diff --git a/composite/iso7816_tx.pio b/composite/iso7816_tx.pio
index ea616b4..0f57960 100644
--- a/composite/iso7816_tx.pio
+++ b/composite/iso7816_tx.pio
@@ -94,14 +94,17 @@ static inline void iso7816_tx_program_puts(PIO pio, uint sm, const uint8_t *s, s
 
 static inline bool iso7816_tx_is_idle(PIO pio, uint sm)
 {
+	uint32_t flag = (1 << (PIO_FDEBUG_TXSTALL_LSB + sm));
+
 	/* how do we know once the PIO unit has completed tx of a character?
 	   - state machine TX fifo must be empty (FSTAT:TXEMPTY), AND
 	   - FDEBUG:TXSTALL bit must be set
 	*/
 
 	/* clear the sticky STALL flag */
-	pio->fdebug |= (sm << 24);
-	if (pio->fstat & (sm << 24) && pio->fdebug & (sm << 24))
+	pio->fdebug |= flag;
+
+	if (pio_sm_is_tx_fifo_empty(pio, sm) && pio->fdebug & flag)
 		return true;
 	else
 		return false;