diff --git a/src/zynq/ddr/mod.rs b/src/zynq/ddr/mod.rs
index d3b8154b..b2c0432e 100644
--- a/src/zynq/ddr/mod.rs
+++ b/src/zynq/ddr/mod.rs
@@ -56,9 +56,9 @@ impl DdrRam {
     /// Zynq-7000 AP SoC Technical Reference Manual:
     /// 10.6.2 DDR IOB Impedance Calibration
     fn calibrate_iob_impedance(clocks: &CpuClocks) {
-        let divisor0 = (clocks.ddr / DCI_FREQ)
+        let divisor0 = ((DCI_FREQ - 1 + clocks.ddr) / DCI_FREQ)
             .max(1).min(63) as u8;
-        let divisor1 = 1 + (clocks.ddr / DCI_FREQ / u32::from(divisor0))
+        let divisor1 = (clocks.ddr / DCI_FREQ / u32::from(divisor0))
             .max(1).min(63) as u8;
 
         let slcr = slcr::RegisterBlock::new();
diff --git a/src/zynq/eth/mod.rs b/src/zynq/eth/mod.rs
index 6b7d339e..ba6a2413 100644
--- a/src/zynq/eth/mod.rs
+++ b/src/zynq/eth/mod.rs
@@ -192,8 +192,8 @@ impl<'r> Eth<'r, (), ()> {
 impl<'r, RX, TX> Eth<'r, RX, TX> {
     pub fn setup_gem0_clock(tx_clock: u32) {
         let io_pll = CpuClocks::get().io;
-        let d0 = (io_pll / tx_clock).min(63);
-        let d1 = (io_pll / tx_clock / d0).min(63);
+        let d0 = ((tx_clock - 1 + io_pll) / tx_clock).max(1).min(63);
+        let d1 = (io_pll / tx_clock / d0).max(1).min(63);
 
         let slcr = slcr::RegisterBlock::new();
         slcr.gem0_clk_ctrl.write(
@@ -215,8 +215,8 @@ impl<'r, RX, TX> Eth<'r, RX, TX> {
 
     pub fn setup_gem1_clock(tx_clock: u32) {
         let io_pll = CpuClocks::get().io;
-        let d0 = (io_pll / tx_clock).min(63);
-        let d1 = (io_pll / tx_clock / d0).min(63);
+        let d0 = ((tx_clock - 1 + io_pll) / tx_clock).max(1).min(63);
+        let d1 = (io_pll / tx_clock / d0).max(1).min(63);
 
         let slcr = slcr::RegisterBlock::new();
         slcr.gem1_clk_ctrl.write(