diff --git a/bsp/rp2040/m_default/bsp-feature.h b/bsp/rp2040/m_default/bsp-feature.h
index 439f020..4de1c15 100644
--- a/bsp/rp2040/m_default/bsp-feature.h
+++ b/bsp/rp2040/m_default/bsp-feature.h
@@ -4,7 +4,6 @@
 
 #define DBOARD_HAS_UART
 #define DBOARD_HAS_CMSISDAP
-// TODO: rename to _SPI
 #define DBOARD_HAS_SPI
 #define DBOARD_HAS_I2C
 #define DBOARD_HAS_TEMPSENSOR
diff --git a/dmctl2.py b/dmctl2.py
index 3141697..a258b22 100755
--- a/dmctl2.py
+++ b/dmctl2.py
@@ -87,17 +87,18 @@ epout.write(b'\x12') # get mode1 features
 print("stat=%d"%stat)
 print(res)
 
-epout.write(b'\x20') # get mode1 name
+print("echo time!")
+epout.write(b'\x14\x00\x42') # I2C echo!
 (stat, res) = rdresp(epin)
 print("stat=%d"%stat)
 print(res)
 
-epout.write(b'\x21') # get mode1 version
+epout.write(b'\x1f\x14\x00\x43')
 (stat, res) = rdresp(epin)
 print("stat=%d"%stat)
 print(res)
 
-epout.write(b'\x22') # get mode1 features
+epout.write(b'\x14\x00\x44') # I2C echo!
 (stat, res) = rdresp(epin)
 print("stat=%d"%stat)
 print(res)
diff --git a/host/modules/dmj.c b/host/modules/dmj.c
index 36296b5..c453a68 100644
--- a/host/modules/dmj.c
+++ b/host/modules/dmj.c
@@ -453,17 +453,19 @@ static int dmj_probe(struct usb_interface *itf, const struct usb_device_id *usb_
 			goto out_free;
 		}
 
-		if (dmj->dmj_mode & DMJ_FEATURE_MODE1_SPI) {
+		if (dmj->dmj_m1feature & DMJ_FEATURE_MODE1_SPI) {
 			// TODO: add SPI MFD
 		}
-		if (dmj->dmj_mode & DMJ_FEATURE_MODE1_I2C) {
+		if (dmj->dmj_m1feature & DMJ_FEATURE_MODE1_I2C) {
 			ret = mfd_add_hotplug_devices(dev, dmj_mfd_i2c, ARRAY_SIZE(dmj_mfd_i2c));
 			if (ret) {
 				dev_err(dev, "failed to add MFD I2C devices\n");
 				goto out_free;
+			} else {
+				dev_warn(dev, "added i2c mfd\n");
 			}
 		}
-		if (dmj->dmj_mode & DMJ_FEATURE_MODE1_TEMPSENSOR) {
+		if (dmj->dmj_m1feature & DMJ_FEATURE_MODE1_TEMPSENSOR) {
 			// TODO: add tempsensor MFD
 		}
 	}
diff --git a/host/modules/i2c-dmj.c b/host/modules/i2c-dmj.c
index c006ab7..3979c46 100644
--- a/host/modules/i2c-dmj.c
+++ b/host/modules/i2c-dmj.c
@@ -219,7 +219,7 @@ static int dmj_i2c_check_hw(struct platform_device *pdev)
 	len = sizeof(curmode);
 	ret = dmj_transfer(pdev, DMJ_CMD_CFG_GET_CUR_MODE,
 			DMJ_XFER_FLAGS_PARSE_RESP, NULL, 0, &curmode, &len);
-	ret = dmj_check_retval(ret, len, dev, "i2c test", true, sizeof(curmode), sizeof(curmode));
+	ret = dmj_check_retval(ret, len, dev, "i2c test 1", true, sizeof(curmode), sizeof(curmode));
 	if (ret < 0) return ret;
 	if (curmode != 0x1) {
 		dev_err(dev, "device must be in mode 1 for ICD to work, but it is in mode %d\n", curmode);
@@ -229,7 +229,7 @@ static int dmj_i2c_check_hw(struct platform_device *pdev)
 	len = sizeof(m1ver);
 	ret = dmj_transfer(pdev, (1<<4) | DMJ_CMD_MODE_GET_VERSION,
 			DMJ_XFER_FLAGS_PARSE_RESP, NULL, 0, &m1ver, &len);
-	ret = dmj_check_retval(ret, len, dev, "i2c test", true, sizeof(m1ver), sizeof(m1ver));
+	ret = dmj_check_retval(ret, len, dev, "i2c test 2", true, sizeof(m1ver), sizeof(m1ver));
 	if (ret < 0) return ret;
 	if (le16_to_cpu(m1ver) > ver_max || le16_to_cpu(m1ver) < ver_min) {
 		dev_err(dev, "bad mode 1 version %04x on device, must be between %04x and %04x\n",
@@ -240,7 +240,7 @@ static int dmj_i2c_check_hw(struct platform_device *pdev)
 	len = sizeof(m1feat);
 	ret = dmj_transfer(pdev, (1<<4) | DMJ_CMD_MODE_GET_FEATURES,
 			DMJ_XFER_FLAGS_PARSE_RESP, NULL, 0, &m1feat, &len);
-	ret = dmj_check_retval(ret, len, dev, "i2c test", true, sizeof(m1feat), sizeof(m1feat));
+	ret = dmj_check_retval(ret, len, dev, "i2c test 3", true, sizeof(m1feat), sizeof(m1feat));
 	if (ret < 0) return ret;
 	if (!(m1feat & DMJ_FEATURE_MODE1_I2C)) {
 		dev_err(dev, "device's mode 1 does not support I2C\n");
@@ -286,6 +286,8 @@ static int dmj_i2c_probe(struct platform_device *pdev)
 	struct dmj_i2c *dmji;
 	struct device *dev = &pdev->dev;
 
+	dev_warn(dev, "i2c probe hi!\n");
+
 	ret = dmj_i2c_check_hw(pdev);
 	if (ret) return -ENODEV;
 
diff --git a/src/m_default/_default.c b/src/m_default/_default.c
index 166ab84..f0caf01 100644
--- a/src/m_default/_default.c
+++ b/src/m_default/_default.c
@@ -29,7 +29,7 @@
 /*#define MODE_ENABLE_I2CTINYUSB*/
 
 enum m_default_cmds {
-    mdef_cmd_spi = mode_cmd__specific | 0x10,
+    mdef_cmd_spi = mode_cmd__specific,
     mdef_cmd_i2c,
     mdef_cmd_tempsense
 };
@@ -137,25 +137,25 @@ static void handle_cmd_cb(uint8_t cmd) {
 #ifdef DBOARD_HAS_SPI
         sp_spi_bulk_cmd();
 #else
-        vnd_cfg_write_resp(cfg_resp_illcmd, 0, NULL);
+        vnd_cfg_write_str(cfg_resp_illcmd, "SPI not implemented on this device");
 #endif
         break;
     case mdef_cmd_i2c:
 #ifdef DBOARD_HAS_I2C
         i2ctu_bulk_cmd();
 #else
-        vnd_cfg_write_resp(cfg_resp_illcmd, 0, NULL);
+        vnd_cfg_write_str(cfg_resp_illcmd, "I2C not implemented on this device");
 #endif
         break;
     case mdef_cmd_tempsense:
 #ifdef DBOARD_HAS_TEMPSENSOR
         tempsense_bulk_cmd();
 #else
-        vnd_cfg_write_resp(cfg_resp_illcmd, 0, NULL);
+        vnd_cfg_write_str(cfg_resp_illcmd, "temperature sensor not implemented on this device");
 #endif
         break;
     default:
-        vnd_cfg_write_resp(cfg_resp_illcmd, 0, NULL);
+        vnd_cfg_write_strf(cfg_resp_illcmd, "unknown mode1 command %02x", cmd);
         break;
     }
 }
diff --git a/src/m_default/vnd_i2ctinyusb.c b/src/m_default/vnd_i2ctinyusb.c
index 6b3ad4d..354cd6e 100644
--- a/src/m_default/vnd_i2ctinyusb.c
+++ b/src/m_default/vnd_i2ctinyusb.c
@@ -276,7 +276,7 @@ void i2ctu_bulk_cmd(void) {
     } break;
 
     default:
-        vnd_cfg_write_resp(cfg_resp_illcmd, 0, NULL);
+        vnd_cfg_write_str(cfg_resp_illcmd, "unknown I2C command");
         break;
     }
 }
diff --git a/src/vnd_cfg.c b/src/vnd_cfg.c
index 623cebf..4346c69 100644
--- a/src/vnd_cfg.c
+++ b/src/vnd_cfg.c
@@ -1,5 +1,7 @@
 // vim: set et:
 
+#include <stdarg.h>
+#include <stdio.h>
 #include <string.h>
 #include <tusb.h>
 
@@ -14,6 +16,8 @@ static uint8_t tx_buf[CFG_TUD_VENDOR_TX_BUFSIZE];
 
 static uint32_t rxavail, rxpos, txpos;
 
+static const int VND_N_CFG;
+
 void vnd_cfg_init(void) {
     rxavail = 0;
     rxpos   = 0;
@@ -22,13 +26,13 @@ void vnd_cfg_init(void) {
 
 uint8_t vnd_cfg_read_byte(void) {
     while (rxavail <= 0) {
-        if (!tud_vendor_n_mounted(0) && !tud_vendor_n_available(0)) {
+        if (!tud_vendor_n_mounted(VND_N_CFG) || !tud_vendor_n_available(VND_N_CFG)) {
             thread_yield();
             continue;
         }
 
         rxpos   = 0;
-        rxavail = tud_vendor_n_read(0, rx_buf, sizeof rx_buf);
+        rxavail = tud_vendor_n_read(VND_N_CFG, rx_buf, sizeof rx_buf);
 
         if (rxavail == 0) thread_yield();
     }
@@ -39,13 +43,24 @@ uint8_t vnd_cfg_read_byte(void) {
 
     return rv;
 }
+void vnd_cfg_drop_incoming(void) {
+    rxavail = 0;
+    rxpos = 0;
+
+    // empty tinyusb internal buffer
+    if (tud_vendor_n_mounted(VND_N_CFG)) {
+        while (tud_vendor_n_available(VND_N_CFG)) {
+            tud_vendor_n_read(VND_N_CFG, rx_buf, sizeof rx_buf);
+        }
+    }
+}
 void vnd_cfg_write_flush(void) {
     // TODO: is this needed?
-    while (tud_vendor_n_write_available(0) < txpos) {
+    while (tud_vendor_n_write_available(VND_N_CFG) < txpos) {
         thread_yield();
     }
 
-    tud_vendor_n_write(0, tx_buf, txpos);
+    tud_vendor_n_write(VND_N_CFG, tx_buf, txpos);
     txpos = 0;
 }
 void vnd_cfg_write_byte(uint8_t v) {
@@ -56,7 +71,7 @@ void vnd_cfg_write_byte(uint8_t v) {
     tx_buf[txpos] = v;
     ++txpos;
 }
-void vnd_cfg_write_resp(enum cfg_resp stat, uint32_t len, const void* data) {
+void vnd_cfg_write_resp_no_drop(enum cfg_resp stat, uint32_t len, const void* data) {
     if (len > 0x3fffff) {
         printf("W: truncating response length from 0x%lx to 0x3fffff\n", len);
         len = 0x3fffff;
@@ -82,6 +97,23 @@ void vnd_cfg_write_resp(enum cfg_resp stat, uint32_t len, const void* data) {
 
     vnd_cfg_write_flush();
 }
+void vnd_cfg_write_resp(enum cfg_resp stat, uint32_t len, const void* data) {
+    if (stat != cfg_resp_ok) vnd_cfg_drop_incoming();
+    vnd_cfg_write_resp_no_drop(stat, len, data);
+}
+void vnd_cfg_write_str(enum cfg_resp stat, const char* str) {
+    vnd_cfg_write_resp(stat, strlen(str), str);
+}
+void vnd_cfg_write_strf(enum cfg_resp stat, const char* fmt, ...) {
+    static char pbuf[64];
+
+    va_list args;
+    va_start(args, fmt);
+    vsnprintf(pbuf, sizeof pbuf, fmt, args);
+    va_end(args);
+
+    vnd_cfg_write_str(stat, pbuf);
+}
 
 void vnd_cfg_task(void) {
     uint8_t cmd = vnd_cfg_read_byte();
@@ -157,11 +189,18 @@ void vnd_cfg_task(void) {
 #else /* CFG_TUD_VENDOR == 0 */
 void vnd_cfg_init(void) { }
 uint8_t vnd_cfg_read_byte(void) { return 0xff; }
+void vnd_cfg_drop_incoming(void) { }
 void vnd_cfg_write_flush(void) { }
 void vnd_cfg_write_byte(uint8_t v) { (void)v; }
 void vnd_cfg_write_resp(enum cfg_resp stat, uint16_t len, const void* data) {
     (void)stat; (void)len; (void)data;
 }
+void vnd_cfg_write_str(enum cfg_resp stat, const char* str) {
+    (void)stat; (void)str;
+}
+void vnd_cfg_write_strf(enum cfg_resp stat, const char* fmt, ...) {
+    (void)stat; (void)str;
+}
 void vnd_cfg_task(void) { }
 #endif /* CFG_TUD_VENDOR */
 
diff --git a/src/vnd_cfg.h b/src/vnd_cfg.h
index 57fa591..82736fb 100644
--- a/src/vnd_cfg.h
+++ b/src/vnd_cfg.h
@@ -52,9 +52,14 @@ enum cfg_resp {
 };
 
 uint8_t vnd_cfg_read_byte (void);
+void    vnd_cfg_drop_incoming(void);
 void    vnd_cfg_write_flush(void);
 void    vnd_cfg_write_byte(uint8_t v);
+void    vnd_cfg_write_resp_no_drop(enum cfg_resp stat, uint32_t len, const void* data);
 void    vnd_cfg_write_resp(enum cfg_resp stat, uint32_t len, const void* data);
+void    vnd_cfg_write_str(enum cfg_resp stat, const char* str);
+__attribute__((__format__(printf, 2, 3)))
+void    vnd_cfg_write_strf(enum cfg_resp stat, const char* fmt, ...);
 
 #endif