diff --git a/src/gateware/zc706.py b/src/gateware/zc706.py
index 55a90de..e33b0d7 100755
--- a/src/gateware/zc706.py
+++ b/src/gateware/zc706.py
@@ -698,8 +698,9 @@ class CXP_FMC():
         self.csr_devices.append("cxp")
 
         # max freq of cxp_gtx_rx = linerate/internal_datawidth = 12.5Gbps/40 = 312.5MHz
-        platform.add_period_constraint(self.cxp.downconn.gtx.cd_cxp_gtx_tx.clk, 3.2)
-        platform.add_period_constraint(self.cxp.downconn.gtx.cd_cxp_gtx_rx.clk, 3.2)
+        # 4.0 works on all CXP linerate, 3.2 has some strange setup/hold time problem even on 12.5Gbps
+        platform.add_period_constraint(self.cxp.downconn.gtx.cd_cxp_gtx_tx.clk, 4.0)
+        platform.add_period_constraint(self.cxp.downconn.gtx.cd_cxp_gtx_rx.clk, 4.0)
         platform.add_false_path_constraints(self.cxp.downconn.gtx.cd_cxp_gtx_tx.clk, self.cxp.downconn.gtx.cd_cxp_gtx_rx.clk)
 
         rtio_channels = []