diff --git a/rtl/frame_monitor.sv b/rtl/frame_monitor.sv
new file mode 100644
index 0000000..5eb9b37
--- /dev/null
+++ b/rtl/frame_monitor.sv
@@ -0,0 +1,48 @@
+module frame_monitor #(
+    parameter int MAX_LENGTH = 32
+) (
+    input logic clk, rst_n, byte_valid,
+    input logic [7:0] byte_data,
+    output logic frame_valid,
+    output logic [15:0] length_errors, parity_errors
+);
+  import frame_types::*;
+  state_t state;
+  logic [7:0] length;
+  logic parity, clear_parity, feed_parity;
+  logic length_error, parity_error;
+
+  assign clear_parity = byte_valid && state == WAIT_START && byte_data == START_BYTE;
+  assign feed_parity = byte_valid && state == COLLECT &&
+                       byte_data != END_BYTE && length < MAX_LENGTH;
+
+  parity_accumulator checksum(clk, rst_n, clear_parity, feed_parity, byte_data, parity);
+  error_counters counters(clk, rst_n, length_error, parity_error, length_errors, parity_errors);
+
+  always_ff @(posedge clk or negedge rst_n) begin
+    if (!rst_n) begin
+      state <= WAIT_START; length <= 0; frame_valid <= 0;
+      length_error <= 0; parity_error <= 0;
+    end else begin
+      frame_valid <= 0;
+      length_error <= 0; parity_error <= 0;
+      if (byte_valid) begin
+        case (state)
+          WAIT_START: if (byte_data == START_BYTE) begin
+            state <= COLLECT; length <= 0;
+          end
+          COLLECT: if (byte_data == END_BYTE) begin
+            if (length == 0) length_error <= 1;
+            else if (parity) parity_error <= 1;
+            else frame_valid <= 1;
+            state <= WAIT_START;
+          end else if (length >= MAX_LENGTH) begin
+            length_error <= 1; state <= DROP;
+          end else begin length <= length + 1'b1; end
+          DROP: if (byte_data == END_BYTE) state <= WAIT_START;
+          default: state <= WAIT_START;
+        endcase
+      end
+    end
+  end
+endmodule