diff --git a/src/libboard_artiq/src/cxp_downconn.rs b/src/libboard_artiq/src/cxp_downconn.rs
new file mode 100644
index 0000000..2ae44b8
--- /dev/null
+++ b/src/libboard_artiq/src/cxp_downconn.rs
@@ -0,0 +1,534 @@
+use embedded_hal::prelude::_embedded_hal_blocking_delay_DelayUs;
+use libboard_zynq::{println, timer::GlobalTimer};
+use log::info;
+
+// use log::info;
+use crate::{cxp_proto,
+            pl::{csr, csr::CXP}};
+
+#[derive(Clone, Copy, Debug)]
+#[allow(non_camel_case_types)]
+pub enum CXP_SPEED {
+    CXP_1,
+    CXP_2,
+    CXP_3,
+    CXP_5,
+    CXP_6,
+    CXP_10,
+    CXP_12,
+}
+
+pub fn loopback_testing(channel: usize, timer: &mut GlobalTimer, speed: CXP_SPEED) {
+    println!("==============================================================================");
+    cxp_gtx::change_linerate(channel, timer, speed);
+
+    unsafe {
+        info!("waiting for tx&rx setup...");
+        timer.delay_us(50_000);
+        info!(
+            "tx_phaligndone = {} | rx_phaligndone = {}",
+            (CXP[channel].downconn_txinit_phaligndone_read)(),
+            (CXP[channel].downconn_rxinit_phaligndone_read)(),
+        );
+
+        // enable txdata tranmission thought MGTXTXP, required by PMA loopback
+        (CXP[channel].downconn_txenable_write)(1);
+
+        info!("waiting for rx to align...");
+        while (CXP[channel].downconn_rx_ready_read)() != 1 {}
+        info!("rx ready!");
+
+        (CXP[channel].downconn_tx_stb_write)(1);
+        cxp_proto::downconn_send_test_packet(channel);
+
+        cxp_proto::downconn_debug_send_trig_ack(channel);
+
+        const DATA_MAXSIZE: usize = 253;
+        let data_size = 4; // no. of bytes
+
+        let data: u32 = 0xDADA as u32;
+        let mut data_slice: [u8; DATA_MAXSIZE] = [0; DATA_MAXSIZE];
+        data_slice[..4].clone_from_slice(&data.to_be_bytes());
+        cxp_proto::downconn_debug_send(
+            channel,
+            &cxp_proto::UpConnPacket::Event {
+                conn_id: 0x1234_5678_u32,
+                packet_tag: 0x69_u8,
+                length: data_size + 3,
+                event_size: data_size,
+                namespace: 0x02_u8,
+                event_id: 0x00_6969u16,
+                timestamp: 0x1234_5678u64,
+                data: data_slice,
+            },
+        )
+        .expect("loopback gtx tx error");
+
+        timer.delay_us(1000); // wait packet has arrive at RX async fifo
+        (CXP[channel].downconn_tx_stb_write)(0);
+
+        info!("trig ack = {}", (CXP[channel].downconn_trigger_ack_read)());
+        (CXP[channel].downconn_trigger_ack_write)(1);
+        info!("after clr trig ack = {}", (CXP[channel].downconn_trigger_ack_read)());
+
+        info!("decoder error = {}", (CXP[channel].downconn_decoder_error_read)());
+        info!("test error = {}", (CXP[channel].downconn_test_error_read)());
+        info!("packet type = {:#06X}", (CXP[channel].downconn_packet_type_read)());
+
+        cxp_proto::receive(channel).expect("loopback gtx rx error");
+        // cxp_proto::downconn_debug_mem_print(channel);
+
+        // DEBUG: print loopback packets
+        const LEN: usize = 20;
+        let mut pak_arr: [u32; LEN] = [0; LEN];
+        let mut k_arr: [u8; LEN] = [0; LEN];
+        let mut i: usize = 0;
+        while (CXP[channel].downconn_debug_out_dout_valid_read)() == 1 {
+            pak_arr[i] = (CXP[channel].downconn_debug_out_dout_pak_read)();
+            k_arr[i] = (CXP[channel].downconn_debug_out_kout_pak_read)();
+            // println!("received {:#04X}", pak_arr[i]);
+            (CXP[channel].downconn_debug_out_inc_write)(1);
+            i += 1;
+            if i == LEN {
+                break;
+            }
+        }
+        cxp_proto::print_packetu32(&pak_arr, &k_arr);
+    }
+}
+
+pub fn setup(timer: &mut GlobalTimer) {
+    // TODO: do a for loop for channel?
+    let channel: usize = 0;
+    unsafe {
+        info!("turning on pmc loopback mode...");
+        (CXP[channel].downconn_loopback_mode_write)(0b010); // Near-End PMA Loopback
+
+        // QPLL setup
+        csr::cxp_phys::downconn_qpll_reset_write(1);
+        info!("waiting for QPLL/CPLL to lock...");
+        while csr::cxp_phys::downconn_qpll_locked_read() != 1 {}
+        info!("QPLL locked");
+
+        // tx/rx setup
+        (CXP[channel].downconn_tx_start_init_write)(1);
+        (CXP[channel].downconn_rx_start_init_write)(1);
+
+        info!("waiting for tx & rx setup...");
+        timer.delay_us(50_000);
+        info!(
+            "tx_phaligndone = {} | rx_phaligndone = {}",
+            (CXP[channel].downconn_txinit_phaligndone_read)(),
+            (CXP[channel].downconn_rxinit_phaligndone_read)(),
+        );
+    }
+
+    cxp_gtx::change_linerate(channel, timer, CXP_SPEED::CXP_1);
+}
+
+pub mod cxp_gtx {
+    use super::*;
+
+    struct CdrConfig {
+        pub cfg_reg0: u16, // addr = 0xA8
+        pub cfg_reg1: u16, // addr = 0xA9
+        pub cfg_reg2: u16, // addr = 0xAA
+        pub cfg_reg3: u16, // addr = 0xAB
+        pub cfg_reg4: u16, // addr = 0xAC
+    }
+
+    pub fn change_linerate(channel: usize, timer: &mut GlobalTimer, speed: CXP_SPEED) {
+        info!("Changing datarate to {:?}", speed);
+        // DEBUG: DRP pll for TXUSRCLK = freq(linerate)/20
+        let settings = txusrclk::get_txusrclk_config(speed);
+        txusrclk::setup(channel, timer, settings);
+
+        change_qpll_fb_divider(speed);
+        change_gtx_divider(channel, speed);
+        change_cdr_cfg(channel, speed);
+
+        unsafe {
+            csr::cxp_phys::downconn_qpll_reset_write(1);
+            info!("waiting for QPLL/CPLL to lock...");
+            while csr::cxp_phys::downconn_qpll_locked_read() != 1 {}
+            info!("QPLL locked");
+        }
+
+        unsafe {
+            (CXP[channel].downconn_tx_restart_write)(1);
+            (CXP[channel].downconn_rx_restart_write)(1);
+        }
+    }
+
+    fn change_qpll_fb_divider(speed: CXP_SPEED) {
+        let qpll_div_reg = match speed {
+            CXP_SPEED::CXP_1 | CXP_SPEED::CXP_2 | CXP_SPEED::CXP_5 | CXP_SPEED::CXP_10 => 0x0120, // FB_Divider = 80
+            CXP_SPEED::CXP_3 | CXP_SPEED::CXP_6 | CXP_SPEED::CXP_12 => 0x0170,                    // FB_Divider = 100
+        };
+
+        println!("0x36 = {:#06x}", qpll_read(0x36));
+        qpll_write(0x36, qpll_div_reg);
+        println!("0x36 = {:#06x}", qpll_read(0x36));
+    }
+
+    fn change_gtx_divider(channel: usize, speed: CXP_SPEED) {
+        let div_reg = match speed {
+            CXP_SPEED::CXP_1 => 0x33,                      // RXOUT_DIV = 8
+            CXP_SPEED::CXP_2 | CXP_SPEED::CXP_3 => 0x22,   // RXOUT_DIV = 4
+            CXP_SPEED::CXP_5 | CXP_SPEED::CXP_6 => 0x11,   // RXOUT_DIV = 2
+            CXP_SPEED::CXP_10 | CXP_SPEED::CXP_12 => 0x00, // RXOUT_DIV = 1
+        };
+
+        println!("0x88 = {:#06x}", gtx_read(channel, 0x88));
+        gtx_write(channel, 0x88, div_reg);
+        println!("0x88 = {:#06x}", gtx_read(channel, 0x88));
+    }
+
+    fn change_cdr_cfg(channel: usize, speed: CXP_SPEED) {
+        let cdr_cfg = match speed {
+            // when RXOUT_DIV = 8
+            CXP_SPEED::CXP_1 => {
+                CdrConfig {
+                    cfg_reg0: 0x0020, //0x0A8
+                    cfg_reg1: 0x1008, //0x0A9
+                    cfg_reg2: 0x23FF, //0x0AA
+                    cfg_reg3: 0x0000, //0x0AB
+                    cfg_reg4: 0x0003, //0x0AC
+                }
+            }
+            // when RXOUT_DIV = 4
+            CXP_SPEED::CXP_2 | CXP_SPEED::CXP_5 => {
+                CdrConfig {
+                    cfg_reg0: 0x0020, //0x0A8
+                    cfg_reg1: 0x1010, //0x0A9
+                    cfg_reg2: 0x23FF, //0x0AA
+                    cfg_reg3: 0x0000, //0x0AB
+                    cfg_reg4: 0x0003, //0x0AC
+                }
+            }
+            // when RXOUT_DIV= 2
+            CXP_SPEED::CXP_3 | CXP_SPEED::CXP_6 => {
+                CdrConfig {
+                    cfg_reg0: 0x0020, //0x0A8
+                    cfg_reg1: 0x1020, //0x0A9
+                    cfg_reg2: 0x23FF, //0x0AA
+                    cfg_reg3: 0x0000, //0x0AB
+                    cfg_reg4: 0x0003, //0x0AC
+                }
+            }
+            // when RXOUT_DIV= 1
+            CXP_SPEED::CXP_10 | CXP_SPEED::CXP_12 => {
+                CdrConfig {
+                    cfg_reg0: 0x0020, //0x0A8
+                    cfg_reg1: 0x1040, //0x0A9
+                    cfg_reg2: 0x23FF, //0x0AA
+                    cfg_reg3: 0x0000, //0x0AB
+                    cfg_reg4: 0x000B, //0x0AC
+                }
+            }
+        };
+
+        gtx_write(channel, 0x0A8, cdr_cfg.cfg_reg0);
+        gtx_write(channel, 0x0A9, cdr_cfg.cfg_reg1);
+        gtx_write(channel, 0x0AA, cdr_cfg.cfg_reg2);
+        gtx_write(channel, 0x0AB, cdr_cfg.cfg_reg3);
+        gtx_write(channel, 0x0AC, cdr_cfg.cfg_reg4);
+    }
+
+    #[allow(dead_code)]
+    fn gtx_read(channel: usize, address: u16) -> u16 {
+        unsafe {
+            (CXP[channel].downconn_gtx_daddr_write)(address);
+            (CXP[channel].downconn_gtx_dread_write)(1);
+            while (CXP[channel].downconn_gtx_dready_read)() != 1 {}
+            (CXP[channel].downconn_gtx_dout_read)()
+        }
+    }
+
+    fn gtx_write(channel: usize, address: u16, value: u16) {
+        unsafe {
+            (CXP[channel].downconn_gtx_daddr_write)(address);
+            (CXP[channel].downconn_gtx_din_write)(value);
+            (CXP[channel].downconn_gtx_din_stb_write)(1);
+            while (CXP[channel].downconn_gtx_dready_read)() != 1 {}
+        }
+    }
+
+    #[allow(dead_code)]
+    fn qpll_read(address: u8) -> u16 {
+        unsafe {
+            csr::cxp_phys::downconn_qpll_daddr_write(address);
+            csr::cxp_phys::downconn_qpll_dread_write(1);
+            while csr::cxp_phys::downconn_qpll_dready_read() != 1 {}
+            csr::cxp_phys::downconn_qpll_dout_read()
+        }
+    }
+
+    fn qpll_write(address: u8, value: u16) {
+        unsafe {
+            csr::cxp_phys::downconn_qpll_daddr_write(address);
+            csr::cxp_phys::downconn_qpll_din_write(value);
+            csr::cxp_phys::downconn_qpll_din_stb_write(1);
+            while csr::cxp_phys::downconn_qpll_dready_read() != 1 {}
+        }
+    }
+}
+
+pub mod txusrclk {
+    use super::*;
+
+    pub struct PLLSetting {
+        pub clkout0_reg1: u16,  //0x08
+        pub clkout0_reg2: u16,  //0x09
+        pub clkfbout_reg1: u16, //0x14
+        pub clkfbout_reg2: u16, //0x15
+        pub div_reg: u16,       //0x16
+        pub lock_reg1: u16,     //0x18
+        pub lock_reg2: u16,     //0x19
+        pub lock_reg3: u16,     //0x1A
+        pub power_reg: u16,     //0x28
+        pub filt_reg1: u16,     //0x4E
+        pub filt_reg2: u16,     //0x4F
+    }
+
+    fn one_clock_cycle(channel: usize) {
+        unsafe {
+            (CXP[channel].downconn_pll_dclk_write)(1);
+            (CXP[channel].downconn_pll_dclk_write)(0);
+        }
+    }
+
+    fn set_addr(channel: usize, address: u8) {
+        unsafe {
+            (CXP[channel].downconn_pll_daddr_write)(address);
+        }
+    }
+
+    fn set_data(channel: usize, value: u16) {
+        unsafe {
+            (CXP[channel].downconn_pll_din_write)(value);
+        }
+    }
+
+    fn set_enable(channel: usize, en: bool) {
+        unsafe {
+            let val = if en { 1 } else { 0 };
+            (CXP[channel].downconn_pll_den_write)(val);
+        }
+    }
+
+    fn set_write_enable(channel: usize, en: bool) {
+        unsafe {
+            let val = if en { 1 } else { 0 };
+            (CXP[channel].downconn_pll_dwen_write)(val);
+        }
+    }
+
+    fn get_data(channel: usize) -> u16 {
+        unsafe { (CXP[channel].downconn_pll_dout_read)() }
+    }
+
+    fn drp_ready(channel: usize) -> bool {
+        unsafe { (CXP[channel].downconn_pll_dready_read)() == 1 }
+    }
+
+    #[allow(dead_code)]
+    fn read(channel: usize, address: u8) -> u16 {
+        set_addr(channel, address);
+        set_enable(channel, true);
+        // Set DADDR on the mmcm and assert DEN for one clock cycle
+        one_clock_cycle(channel);
+
+        set_enable(channel, false);
+        while !drp_ready(channel) {
+            // keep the clock signal until data is ready
+            one_clock_cycle(channel);
+        }
+        get_data(channel)
+    }
+
+    fn write(channel: usize, address: u8, value: u16) {
+        set_addr(channel, address);
+        set_data(channel, value);
+        set_write_enable(channel, true);
+        set_enable(channel, true);
+        // Set DADDR, DI on the mmcm and assert DWE, DEN for one clock cycle
+        one_clock_cycle(channel);
+
+        set_write_enable(channel, false);
+        set_enable(channel, false);
+        while !drp_ready(channel) {
+            // keep the clock signal until write is finished
+            one_clock_cycle(channel);
+        }
+    }
+
+    fn reset(channel: usize, rst: bool) {
+        unsafe {
+            let val = if rst { 1 } else { 0 };
+            (CXP[channel].downconn_txpll_reset_write)(val)
+        }
+    }
+
+    pub fn setup(channel: usize, timer: &mut GlobalTimer, settings: PLLSetting) {
+        if false {
+            info!("0x08 = {:#06x}", read(channel, 0x08));
+            info!("0x09 = {:#06x}", read(channel, 0x09));
+            info!("0x14 = {:#06x}", read(channel, 0x14));
+            info!("0x15 = {:#06x}", read(channel, 0x15));
+            info!("0x16 = {:#06x}", read(channel, 0x16));
+            info!("0x18 = {:#06x}", read(channel, 0x18));
+            info!("0x19 = {:#06x}", read(channel, 0x19));
+            info!("0x1A = {:#06x}", read(channel, 0x1A));
+            info!("0x28 = {:#06x}", read(channel, 0x28));
+            info!("0x4E = {:#06x}", read(channel, 0x4E));
+            info!("0x4F = {:#06x}", read(channel, 0x4F));
+        } else {
+            // Based on "DRP State Machine" from XAPP888
+            // hold reset HIGH during pll config
+            reset(channel, true);
+            write(channel, 0x08, settings.clkout0_reg1);
+            write(channel, 0x09, settings.clkout0_reg2);
+            write(channel, 0x14, settings.clkfbout_reg1);
+            write(channel, 0x15, settings.clkfbout_reg2);
+            write(channel, 0x16, settings.div_reg);
+            write(channel, 0x18, settings.lock_reg1);
+            write(channel, 0x19, settings.lock_reg2);
+            write(channel, 0x1A, settings.lock_reg3);
+            write(channel, 0x28, settings.power_reg);
+            write(channel, 0x4E, settings.filt_reg1);
+            write(channel, 0x4F, settings.filt_reg2);
+            reset(channel, false);
+
+            // wait for the pll to lock
+            timer.delay_us(100);
+
+            let locked = unsafe { (CXP[channel].downconn_txpll_locked_read)() == 1 };
+            info!("txusrclk locked = {}", locked);
+        }
+    }
+
+    pub fn get_txusrclk_config(speed: CXP_SPEED) -> PLLSetting {
+        match speed {
+            CXP_SPEED::CXP_1 => {
+                // CLKFBOUT_MULT = 8, DIVCLK_DIVIDE = 1 , CLKOUT0_DIVIDE = 32
+                // TXUSRCLK=62.5MHz
+                PLLSetting {
+                    clkout0_reg1: 0x1410,  //0x08
+                    clkout0_reg2: 0x0000,  //0x09
+                    clkfbout_reg1: 0x1104, //0x14
+                    clkfbout_reg2: 0x0000, //0x15
+                    div_reg: 0x1041,       //0x16
+                    lock_reg1: 0x03e8,     //0x18
+                    lock_reg2: 0x5801,     //0x19
+                    lock_reg3: 0xdbe9,     //0x1A
+                    power_reg: 0x0000,     //0x28
+                    filt_reg1: 0x9808,     //0x4E
+                    filt_reg2: 0x9100,     //0x4F
+                }
+            }
+            CXP_SPEED::CXP_2 => {
+                // CLKFBOUT_MULT = 8, DIVCLK_DIVIDE = 1 , CLKOUT0_DIVIDE = 16
+                // TXUSRCLK=62.5MHz
+                PLLSetting {
+                    clkout0_reg1: 0x1208,  //0x08
+                    clkout0_reg2: 0x0000,  //0x09
+                    clkfbout_reg1: 0x1104, //0x14
+                    clkfbout_reg2: 0x0000, //0x15
+                    div_reg: 0x1041,       //0x16
+                    lock_reg1: 0x03e8,     //0x18
+                    lock_reg2: 0x5801,     //0x19
+                    lock_reg3: 0xdbe9,     //0x1A
+                    power_reg: 0x0000,     //0x28
+                    filt_reg1: 0x9808,     //0x4E
+                    filt_reg2: 0x9100,     //0x4F
+                }
+            }
+            CXP_SPEED::CXP_3 => {
+                // CLKFBOUT_MULT = 10, DIVCLK_DIVIDE = 1 , CLKOUT0_DIVIDE = 16
+                // TXUSRCLK=78.125MHz
+                PLLSetting {
+                    clkout0_reg1: 0x1208,  //0x08
+                    clkout0_reg2: 0x0000,  //0x09
+                    clkfbout_reg1: 0x1145, //0x14
+                    clkfbout_reg2: 0x0000, //0x15
+                    div_reg: 0x1041,       //0x16
+                    lock_reg1: 0x03e8,     //0x18
+                    lock_reg2: 0x7001,     //0x19
+                    lock_reg3: 0xf3e9,     //0x1A
+                    power_reg: 0x0000,     //0x28
+                    filt_reg1: 0x9908,     //0x4E
+                    filt_reg2: 0x1900,     //0x4F
+                }
+            }
+            CXP_SPEED::CXP_5 => {
+                // CLKFBOUT_MULT = 8, DIVCLK_DIVIDE = 1 , CLKOUT0_DIVIDE = 8
+                // TXUSRCLK=125MHz
+                PLLSetting {
+                    clkout0_reg1: 0x1104,  //0x08
+                    clkout0_reg2: 0x0000,  //0x09
+                    clkfbout_reg1: 0x1104, //0x14
+                    clkfbout_reg2: 0x0000, //0x15
+                    div_reg: 0x1041,       //0x16
+                    lock_reg1: 0x03e8,     //0x18
+                    lock_reg2: 0x5801,     //0x19
+                    lock_reg3: 0xdbe9,     //0x1A
+                    power_reg: 0x0000,     //0x28
+                    filt_reg1: 0x9808,     //0x4E
+                    filt_reg2: 0x9100,     //0x4F
+                }
+            }
+            CXP_SPEED::CXP_6 => {
+                // CLKFBOUT_MULT = 10, DIVCLK_DIVIDE = 1 , CLKOUT0_DIVIDE = 8
+                // TXUSRCLK=156.25MHz
+                PLLSetting {
+                    clkout0_reg1: 0x1104,  //0x08
+                    clkout0_reg2: 0x0000,  //0x09
+                    clkfbout_reg1: 0x1145, //0x14
+                    clkfbout_reg2: 0x0000, //0x15
+                    div_reg: 0x1041,       //0x16
+                    lock_reg1: 0x03e8,     //0x18
+                    lock_reg2: 0x7001,     //0x19
+                    lock_reg3: 0xf3e9,     //0x1A
+                    power_reg: 0x0000,     //0x28
+                    filt_reg1: 0x9908,     //0x4E
+                    filt_reg2: 0x1900,     //0x4F
+                }
+            }
+            CXP_SPEED::CXP_10 => {
+                // CLKFBOUT_MULT = 8, DIVCLK_DIVIDE = 1 , CLKOUT0_DIVIDE = 4
+                // TXUSRCLK=250MHz
+                PLLSetting {
+                    clkout0_reg1: 0x1082,  //0x08
+                    clkout0_reg2: 0x0000,  //0x09
+                    clkfbout_reg1: 0x1104, //0x14
+                    clkfbout_reg2: 0x0000, //0x15
+                    div_reg: 0x1041,       //0x16
+                    lock_reg1: 0x03e8,     //0x18
+                    lock_reg2: 0x5801,     //0x19
+                    lock_reg3: 0xdbe9,     //0x1A
+                    power_reg: 0x0000,     //0x28
+                    filt_reg1: 0x9808,     //0x4E
+                    filt_reg2: 0x9100,     //0x4F
+                }
+            }
+            CXP_SPEED::CXP_12 => {
+                // CLKFBOUT_MULT = 10, DIVCLK_DIVIDE = 1 , CLKOUT0_DIVIDE = 4
+                // TXUSRCLK=312.5MHz
+                PLLSetting {
+                    clkout0_reg1: 0x1082,  //0x08
+                    clkout0_reg2: 0x0000,  //0x09
+                    clkfbout_reg1: 0x1145, //0x14
+                    clkfbout_reg2: 0x0000, //0x15
+                    div_reg: 0x1041,       //0x16
+                    lock_reg1: 0x03e8,     //0x18
+                    lock_reg2: 0x7001,     //0x19
+                    lock_reg3: 0xf3e9,     //0x1A
+                    power_reg: 0x0000,     //0x28
+                    filt_reg1: 0x9908,     //0x4E
+                    filt_reg2: 0x1900,     //0x4F
+                }
+            }
+        }
+    }
+}
diff --git a/src/libboard_artiq/src/lib.rs b/src/libboard_artiq/src/lib.rs
index 81cac5d..3868efd 100644
--- a/src/libboard_artiq/src/lib.rs
+++ b/src/libboard_artiq/src/lib.rs
@@ -42,6 +42,8 @@ pub mod si5324;
 pub mod si549;
 use core::{cmp, str};
 
+#[cfg(has_cxp_phys)]
+pub mod cxp_downconn;
 #[cfg(has_cxp_phys)]
 pub mod cxp_upconn;