diff --git a/avr/twimaster.asm b/avr/twimaster.asm
index 6f20562..5b156a3 100644
--- a/avr/twimaster.asm
+++ b/avr/twimaster.asm
@@ -2,7 +2,7 @@
 ; ***************************************************************************
 ; defines
 
-.equ TWI_BIT_LENGTH = 100000 ; 100000 and 200000 works for display
+.equ TWI_BIT_LENGTH = 10000 ; 100000 and 200000 works for display: 10000, 100000, 200000
 
 
 
@@ -285,6 +285,30 @@ twiSendByte_error:
 
 
 
+; ---------------------------------------------------------------------------
+; twiSendByteExpectAck
+;
+; Write a byte to the bus and expect ACK response.
+;
+; IN:
+; - R16: byte to send
+; OUT:
+; - CARRY flag set: okay, cleared on error (including case of non-ACK response)
+; USED: R16, R17, R18, R22
+
+twiSendByteExpectAck:
+  rcall twiSendByte
+  brcc twiSendByteExpectAck_error
+  tst r16
+  brne twiSendByteExpectAck_error
+  sec
+  ret
+twiSendByteExpectAck_error:
+  clc
+  ret
+
+
+
 ; ---------------------------------------------------------------------------
 ; twiReceiveByte
 ;