO("%u: computed F(%u)/D(%u) ratio: %d\r\n", ch->num,
 			   ch->F_index, ch->D_index, rc);
 		/* make sure UART uses new F/D ratio */
diff --git a/firmware/libcommon/source/mode_cardemu.c b/firmware/libcommon/source/mode_cardemu.c
index a5485e4d..3a0808d9 100644
--- a/firmware/libcommon/source/mode_cardemu.c
+++ b/firmware/libcommon/source/mode_cardemu.c
@@ -387,7 +387,7 @@ int card_emu_uart_update_fidi(uint8_t uart_chan, unsigned int fidi)
 	Usart *usart = get_usart_by_chan(uart_chan);
 
 	usart->US_CR |= US_CR_RXDIS | US_CR_RSTRX;
-	usart->US_FIDI = fidi & 0x3ff;
+	usart->US_FIDI = US_FIDI_FI_DI_RATIO(fidi);
 	usart->US_CR |= US_CR_RXEN | US_CR_STTTO;
 	return 0;
 }
diff --git a/firmware/libcommon/source/simtrace_iso7816.c b/firmware/libcommon/source/simtrace_iso7816.c
index 27677a67..f231faa5 100644
--- a/firmware/libcommon/source/simtrace_iso7816.c
+++ b/firmware/libcommon/source/simtrace_iso7816.c
@@ -127,14 +127,14 @@ void update_fidi(Usart_info *usart, uint8_t fidi)
 	uint8_t di = fidi & 0xf;
 	int ratio = iso7816_3_compute_fd_ratio(fi, di);
 
-	if (ratio > 0 && ratio < 0x8000) {
+	if (ratio > 0 && ratio <= (US_FIDI_FI_DI_RATIO_Msk >> US_FIDI_FI_DI_RATIO_Pos)) {
 		/* make sure USART uses new F/D ratio */
 		usart->base->US_CR |= US_CR_RXDIS | US_CR_RSTRX;
 		/* disable write protection */
 		if (usart->base->US_WPMR) {
 			usart->base->US_WPMR = US_WPMR_WPKEY(0x555341);
 		}
-		usart->base->US_FIDI = (ratio & 0x7ff);
+		usart->base->US_FIDI = US_FIDI_FI_DI_RATIO(ratio);
 		usart->base->US_CR |= US_CR_RXEN | US_CR_STTTO;
 		//TRACE_INFO("updated USART(%u) Fi(%u)/Di(%u) ratio(%d): %u\n\r", usart->id, fi, di, ratio, usart->base->US_FIDI);
 	} else {