diff --git a/artiq/firmware/libboard_artiq/hmc542.rs b/artiq/firmware/libboard_artiq/hmc542.rs
index d7a0c48b7..f395e0ede 100644
--- a/artiq/firmware/libboard_artiq/hmc542.rs
+++ b/artiq/firmware/libboard_artiq/hmc542.rs
@@ -25,27 +25,29 @@ fn set_pins(card_index: usize, chan_index: usize, pins: u32) {
 pub fn program(card_index: usize, chan_index: usize, atten: u8) {
     assert!(card_index < 4 && chan_index < 2);
 
-    // 0b111111 = 1dB
+    info!("card {} channel {} set to {}{} dB",
+          card_index, chan_index,
+          atten / 2, if atten % 2 != 0 { ".5" } else { "" });
+
+    // 0b111111 = 0dB
     // 0b111110 = 0.5dB
     // 0b111101 = 1dB
+    // 0b111100 = 1.5dB
     // ...
     // 0b011111 = 16dB
+    // ...
     // 0b000000 = 31.5dB
-    let atten = !(atten << 2);
+    let atten = !atten << 2;
 
     let set_pins = |pins| set_pins(card_index, chan_index, pins);
     set_pins(PIN_RST);
     set_pins(0);
-    for n in 0..8 {
+    for n in (0..8).rev() {
         let sin = if atten & 1 << n != 0 { PIN_SIN } else { 0 };
         set_pins(sin);
         set_pins(sin | PIN_CLK);
     }
     set_pins(PIN_LE);
-
-    info!("card {} channel {} set to {}{} dB",
-          card_index, chan_index,
-          atten / 2, if atten % 2 != 0 { ".5" } else { "" });
 }
 
 /// See `program`.