diff --git a/Makefile b/Makefile
index 2f07600..d451e18 100644
--- a/Makefile
+++ b/Makefile
@@ -18,7 +18,7 @@
 # be silent per default, but 'make V=1' will show all compiler calls.
 ifneq ($(V),1)
 Q := @
-NULL := 2> /dev/null
+NULL := 1> /dev/null 2> /dev/null
 endif
 
 # the final binary name (without extension)
@@ -94,6 +94,9 @@ OOCD ?= openocd
 OOCD_INTERFACE ?= stlink-v2
 OOCD_TARGET ?= stm32f1x
 
+# which USB CDC ACM port is used bu the device, so we can reset it
+ACMPORT = /dev/ttyACM0
+
 # board specific USB DFU bootloader
 BOOTLOADERS = STM32duino-bootloader
 BOOTLOADER = $(BOOTLOADERS)/STM32F1/binaries/generic_boot20_pa1.bin
@@ -129,12 +132,12 @@ list: $(BINARY).list
 %.map: %.elf
 	@# it's generated along with the elf
 
-%.elf: $(OBJ) $(LDSCRIPT) $(LIB_DIR)/lib$(LIBNAME).a
+%.elf: $(LDSCRIPT) $(LIB_DIR)/lib$(LIBNAME).a $(OBJ)
 	$(info compiling $(@))
 	$(Q)$(LD) $(LDFLAGS) $(ARCH_FLAGS) $(OBJ) $(LDLIBS) -o $(@)
+	$(Q)size $(@)
 
 %.o: %.c
-	@#printf "  CC      $(*).c\n"
 	$(Q)$(CC) $(CFLAGS) $(ARCH_FLAGS) -o $(@) -c $(<)
 
 clean:
@@ -155,9 +158,14 @@ flash-swd: $(BINARY).hex
 	$(info flashing $(<) using SWD)
 	$(Q)$(OOCD) --file interface/$(OOCD_INTERFACE).cfg --file target/$(OOCD_TARGET).cfg --command "init" --command "reset init" --command "flash write_image erase $(<)" --command "reset" --command "shutdown" $(NULL)
 
-flash-dfu: $(BINARY).bin
+# reset device by setting the data width to 5 bis on the USB CDC ACM port
+reset:
+	$(Q)stty --file /dev/ttyACM0 115200 raw cs5
+	$(Q)sleep 0.5
+	
+flash-dfu: $(BINARY).bin reset
 	$(info flashing $(<) using DFU)
-	$(Q)dfu-util --device 1eaf:0003 --cfg 1 --intf 0 --alt 2 --reset --download $(<)
+	$(Q)dfu-util --device 1eaf:0003 --cfg 1 --intf 0 --alt 2 --reset --download $(<) $(NULL)
 
 .PHONY: clean elf bin hex srec list bootloader flash flash-swd flash-dfu
 .SECONDARY:
diff --git a/main.h b/global.h
similarity index 85%
rename from main.h
rename to global.h
index 6d2e1eb..559ae06 100644
--- a/main.h
+++ b/global.h
@@ -14,6 +14,12 @@
  */
 /* Copyright (c) 2016 King Kévin <kingkevin@cuvoodoo.info> */
 
+/* get the length of an array */
+#define LENGTH(x) (sizeof(x) / sizeof((x)[0]))
+
+/* system clock frequency in Hz */
+#define SYSTEM_CLOCK_FREQ 72000000
+
 /* LED is on pin 11/PA1 */
 #define LED_RCC RCC_GPIOA
 #define LED_PORT GPIOA
@@ -21,3 +27,4 @@
 
 /* default output (i.e. for printf) */
 int _write(int file, char *ptr, int len);
+
diff --git a/lib/usb_cdcacm.c b/lib/usb_cdcacm.c
index 27f3859..9f3d01e 100644
--- a/lib/usb_cdcacm.c
+++ b/lib/usb_cdcacm.c
@@ -24,20 +24,23 @@
 #include <libopencm3/stm32/rcc.h> // real-time control clock library
 #include <libopencm3/stm32/gpio.h> // general purpose input output library
 #include <libopencm3/cm3/nvic.h> // interrupt handler
+#include <libopencm3/cm3/scb.h> // reset utilities
 #include <libopencmsis/core_cm3.h> // Cortex M3 utilities
 #include <libopencm3/usb/usbd.h> // USB library
 #include <libopencm3/usb/cdc.h> // USB CDC library
 
 #include "usb_cdcacm.h" // USB CDC ACM header and definitions
 
-/* USB descriptor */
+/* USB devices descriptor
+ * as defined in USB CDC specification section 5
+ */
 static const struct usb_device_descriptor device_descriptor = {
 	.bLength = USB_DT_DEVICE_SIZE, // the size of this header in bytes, 18
 	.bDescriptorType = USB_DT_DEVICE, // a value of 1 indicates that this is a device descriptor
 	.bcdUSB = 0x0200, // this device supports USB 2.0
 	.bDeviceClass = USB_CLASS_CDC, // use the CDC device class
-	.bDeviceSubClass = USB_CDC_SUBCLASS_ACM, // use the ACM sub-class
-	.bDeviceProtocol = USB_CDC_PROTOCOL_NONE, // use no specific protocol
+	.bDeviceSubClass = 0, // unused
+	.bDeviceProtocol = 0, // unused
 	.bMaxPacketSize0 = 64, // packet size for endpoint zero in bytes
 	.idVendor = 0xc440, // Vendor ID (CuVo...)
 	.idProduct = 0x0d00, // product ID within the Vendor ID space (...odoo)
@@ -77,6 +80,9 @@ static const struct usb_endpoint_descriptor communication_endpoints[] = {{
 	.bInterval = 255, // the frequency, in number of frames, that we're going to be sending data
 }};
 
+/* functional descriptor
+ * as defined in USB CDC specification section 5.2.3
+ */
 static const struct {
 	struct usb_cdc_header_descriptor header;
 	struct usb_cdc_call_management_descriptor call_mgmt;
@@ -90,8 +96,7 @@ static const struct {
 		.bcdCDC = 0x0110,
 	},
 	.call_mgmt = {
-		.bFunctionLength =
-			sizeof(struct usb_cdc_call_management_descriptor),
+		.bFunctionLength = sizeof(struct usb_cdc_call_management_descriptor),
 		.bDescriptorType = CS_INTERFACE,
 		.bDescriptorSubtype = USB_CDC_TYPE_CALL_MANAGEMENT,
 		.bmCapabilities = 0,
@@ -112,6 +117,9 @@ static const struct {
 	 },
 };
 
+/* communication class interface descriptor
+ * as defined in USB CDC specification section 5.1.3
+ */
 static const struct usb_interface_descriptor communication_interface[] = {{
 	.bLength = USB_DT_INTERFACE_SIZE,
 	.bDescriptorType = USB_DT_INTERFACE,
@@ -129,6 +137,9 @@ static const struct usb_interface_descriptor communication_interface[] = {{
 	.extralen = sizeof(cdcacm_functional_descriptors),
 }};
 
+/* data class interface descriptor
+ * as defined in USB CDC specification section 5.1.3
+ */
 static const struct usb_interface_descriptor data_interface[] = {{
 	.bLength = USB_DT_INTERFACE_SIZE,
 	.bDescriptorType = USB_DT_INTERFACE,
@@ -158,7 +169,7 @@ static const struct usb_config_descriptor config = {
 	.bNumInterfaces = 2, // the number of interfaces in this configuration 
 	.bConfigurationValue = 1, // the index of this configuration
 	.iConfiguration = 0, // a string index describing this configuration (zero means not provided)
-	.bmAttributes = 0x80, // self powered (0<<6), supports remote wakeup (5<<0)
+	.bmAttributes = 0x80, // self powered (0<<6), supports remote wakeup (0<<5)
 	.bMaxPower = 0x32, // the maximum amount of current that this device will draw in 2mA units
 	// end of header
 	.interface = interfaces, // pointer to an array of interfaces
@@ -181,6 +192,9 @@ static usbd_device *usb_device;
 static uint8_t rx_buffer[CDCACM_BUFFER] = {0};
 static volatile uint8_t rx_i = 0;
 static volatile uint8_t rx_used = 0;
+static uint8_t tx_buffer[CDCACM_BUFFER] = {0};
+static volatile uint8_t tx_i = 0;
+static volatile uint8_t tx_used = 0;
 /* show the user how much data received over USB is ready */
 volatile uint8_t cdcacm_received = 0; // same as rx_used, but since the user can write this variable we don't rely on it
 
@@ -191,30 +205,35 @@ static int cdcacm_control_request(usbd_device *usbd_dev, struct usb_setup_data *
 	(void)usbd_dev;
 
 	switch (req->bRequest) {
-	case USB_CDC_REQ_SET_CONTROL_LINE_STATE: {
-		/*
-		 * This Linux cdc_acm driver requires this to be implemented
-		 * even though it's optional in the CDC spec, and we don't
-		 * advertise it in the ACM functional descriptor.
-		 */
-		char local_buf[10];
-		struct usb_cdc_notification *notif = (void *)local_buf;
-
-		/* We echo signals back to host as notification. */
-		notif->bmRequestType = 0xA1;
-		notif->bNotification = USB_CDC_NOTIFY_SERIAL_STATE;
-		notif->wValue = 0;
-		notif->wIndex = 0;
-		notif->wLength = 2;
-		local_buf[8] = req->wValue & 3;
-		local_buf[9] = 0;
-		// usbd_ep_write_packet(0x83, buf, 10);
-		return 1;
+		case USB_CDC_REQ_SET_CONTROL_LINE_STATE: {
+			/*
+			bool dtr = (req->wValue & (1 << 0)) ? true : false;
+			bool rts = (req->wValue & (1 << 1)) ? true : false;
+			*/
+			/* this Linux cdc_acm driver requires this to be implemented
+			 * even though it's optional in the CDC spec, and we don't
+			 * advertise it in the ACM functional descriptor.
+			 */
+			return 1;
 		}
-	case USB_CDC_REQ_SET_LINE_CODING:
-		if (*len < sizeof(struct usb_cdc_line_coding))
+		case USB_CDC_REQ_SET_LINE_CODING:
+			// ignore if length is wrong
+			if (*len < sizeof(struct usb_cdc_line_coding)) {
+				return 0;
+			}
+			// get the line coding
+			struct usb_cdc_line_coding *coding = (struct usb_cdc_line_coding *)*buf;
+			/* reset device is the data bits is set to 5
+			 * this is used to allowing rebooting the device in DFU mode for reflashing
+			 * to reset the device from the host you can use stty --file /dev/ttyACM0 115200 raw cs5
+			 */
+			if (coding->bDataBits==5) {
+				scb_reset_system(); // reset device
+				while (true); // wait for the reset to happen
+			}
+			return 1;
+		default:
 			return 0;
-		return 1;
 	}
 	return 0;
 }
@@ -238,13 +257,35 @@ static void cdcacm_data_rx_cb(usbd_device *usbd_dev, uint8_t ep)
 	}
 }
 
+static void cdcacm_data_tx_cb(usbd_device *usbd_dev, uint8_t ep)
+{
+	(void)ep;
+	(void)usbd_dev;
+
+	char usb_data[64] = {0}; // buffer to send data
+	uint16_t usb_length = 0; // length of transmitted data
+	
+	/* transmit data */
+	if (tx_used) { // copy received data
+		for (usb_length=0; usb_length<sizeof(usb_data) && usb_length<tx_used; usb_length++) { // only until buffer is full
+			usb_data[usb_length] = tx_buffer[(tx_i+usb_length)%sizeof(tx_buffer)]; // put data in transmit data
+		}
+		// this could lead to a lock down
+		// while(usbd_ep_write_packet(usb_device, 0x82, usb_data, usb_length)==0);
+		// this is less critical
+		uint8_t transmitted = usbd_ep_write_packet(usb_device, 0x82, usb_data, usb_length); // try to transmit data
+		tx_i = (tx_i+transmitted)%sizeof(rx_buffer); // update location on buffer
+		tx_used -= transmitted; // update used size
+	}
+}
+
 static void cdcacm_set_config(usbd_device *usbd_dev, uint16_t wValue)
 {
 	(void)wValue;
 	(void)usbd_dev;
 
 	usbd_ep_setup(usbd_dev, 0x01, USB_ENDPOINT_ATTR_BULK, 64, cdcacm_data_rx_cb);
-	usbd_ep_setup(usbd_dev, 0x82, USB_ENDPOINT_ATTR_BULK, 64, NULL);
+	usbd_ep_setup(usbd_dev, 0x82, USB_ENDPOINT_ATTR_BULK, 64, cdcacm_data_tx_cb);
 	usbd_ep_setup(usbd_dev, 0x83, USB_ENDPOINT_ATTR_INTERRUPT, 16, NULL);
 
 	usbd_register_control_callback( usbd_dev, USB_REQ_TYPE_CLASS | USB_REQ_TYPE_INTERFACE, USB_REQ_TYPE_TYPE | USB_REQ_TYPE_RECIPIENT, cdcacm_control_request);
@@ -272,13 +313,16 @@ void cdcacm_setup(void)
 	rx_i = 0;
 	rx_used = 0;
 	cdcacm_received = 0;
+	
+	/* start sending */
+	usbd_ep_write_packet(usb_device, 0x82, NULL, 0);
 }
 
 /* get character from USB CDC ACM (blocking) */
 char cdcacm_getchar(void)
 {
 	while (!rx_used) { // idle until data is available
-		__WFI(); // sleep until interrupt;
+		__WFI(); // sleep until interrupt (not sure if it's a good idea here
 	}
 	char to_return = rx_buffer[rx_i]; // get the next available character
 	rx_i = (rx_i+1)%sizeof(rx_buffer); // update used buffer
@@ -290,7 +334,14 @@ char cdcacm_getchar(void)
 /* put character on USB CDC ACM (blocking) */
 void cdcacm_putchar(char c)
 {
-	while(usbd_ep_write_packet(usb_device, 0x82, &c, 1)==0); // sending single characters at a time isn't very optimal, but I didn't find a way to do it USB request based
+	if (tx_used<sizeof(tx_buffer)) { // buffer not full
+		tx_buffer[(tx_i+tx_used)%sizeof(tx_buffer)] = c; // put character in buffer
+		tx_used++; // update used buffer
+	} else { // buffer full
+		tx_i = (tx_i+1)%sizeof(tx_buffer); // shift start
+		tx_buffer[(tx_i+tx_used)%sizeof(tx_buffer)] = c; // overwrite old data
+	}
+	usbd_ep_write_packet(usb_device, 0x82, NULL, 0); // trigger tx (not sure why cdcacm_data_tx_cb doesn't work else)
 }
 
 void usb_wakeup_isr(void) {
diff --git a/lib/vfd_hv518.c b/lib/vfd_hv518.c
new file mode 100644
index 0000000..03b9952
--- /dev/null
+++ b/lib/vfd_hv518.c
@@ -0,0 +1,523 @@
+/* This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+/* Copyright (c) 2016 King Kévin <kingkevin@cuvoodoo.info> */
+/* this library is used to drive the vacuum fluorescent display extracted from a Samsung SER-6500 cash register
+ * it uses three chained supertex HV518 shift register VFD drivers */
+
+/* standard libraries */
+#include <stdint.h> // standard integer types
+#include <stdlib.h> // general utilities
+
+/* STM32 (including CM3) libraries */
+#include <libopencm3/stm32/rcc.h> // real-time control clock library
+#include <libopencm3/stm32/gpio.h> // general purpose input output library
+#include <libopencm3/stm32/spi.h> // SPI library
+#include <libopencm3/stm32/timer.h> // timer library
+#include <libopencm3/cm3/nvic.h> // interrupt handler
+
+#include "global.h" // global definitions
+#include "vfd_hv518.h" // VFD library API
+
+/* supertex HV518 VFD driver pins */
+/* port on which the pins to control the supertex HV518 VFD driver are
+ * we use port A because of the SPI interface */
+#define VFD_PORT GPIOA
+#define VFD_PORT_RCC RCC_GPIOA
+/* SPI port to use */
+#define VFD_SPI SPI1
+#if (VFD_SPI==SPI1)
+#define VFD_SPI_RCC RCC_SPI1
+#define VFD_SPI_IRQ NVIC_SPI1_IRQ
+#elif (VFD_SPI==SPI2)
+#define VFD_SPI_RCC RCC_SPI2
+#define VFD_SPI_IRQ NVIC_SPI2_IRQ
+#endif
+/* strobe pin to enable high voltage output
+ * high voltage is output on low
+ * drive using a GPIO PA6 (normally MISO) */
+#define VFD_STR GPIO6
+/* latch enable pin
+ * store the shifted data on low
+ * output the parallel data on high
+ * use GPIO (PA4) (NSS does not work as SS) */
+#define VFD_NLE GPIO4
+/* clock signal
+ * drive using SPI SCK (PA5) */
+#define VFD_CLK GPIO_SPI1_SCK
+/* data input, where the data is shifted to
+ * drive using SPI MOSI (PA7) */
+#define VFD_DIN GPIO_SPI1_MOSI
+/* timer for automatic refresh */
+#define VFD_TIMER TIM2
+#if (VFD_TIMER==TIM2)
+#define VFD_TIMER_RCC RCC_TIM2
+#define VFD_TIMER_IRQ NVIC_TIM2_IRQ
+#elif (VFD_TIMER==TIM3)
+#define VFD_TIMER_RCC RCC_TIM3
+#define VFD_TIMER_IRQ NVIC_TIM3_IRQ
+#elif (VFD_TIMER==TIM4)
+#define VFD_TIMER_RCC RCC_TIM4
+#define VFD_TIMER_IRQ NVIC_TIM4_IRQ
+#elif (VFD_TIMER==TIM5)
+#define VFD_TIMER_RCC RCC_TIM5
+#define VFD_TIMER_IRQ NVIC_TIM5_IRQ
+#endif
+
+/* ASCII characters encoded for 7 segments display
+ * starts with space
+ */
+static const uint8_t ascii_7segments[] = {
+	0b00000000, // space
+	0b00110000, // ! (I)
+	0b00100010, // "
+	0b01011100, // # (o)
+	0b01101101, // $ (s)
+	0b01010010, // % (/)
+	0b01111101, // & (6)
+	0b00100000, // '
+	0b00111001, // ( ([)
+	0b00001111, // )
+	0b01110000, // *
+	0b01000110, // +
+	0b00010000, // ,
+	0b01000000, // -
+	0b00010000, // . (,)
+	0b01010010, // /
+	0b00111111, // 0
+	0b00000110, // 1
+	0b01011011, // 2
+	0b01001111, // 3
+	0b01100110, // 4
+	0b01101101, // 5
+	0b01111101, // 6
+	0b00000111, // 7
+	0b01111111, // 8
+	0b01101111, // 9
+	0b01001000, // : (=)
+	0b01001000, // ; (=)
+	0b01011000, // <
+	0b01001000, // =
+	0b01001100, // >
+	0b01010011, // ?
+	0b01111011, // @
+	0b01110111, // A
+	0b01111111, // B
+	0b00111001, // C
+	0b01011110, // D
+	0b01111001, // E
+	0b01110001, // F
+	0b00111101, // G
+	0b01110110, // H
+	0b00110000, // I
+	0b00011110, // J
+	0b01110110, // K
+	0b00111000, // L
+	0b00110111, // M
+	0b00110111, // N
+	0b00111111, // O
+	0b01110011, // P
+	0b01101011, // Q
+	0b00110011, // R
+	0b01101101, // S
+	0b01111000, // T
+	0b00111110, // U
+	0b00111110, // V (U)
+	0b00111110, // W (U)
+	0b01110110, // X (H)
+	0b01101110, // Y
+	0b01011011, // Z
+	0b00111001, // [
+	0b01100100, // '\'
+	0b00001111, // /
+	0b00100011, // ^
+	0b00001000, // _
+	0b00000010, // `
+	0b01011111, // a
+	0b01111100, // b
+	0b01011000, // c
+	0b01011110, // d
+	0b01111011, // e
+	0b01110001, // f
+	0b01101111, // g
+	0b01110100, // h
+	0b00010000, // i
+	0b00001100, // j
+	0b01110110, // k
+	0b00110000, // l
+	0b01010100, // m
+	0b01010100, // n
+	0b01011100, // o
+	0b01110011, // p
+	0b01100111, // q
+	0b01010000, // r
+	0b01101101, // s
+	0b01111000, // t
+	0b00011100, // u
+	0b00011100, // v (u)
+	0b00011100, // w (u)
+	0b01110110, // x
+	0b01101110, // y
+	0b01011011, // z
+	0b00111001, // { ([)
+	0b00110000, // |
+	0b00001111, // } ([)
+	0b01000000, // ~
+};
+
+/* font for the 5x7 dot matrix display
+ * from http://sunge.awardspace.com/glcd-sd/node4.html
+ * first value is left-most line
+ * LSB is top dot, MSB is not used
+ */
+static const uint8_t font5x7[][5] = {
+	{0x00, 0x00, 0x00, 0x00, 0x00}, // (space)
+	{0x00, 0x00, 0x5F, 0x00, 0x00}, // !
+	{0x00, 0x07, 0x00, 0x07, 0x00}, // "
+	{0x14, 0x7F, 0x14, 0x7F, 0x14}, // #
+	{0x24, 0x2A, 0x7F, 0x2A, 0x12}, // $
+	{0x23, 0x13, 0x08, 0x64, 0x62}, // %
+	{0x36, 0x49, 0x55, 0x22, 0x50}, // &
+	{0x00, 0x05, 0x03, 0x00, 0x00}, // '
+	{0x00, 0x1C, 0x22, 0x41, 0x00}, // (
+	{0x00, 0x41, 0x22, 0x1C, 0x00}, // )
+	{0x08, 0x2A, 0x1C, 0x2A, 0x08}, // *
+	{0x08, 0x08, 0x3E, 0x08, 0x08}, // +
+	{0x00, 0x50, 0x30, 0x00, 0x00}, // ,
+	{0x08, 0x08, 0x08, 0x08, 0x08}, // -
+	{0x00, 0x60, 0x60, 0x00, 0x00}, // .
+	{0x20, 0x10, 0x08, 0x04, 0x02}, // /
+	{0x3E, 0x51, 0x49, 0x45, 0x3E}, // 0
+	{0x00, 0x42, 0x7F, 0x40, 0x00}, // 1
+	{0x42, 0x61, 0x51, 0x49, 0x46}, // 2
+	{0x21, 0x41, 0x45, 0x4B, 0x31}, // 3
+	{0x18, 0x14, 0x12, 0x7F, 0x10}, // 4
+	{0x27, 0x45, 0x45, 0x45, 0x39}, // 5
+	{0x3C, 0x4A, 0x49, 0x49, 0x30}, // 6
+	{0x01, 0x71, 0x09, 0x05, 0x03}, // 7
+	{0x36, 0x49, 0x49, 0x49, 0x36}, // 8
+	{0x06, 0x49, 0x49, 0x29, 0x1E}, // 9
+	{0x00, 0x36, 0x36, 0x00, 0x00}, // :
+	{0x00, 0x56, 0x36, 0x00, 0x00}, // ;
+	{0x00, 0x08, 0x14, 0x22, 0x41}, // <
+	{0x14, 0x14, 0x14, 0x14, 0x14}, // =
+	{0x41, 0x22, 0x14, 0x08, 0x00}, // >
+	{0x02, 0x01, 0x51, 0x09, 0x06}, // ?
+	{0x32, 0x49, 0x79, 0x41, 0x3E}, // @
+	{0x7E, 0x11, 0x11, 0x11, 0x7E}, // A
+	{0x7F, 0x49, 0x49, 0x49, 0x36}, // B
+	{0x3E, 0x41, 0x41, 0x41, 0x22}, // C
+	{0x7F, 0x41, 0x41, 0x22, 0x1C}, // D
+	{0x7F, 0x49, 0x49, 0x49, 0x41}, // E
+	{0x7F, 0x09, 0x09, 0x01, 0x01}, // F
+	{0x3E, 0x41, 0x41, 0x51, 0x32}, // G
+	{0x7F, 0x08, 0x08, 0x08, 0x7F}, // H
+	{0x00, 0x41, 0x7F, 0x41, 0x00}, // I
+	{0x20, 0x40, 0x41, 0x3F, 0x01}, // J
+	{0x7F, 0x08, 0x14, 0x22, 0x41}, // K
+	{0x7F, 0x40, 0x40, 0x40, 0x40}, // L
+	{0x7F, 0x02, 0x04, 0x02, 0x7F}, // M
+	{0x7F, 0x04, 0x08, 0x10, 0x7F}, // N
+	{0x3E, 0x41, 0x41, 0x41, 0x3E}, // O
+	{0x7F, 0x09, 0x09, 0x09, 0x06}, // P
+	{0x3E, 0x41, 0x51, 0x21, 0x5E}, // Q
+	{0x7F, 0x09, 0x19, 0x29, 0x46}, // R
+	{0x46, 0x49, 0x49, 0x49, 0x31}, // S
+	{0x01, 0x01, 0x7F, 0x01, 0x01}, // T
+	{0x3F, 0x40, 0x40, 0x40, 0x3F}, // U
+	{0x1F, 0x20, 0x40, 0x20, 0x1F}, // V
+	{0x7F, 0x20, 0x18, 0x20, 0x7F}, // W
+	{0x63, 0x14, 0x08, 0x14, 0x63}, // X
+	{0x03, 0x04, 0x78, 0x04, 0x03}, // Y
+	{0x61, 0x51, 0x49, 0x45, 0x43}, // Z
+	{0x00, 0x00, 0x7F, 0x41, 0x41}, // [
+	{0x02, 0x04, 0x08, 0x10, 0x20}, // '\'
+	{0x41, 0x41, 0x7F, 0x00, 0x00}, // ]
+	{0x04, 0x02, 0x01, 0x02, 0x04}, // ^
+	{0x40, 0x40, 0x40, 0x40, 0x40}, // _
+	{0x00, 0x01, 0x02, 0x04, 0x00}, // `
+	{0x20, 0x54, 0x54, 0x54, 0x78}, // a
+	{0x7F, 0x48, 0x44, 0x44, 0x38}, // b
+	{0x38, 0x44, 0x44, 0x44, 0x20}, // c
+	{0x38, 0x44, 0x44, 0x48, 0x7F}, // d
+	{0x38, 0x54, 0x54, 0x54, 0x18}, // e
+	{0x08, 0x7E, 0x09, 0x01, 0x02}, // f
+	{0x08, 0x14, 0x54, 0x54, 0x3C}, // g
+	{0x7F, 0x08, 0x04, 0x04, 0x78}, // h
+	{0x00, 0x44, 0x7D, 0x40, 0x00}, // i
+	{0x20, 0x40, 0x44, 0x3D, 0x00}, // j
+	{0x00, 0x7F, 0x10, 0x28, 0x44}, // k
+	{0x00, 0x41, 0x7F, 0x40, 0x00}, // l
+	{0x7C, 0x04, 0x18, 0x04, 0x78}, // m
+	{0x7C, 0x08, 0x04, 0x04, 0x78}, // n
+	{0x38, 0x44, 0x44, 0x44, 0x38}, // o
+	{0x7C, 0x14, 0x14, 0x14, 0x08}, // p
+	{0x08, 0x14, 0x14, 0x18, 0x7C}, // q
+	{0x7C, 0x08, 0x04, 0x04, 0x08}, // r
+	{0x48, 0x54, 0x54, 0x54, 0x20}, // s
+	{0x04, 0x3F, 0x44, 0x40, 0x20}, // t
+	{0x3C, 0x40, 0x40, 0x20, 0x7C}, // u
+	{0x1C, 0x20, 0x40, 0x20, 0x1C}, // v
+	{0x3C, 0x40, 0x30, 0x40, 0x3C}, // w
+	{0x44, 0x28, 0x10, 0x28, 0x44}, // x
+	{0x0C, 0x50, 0x50, 0x50, 0x3C}, // y
+	{0x44, 0x64, 0x54, 0x4C, 0x44}, // z
+	{0x00, 0x08, 0x36, 0x41, 0x00}, // {
+	{0x00, 0x00, 0x7F, 0x00, 0x00}, // |
+	{0x00, 0x41, 0x36, 0x08, 0x00}, // }
+	{0b00001000, 0b00000100, 0b00001100, 0b00001000, 0b00000100} // ~
+};
+
+/* pictures for the 5x7 dot matrix display
+ * first value is left-most line
+ * LSB is top dot, MSB is not used
+ */
+static const uint8_t pict5x7[][5] = {
+	{0x08, 0x08, 0x2A, 0x1C, 0x08}, // ->
+	{0x08, 0x1C, 0x2A, 0x08, 0x08}, // <-
+	{0b01110000, 0b01110000, 0b01111010, 0b01111100, 0b01011000}, // bunny side 1
+	{0b00100000, 0b01110000, 0b01110010, 0b01111100, 0b01011000}, // bunny side 2
+	{0b00111110, 0b01001001, 0b01010110, 0b01001001, 0b00111110}, // bunny face 1
+	{0b00111110, 0b01010001, 0b01100110, 0b01010001, 0b00111110}, // bunny face 2
+	{0b00111000, 0b01010111, 0b01100100, 0b01010111, 0b00111000}, // bunny face 3
+	{0b00111000, 0b01001111, 0b01010100, 0b01001111, 0b00111000}, // bunny face 4
+	{0b00111000, 0b01011110, 0b01101000, 0b01011110, 0b00111000}, // bunny face 5
+	{0b01000001, 0b00110110, 0b00001000, 0b00110110, 0b01000001}, // cross 1
+	{~0b01000001, ~0b00110110, ~0b00001000, ~0b00110110, ~0b01000001}, // cross 1 negated
+	{0b00100010, 0b00010100, 0b00001000, 0b00010100, 0b00100010}, // cross 2
+	{~0b00100010, ~0b00010100, ~0b00001000, ~0b00010100, ~0b00100010}, // cross 2 negated
+	{0x00, 0x00, 0x00, 0x00, 0x00} // nothing
+};
+
+/* the 32 bits values to be shifted out to the VFD driver
+ * split into 16 bit for SPI transfer
+ * since the bits for digits and matrix are independent, they can be combined
+ * we have more matrix (12) than digits (10)
+ */
+static uint16_t driver_data[VFD_MATRIX][VFD_DRIVERS*2] = {0};
+static volatile uint8_t spi_i = 0; // which driver data is being transmitted
+static volatile uint8_t vfd_grid = 0; // which grid/part to activate (single digits and matrix can be combined)
+static const uint32_t digit_mask = 0x00fffff0; // the bits used for selecting then digit and 7 segment anodes (for the second driver)
+
+/* set digit <nb> to ASCII character <c>
+ * use the MSB of <c> to enable the dot */
+void vfd_digit(uint8_t nb, char c)
+{
+	if (!(nb<VFD_DIGITS)) { // check the digit exists
+		return;
+	}
+
+	uint32_t digit_data = 0; // the data to be shifted out for the driver (for the second driver)
+	
+	digit_data = 1<<(4+(9-nb)); // select digit
+	/* encode segment
+	 * here the bit order (classic 7 segment + underline and dot)
+	 *   3_
+	 * 8|9_|4
+	 * 7|6_|5.1
+	 *   0_2,
+	 * */
+	if (false) { // add the underline (not encoded)
+		digit_data |= (1<<(14));
+	}
+	if (c&0x80) { // add the dot (encoded in the 8th bit)
+		digit_data |= (1<<(15));
+	}
+	if (false) { // add the comma (not encoded)
+		digit_data |= (1<<(16));
+	}
+	
+	c &= 0x7f; // only take the ASCII part
+	if (c>=' ') { // only take printable characters
+		uint8_t i = c-' '; // get index for character
+		if (i<LENGTH(ascii_7segments)) {
+			digit_data |= (ascii_7segments[i]<<(17)); // add encoded segments to memory
+		}
+	}
+
+	digit_data &= digit_mask; // be sure only the bits for the digit are used
+	digit_data |= (driver_data[nb][2]+(driver_data[nb][3]<<16))&~digit_mask; // get the existing data and add the bits for the digit
+	driver_data[nb][2] = digit_data; // write back data (least significant half)
+	driver_data[nb][3] = (digit_data>>16); // write back data (most significant half)
+}
+
+/* set dot matrix <nb> to ASCII character <c>
+ * non ASCII characters are used for pictures */
+void vfd_matrix(uint8_t nb, char c)
+{
+	// check the matrix exists
+	if (!(nb<VFD_MATRIX)) { 
+		return;
+	} 
+	
+	uint32_t matrix_data[VFD_DRIVERS] = {0}; // the data to be shifted out for the driver
+	
+	// select matrix
+	if (nb<4) {
+		matrix_data[1] = 1<<(3-nb); 
+	} else {
+		matrix_data[0] = 1<<(35-nb);
+	}
+	
+	if ((c<0x80) && (c>=' ')) { // only take printable characters
+		uint8_t i = c-' '; // get index for character
+		if (i<LENGTH(font5x7)) {
+			matrix_data[1] |= font5x7[i][0]<<24;
+			matrix_data[2] |= font5x7[i][1]<<0;
+			matrix_data[2] |= font5x7[i][2]<<8;
+			matrix_data[2] |= font5x7[i][3]<<16;
+			matrix_data[2] |= font5x7[i][4]<<24;
+		}
+	} else if (c>0x7f) { // the non ASCII character are used for pictures
+		uint8_t i = c-0x80; // get index for character
+		if (i<LENGTH(pict5x7)) {
+			matrix_data[1] |= pict5x7[i][0]<<24;
+			matrix_data[2] |= pict5x7[i][1]<<0;
+			matrix_data[2] |= pict5x7[i][2]<<8;
+			matrix_data[2] |= pict5x7[i][3]<<16;
+			matrix_data[2] |= pict5x7[i][4]<<24;
+		}
+	}
+
+	matrix_data[1] &= ~digit_mask; // be sure only the bits for the matrix are used
+	matrix_data[1] |= (driver_data[nb][2]+(driver_data[nb][3]<<16))&digit_mask; // get the existing data for the digit
+	// prepare the data for SPI to shift it out
+	for (uint8_t i=0; i<LENGTH(matrix_data); i++) {
+		driver_data[nb][i*2] = matrix_data[i];
+		driver_data[nb][i*2+1] = matrix_data[i]>>16;
+	}
+}
+
+/* clear VFD display */
+void vfd_clear(void)
+{
+	for (uint8_t i=0; i<LENGTH(driver_data); i++) {
+		for (uint8_t j=0; j<LENGTH(driver_data[0]); j++) {
+			driver_data[i][j] = 0;
+		}
+	}
+}
+
+/* test VFD display (light up all anodes) */
+void vfd_test(void)
+{
+	for (uint8_t i=0; i<LENGTH(driver_data); i++) {
+		for (uint8_t j=0; j<LENGTH(driver_data[0]); j++) {
+			driver_data[i][j] = ~0;
+		}
+	}
+}
+
+/* switch VFD display on */
+void vfd_on(void)
+{
+	gpio_clear(VFD_PORT, VFD_STR); // enable HV output
+	timer_enable_counter(VFD_TIMER); // start timer to periodically output that to the parts
+}
+
+/* switch VFD display off */
+void vfd_off(void)
+{
+	gpio_set(VFD_PORT, VFD_STR); // disable HV output
+	timer_disable_counter(VFD_TIMER); // stop timer to periodically output that to the parts
+}
+
+/* setup VFD */
+void vfd_setup(void)
+{
+	/* setup GPIO to control the VFD */
+	rcc_periph_clock_enable(VFD_PORT_RCC); // enable clock for VFD GPIO
+	gpio_set_mode(VFD_PORT, GPIO_MODE_OUTPUT_50_MHZ, GPIO_CNF_OUTPUT_PUSHPULL, VFD_STR); // set VFD pin to output push-pull
+	gpio_set_mode(VFD_PORT, GPIO_MODE_OUTPUT_50_MHZ, GPIO_CNF_OUTPUT_PUSHPULL, VFD_NLE); // set VFD pin to output push-pull
+	
+	gpio_set(VFD_PORT, VFD_STR); // disable HV output
+	gpio_clear(VFD_PORT, VFD_NLE); // do not output latched data
+	
+	/* setup SPI to transmit data */
+	rcc_periph_clock_enable(VFD_SPI_RCC); // enable SPI clock
+	gpio_set_mode(VFD_PORT, GPIO_MODE_OUTPUT_50_MHZ, GPIO_CNF_OUTPUT_ALTFN_PUSHPULL, VFD_CLK); // set VFD pin to alternative function push-pull
+	gpio_set_mode(VFD_PORT, GPIO_MODE_OUTPUT_50_MHZ, GPIO_CNF_OUTPUT_ALTFN_PUSHPULL, VFD_DIN); // set VFD pin to alternative function push-pull
+
+	spi_reset(VFD_SPI); // clear SPI values
+	/* set SPI:
+	 * - use VFD_SPI port
+	 * - divide clock by 8 for generating the baudrate (F_PCLK1 is 36MHz, max HV518 is 6MHz)
+	 * - clock idle high polarity
+	 * - data is valid on rising edge (second clock phase)
+	 * - send 16 bits at a time
+	 * - send least significant bit first (that's how I coded the data)
+	 */
+	spi_init_master(VFD_SPI, SPI_CR1_BAUDRATE_FPCLK_DIV_8, SPI_CR1_CPOL_CLK_TO_1_WHEN_IDLE, SPI_CR1_CPHA_CLK_TRANSITION_2, SPI_CR1_DFF_16BIT, SPI_CR1_LSBFIRST);
+	//spi_set_bidirectional_transmit_only_mode(VFD_SPI); // only use MOSI to transmit
+	spi_set_unidirectional_mode(VFD_SPI); // MISO is unused
+	/* set NSS high to enable transmission
+	 * the NSS in STM32 can not be used as hardware slave select
+	 * RM0008 reference manual 25.3.1 is misleading
+	 * when hardware NSS is used and output is enabled NSS never goes up after transmission, even if SPI is disabled
+	 * when software NSS is used, NSS can not be set high again, even when writing to the register
+	 * the slave select must be done manually using GPIO */
+	spi_enable_software_slave_management(VFD_SPI); 
+	spi_set_nss_high(VFD_SPI); // set NSS high
+	
+    nvic_enable_irq(VFD_SPI_IRQ); // enable SPI interrupt
+	spi_enable(VFD_SPI); // enable SPI (the tx empty interrupt will trigger)
+	
+	/* setup timer to refresh display */
+	rcc_periph_clock_enable(VFD_TIMER_RCC); // enable clock for timer block
+	timer_reset(VFD_TIMER); // reset timer state
+	timer_set_mode(VFD_TIMER, TIM_CR1_CKD_CK_INT, TIM_CR1_CMS_EDGE, TIM_CR1_DIR_UP); // set timer mode, use undivided timer clock,edge alignment (simple count), and count up
+	timer_set_prescaler(VFD_TIMER, (SYSTEM_CLOCK_FREQ/(1<<16))-1); // set the prescaler so this 16 bits timer overflows at 1Hz
+	timer_set_period(VFD_TIMER, 0xffff/LENGTH(driver_data)/100); // set the refresh frequency
+	timer_enable_irq(VFD_TIMER, TIM_DIER_UIE); // enable interrupt for timer
+	nvic_enable_irq(VFD_TIMER_IRQ); // allow interrupt for timer
+	
+	vfd_clear(); // initialize values
+}
+
+#if (VFD_SPI==SPI1)
+void spi1_isr(void)
+#elif (VFD_SPI==SPI2)
+void spi2_isr(void)
+#endif
+{
+	if (SPI_SR(VFD_SPI) & SPI_SR_TXE) { // transmission buffer empty
+		if (spi_i<LENGTH(driver_data[0])) { // check if data is available
+			gpio_clear(VFD_PORT, VFD_NLE); // slave select to latch data
+			spi_send(VFD_SPI, driver_data[vfd_grid][spi_i++]); // send next data
+		} else { // all data transmitted
+			spi_disable_tx_buffer_empty_interrupt(VFD_SPI); // no need to wait for new data
+			while (SPI_SR(VFD_SPI) & SPI_SR_BSY); // wait for data to be shifted out
+			spi_disable_tx_buffer_empty_interrupt(VFD_SPI); // no need to wait for new data
+			gpio_set(VFD_PORT, VFD_NLE); // output latched data
+		}
+	}
+}
+
+#if (VFD_TIMER==TIM2)
+void tim2_isr(void)
+#elif (VFD_TIMER==TIM3)
+void tim3_isr(void)
+#elif (VFD_TIMER==TIM4)
+void tim4_isr(void)
+#elif (VFD_TIMER==TIM5)
+void tim5_isr(void)
+#endif
+{
+	if (timer_get_flag(VFD_TIMER, TIM_SR_UIF)) { // overflow even happened
+		timer_clear_flag(VFD_TIMER, TIM_SR_UIF); // clear flag
+    	spi_i = 0; // set the register to shift out
+		spi_enable_tx_buffer_empty_interrupt(VFD_SPI); // enable TX empty interrupt
+		vfd_grid = (vfd_grid+1)%LENGTH(driver_data); // got to next segment
+	}
+}
diff --git a/lib/vfd_hv518.h b/lib/vfd_hv518.h
new file mode 100644
index 0000000..7e6db7f
--- /dev/null
+++ b/lib/vfd_hv518.h
@@ -0,0 +1,39 @@
+/* This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+/* Copyright (c) 2016 King Kévin <kingkevin@cuvoodoo.info> */
+/* this library is used to drive the vacuum fluorescent display extracted from a Samsung SER-6500 cash register
+ * it uses three chained supertex HV518 shift register VFD drivers */
+
+/* the number of blocks available on the VFD */
+#define VFD_DRIVERS 3
+#define VFD_DIGITS 10
+#define VFD_MATRIX 12
+
+/* set digit <nb> to ASCII character <c>
+ * use the MSB of <c> to enable the dot */
+void vfd_digit(uint8_t nb, char c);
+/* set dot matrix <nb> to ASCII character <c>
+ * non ASCII characters are used for pictures */
+void vfd_matrix(uint8_t nb, char c);
+/* clear VFD display */
+void vfd_clear(void);
+/* test VFD display (light up all anodes) */
+void vfd_test(void);
+/* transmit every digit and matrix */
+void vfd_on(void);
+/* switch VFD display off */
+void vfd_off(void);
+/* setup VFD */
+void vfd_setup(void);
diff --git a/main.c b/main.c
index f8ecd15..1e957a2 100644
--- a/main.c
+++ b/main.c
@@ -28,7 +28,7 @@
 #include <libopencmsis/core_cm3.h> // Cortex M3 utilities
 
 /* own libraries */
-#include "main.h" // board definitions
+#include "global.h" // board definitions
 #include "usart.h" // USART utilities
 #include "usb_cdcacm.h" // USB CDC ACM utilities