diff --git a/embassy-rp/src/usb.rs b/embassy-rp/src/usb.rs
index 2710c4ad..8eec55b4 100644
--- a/embassy-rp/src/usb.rs
+++ b/embassy-rp/src/usb.rs
@@ -219,14 +219,16 @@ impl<'d, T: Instance> Driver<'d, T> {
         let (index, ep) = index.ok_or(EndpointAllocError)?;
         assert!(!ep.used);
 
-        if max_packet_size > 64 {
+        // as per datasheet, the maximum buffer size is 64, except for isochronous
+        // endpoints, which are allowed to be up to 1023 bytes.
+        if (ep_type != EndpointType::Isochronous && max_packet_size > 64) || max_packet_size > 1023 {
             warn!("max_packet_size too high: {}", max_packet_size);
             return Err(EndpointAllocError);
         }
 
         // ep mem addrs must be 64-byte aligned, so there's no point in trying
         // to allocate smaller chunks to save memory.
-        let len = 64;
+        let len = (max_packet_size + 63) / 64 * 64;
 
         let addr = self.ep_mem_free;
         if addr + len > EP_MEMORY_SIZE as _ {