diff --git a/src/hw/zotino_fastino.md b/src/hw/zotino_fastino.md
index 29e4439..c486934 100644
--- a/src/hw/zotino_fastino.md
+++ b/src/hw/zotino_fastino.md
@@ -16,6 +16,7 @@
 {
   "type": "fastino",
   "hw_rev": "v1.2", // optional
+  "log2_width": <0 to 5, default 0>, // pack multiple (in powers of 2) DAC channels into one RTIO write
   "ports": [<port num>]
 }
 ```