Author Archives: vonger

VoCore2 SPI: full duplex

mt7628a has a wafer/silicon bug on SPI, we can not use its SPI in full duplex mode, but we can use bitbang for that mode. For some device do not support half-duplex, maybe this is the only way.

To enable bitbang on VoCore2, we need to modify Linux kernel code, VOCORE2.DTS and kernel setting.

Setup Kernel Setting

  1. call “make kernel_menuconfig”
  2. In menu, select “Device Drivers” => “SPI Support” => <*>   GPIO-based bitbanging SPI Master


This file is in target/linux/ramips/dts, example of the DTS. We need to set the driver to “spi-gpio” and pinctrl set the pins to GPIO mode.


#include "VOCORE2.dtsi"

#include <dt-bindings/gpio/gpio.h>

/ {
	compatible = "vocore,vocore2", "mediatek,mt7628an-soc";
	model = "VoCore2";

	gpio-leds {
		compatible = "gpio-leds";

		status {
			label = "vocore2:fuchsia:status";
			gpios = <&gpio1 12 GPIO_ACTIVE_HIGH>;

	spi1: spi-gpio {
		status = "okay";

		compatible = "spi-gpio";

		gpio-sck = <&gpio0 7 1>;
        	gpio-miso = <&gpio0 9 1>;
        	gpio-mosi = <&gpio0 8 1>;

        	cs-gpios = <&gpio0 10 1>, <&gpio0 6 1>;
        	num-chipselects = <2>;

&pinctrl {
        state_default: pinctrl0 {
                spi {
                        ralink,group = "spi";
                        ralink,function = "gpio";
                spi_cs1 {
                        ralink,group = "spi cs1";
                        ralink,function = "gpio";

&i2c {
	status = "okay";

&spi1 {
	m25p80@0 {
		#address-cells = <1>;
		#size-cells = <1>;
		compatible = "jedec,spi-nor";
		reg = <0>;
		spi-max-frequency = <10000000>;
		m25p,chunked-io = <32>;

		partition@0 {
			label = "u-boot";
			reg = <0x0 0x30000>;

		partition@30000 {
			label = "u-boot-env";
			reg = <0x30000 0x10000>;

		factory: partition@40000 {
			label = "factory";
			reg = <0x40000 0x10000>;

		partition@50000 {
			label = "firmware";
			reg = <0x50000 0xfb0000>;

	spidev@1 {
		#address-cells = <1>;
		#size-cells = <1>;
		compatible = "rohm,dh2228fv";
		reg = <1>;
		spi-max-frequency = <10000000>;

Patch Linux spi-gpio.c

This is because CS1 pins has a default pull low resistor. If we do not patch it, CS1 will be low at startup, conflict with the flash driver who use CS0. So we must at startup setup CS1 pin to output and value to high.

                if (!SPI_N_CHIPSEL)
                        spi_gpio->cs_gpios[0] = SPI_GPIO_NO_CHIPSELECT;
                        for (i = 0; i < SPI_N_CHIPSEL; i++) {
                                status = of_get_named_gpio(np, "cs-gpios", i);
                                if (status < 0) {
                                                "invalid cs-gpios property\n");
                                        goto gpio_free;
                                spi_gpio->cs_gpios[i] = status;

+                               gpio_request(status, dev_name(&pdev->dev));
+                               gpio_direction_output(status, 1);
+                               gpio_free(status);

        spi_gpio->bitbang.master = master;
        spi_gpio->bitbang.chipselect = spi_gpio_chipselect;

That is all 🙂 Now the firmware is able to support SPI device in full duplex mode.

Submit patch for OpenWrt on Github

Git is good, but for beginner like me, always feeling no way to give the first bite. I am confused about fork, branch, signed-off-by, push, merge and many other commands, maybe it is pretty clear for the clever boys but not for me. Thanks to adrianschmutzler comment at, I start to understand how it works.


  1. click the “Fork” button on
  2. goto your forked openwrt, click on “Clone or download”, get the address and clone to your local.
  3. make the change on your local source code.
  4. setup your name and email on your local by command git config –global and git config –global “your name”
  5. submit your change by command “git commit -s“, remember to keep that Signed-off-By line and add the reason of the changes to comment.
  6. push the commit by command “git push origin
  7. finally, go to github, create a merge request.

Signed-Off-By is just a text line in the comment to tell other who submit that patch, I am confused about that a long time, I thought it is added by some magic :p

PS: In old good time I only use git init, git commit and git clone. Now good new day comes.

VoCore2: OpenWrt 19.07.2

Recently I try latest stable version openwrt, looks like the open source MT76 driver is much stable and fast now.

iperf3 -c
Connecting to host, port 5201
[  5] local port 60121 connected to port 5201
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec  5.33 MBytes  44.7 Mbits/sec                  
[  5]   1.00-2.00   sec  5.50 MBytes  46.1 Mbits/sec                  
[  5]   2.00-3.00   sec  5.34 MBytes  44.7 Mbits/sec                  
[  5]   3.00-4.00   sec  4.67 MBytes  39.3 Mbits/sec                  
[  5]   4.00-5.00   sec  5.47 MBytes  45.8 Mbits/sec                  
[  5]   5.00-6.00   sec  5.03 MBytes  42.3 Mbits/sec                  
[  5]   6.00-7.00   sec  4.93 MBytes  41.3 Mbits/sec                  
[  5]   7.00-8.00   sec  5.07 MBytes  42.6 Mbits/sec                  
[  5]   8.00-9.00   sec  4.95 MBytes  41.5 Mbits/sec                  
[  5]   9.00-10.00  sec  5.13 MBytes  43.0 Mbits/sec                  
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-10.00  sec  51.4 MBytes  43.1 Mbits/sec                  sender
[  5]   0.00-10.09  sec  51.3 MBytes  42.7 Mbits/sec                  receiver

iperf Done.

This is good 🙂 Next firmware will base on this driver and I do not have to patch it anymore.

VoCore2: About Pin Share

Pin share or I call it pinmux, it is a common feature of most MCU chips. Normally a pin we can use it as GPIO or some special functions.

In picture, the orange number is each pins GPIO number. And in the color block, there is its special function.

Here is a table from, for example, the pin 29 on the chip has three functions, one is used as GPIO, one is system reference clock, final one is SDcard CLK.

+3.3VO+3.3V outputmax output current 100mA
30TXD0GPIO12, UART0 Lite TXDpull down to GND, Test/Normal mode pin.
31RXD0GPIO13, UART0 Lite RXD3.3V
29REFCLKGPIO11, reference clock, SDXC CLK*
33P0RP10/100 PHY Port #0 RXP
34P0RN10/100 PHY Port #0 RXN
35P0TP10/100 PHY Port #0 TXP
36P0TN10/100 PHY Port #0 TXN
43TXD210/100 PHY Port #1 TXN, GPIO15, PWM Channel, SPI Slave Clock
44RXD210/100 PHY Port #1 TXP, GPIO14, PWM Channel, SPI Slave Chip Select
40PWM010/100 PHY Port #1 RXN, GPIO17, UART2 Lite TXD, SPI Slave MOSI4.7K pull down to GND
42PWM110/100 PHY Port #1 RXP, GPIO16, UART2 Lite RXD, SPI Slave MISO
45P2RP10/100 PHY Port #2 RXP, GPIO18, PWM Channel
46P2RN10/100 PHY Port #2 RXN, GPIO19, PWM Channel
47P2TP10/100 PHY Port #2 TXP, GPIO20, PWM Channel
48P2TN10/100 PHY Port #2 TXN, GPIO21, PWM Channel
49SD WPGPIO22, 10/100 PHY Port #3 TXP, SDXC WP
50SD CDGPIO23, 10/100 PHY Port #3 TXN, SDXC CD
28SPI CS0SPI Master Chip Select 04.7K pull up to 3.3V
25SPI CLKSPI Master Clock4.7K pull up to 3.3V
26SPI MISOSPI Master In Slave Out
27SPI MOSISPI Master Out Slave In4.7K pull down to GND
24SPI CS1SPI Master Chip Select 14.7K pull down to GND
21I2C SDGPIO5, I2C Data, SDXC D2*4.7K pull up to 3.3V
20I2C CLKGPIO4, I2C Clock, SDXC D3*4.7K pull up to 3.3V
19I2S CLKGPIO3, I2S Bit Clock, SDXC CMD*
18I2S WSGPIO2, I2S L/R Clock, SDXC D0*
16I2S SDIGPIO0, I2S Data In, SDXC D1*
17I2S SDOGPIO1, I2S Data Out4.7K pull down to GND
147TXD1GPIO13, UART1 Lite TXDpull up to 3.3V, GPIO/JTAG mode pin
141GPIO41GPIO41, JTAG TMSJTAG needs 10K pull up, R9 -> R6
140GPIO40GPIO40, JTAG ClockJTAG needs 10K pull up, R9 -> R6
139GPIO39GPIO39, JTAG ResetJTAG needs 10K pull up, R9 -> R6
143GPIO43GPIO43, JTAG TDOJTAG needs 10K pull up, R9 -> R6
142GPIO42GPIO42, JTAG TDIJTAG needs 10K pull up, R9 -> R6
138RSTReset, touch GND to reset
+3.3VO+3.3V outputmax output current 100mA
+1.8VO+1.8V outputmax output current 100mA
51SD D1GPIO24, 10/100 PHY Port #3 RXP, SDXC D1
52SD D0GPIO25, 10/100 PHY Port #3 RXN, SDXC D0
54SD CLKGPIO26, 10/100 PHY Port #4 RXP, SDXC CLK
55SD CMDGPIO27, 10/100 PHY Port #4 RXN, SDXC CMD
56SD D3GPIO28, 10/100 PHY Port #4 TXP, SDXC D3
57SD D2GPIO29, 10/100 PHY Port #4 TXN, SDXC D2
+3.3VO+3.3V outputmax output current 100mA
61USB DPUSB2.0 Data+
62USB DMUSB2.0 Data-
+5.0VI+5.0V inputmin input current 500mA
+5.0VI+5.0V inputmin input current 500mA
133PCI CKPPCI Express External Reference Clock Output+
132PCI CKNPCI Express External Reference Clock Output-
130PCI RXNPCI Express Differential Receiver RX-
129PCI RXPPCI Express Differential Receiver RX+
127PCI TXPPCI Express Differential Transmit TX+
126PCI TXNPCI Express Differential Transmit TX-
135PCI RSTPCI Express Device Reset

On VoCore2 Ultimate, ES8388 do not need external clock because we can use VoCore2 system reference clock as its source, in most situation, that will save cost of the BOM.

Set the Pin Function

now, question come, how do we setup every pins function? Sometimes you may want use it as GPIO but sometimes we need to use as clock.

Actually it is same as other MCU(like STM32 and ATMEGA–Arduino) , we have register for every function.

Use this REFCLK pin as example, we can find on MT7628AN datasheet page 117, 2.2 System Control Section, 0x10000060 is that control register.

Its register is 1:0 two bits. (note: on this datasheet, its name is GPIO mode)

We can write 10b to this register, then it will be REFCLK mode.

and after it into REFCLK mode, we can change its frequency, at page 110, 0x1000002c register, bit 11:9 REFCLK0_RATE

And you can use same way to change other pins function 🙂

Also I have a simple tool to do this, please download at, Directly write to memory/register Section.

VoCore2 + VoCam264: Push Video Stream

mjpeg-streamer is an easy way, but it only able to push mjpeg data, it is a waste because VoCam264 already has h264 stream inside.

So I plan to push h264 data to a RTMP server like nginx-rtmp(-module).

The first way in my mind is to use ffmpeg directly.

Just one command, and it should work: ffmpeg -re -f v4l2 -i /dev/video1 -c:v copy -f flv rtmp://

My server is setup at rtmp://

Note1: old version ffmpeg do not work well, I am using version 4.2.2. Currently openwrt package has ffmpeg, but it is 3.4.2, we must modify the Makefile and compile again to use latest version. I ave upload Makefile to, in utils folder.

Trying to set codec:h264 pix_fmt:none
parser not found for codec h264, packets or times may be invalid.

Note2: default Makefile will show error about parser, we need to enable h264 parser.

Then ffmpeg is able to push h264 video from camera to server.

Unfortunately, the speed is very slow and video is laggy…Maybe there is something wrong, because it works well on a Linux PC..

I have to find some other way.

My compiled ffmpeg ipk download at here:

To be continue…

VoCore2 Ultimate: AD/DA usage 2

Forget Keil, I love GCC. Makefile is enough for the ARM based chip GD32F150 🙂

Link to my source code:

To get AD values from UART2. Here is the Makefile. Source code please use project/adc1.

NAME = $(notdir $(CURDIR))
CMSIS = $(CURDIR)/../../GD32F1x0_Firmware_Library_v3.1.0/Firmware/CMSIS
PERIP = $(CURDIR)/../../GD32F1x0_Firmware_Library_v3.1.0/Firmware/GD32F1x0_standard_peripheral
TOOLCHAIN = $(CURDIR)/../../toolchain/mac/bin/arm-none-eabi

CP = $(TOOLCHAIN)-objcopy


	-I$(CURDIR)/../core \
	-I$(CMSIS)/GD/GD32F1x0/Include \
	-I$(PERIP)/Include \

	$(CURDIR)/../core/core_cm3.c \
	$(CURDIR)/../core/startup_gd32f1x0.s \
	$(CMSIS)/GD/GD32F1x0/Source/system_gd32f1x0.c \
	$(wildcard $(PERIP)/Source/*.c) \
	$(wildcard $(CURDIR)/*.c)

OBJECTS = $(SOURCES:%.c=%.o)

	-mcpu=cortex-m3 -mthumb -mlittle-endian -mthumb-interwork \
	-ffast-math -fdata-sections -ffunction-sections \
	-Wl,-T,$(CURDIR)/../core/gd32f150g8.ld,-Map,$(NAME).map,--gc-sections \
	-Wall -std=gnu99 -O2 $(DEFINES) $(INCLUDES) 

	@$(CC) $(CFLAGS) $^ -lm -lnosys -o $(CURDIR)/$@
	@$(CP) -O ihex $(CURDIR)/$@ $(CURDIR)/$@.hex

	@echo $(OBJECTS)

	@rm -f $(CURDIR)/$(NAME)
	@rm -f $(CURDIR)/$(NAME).map

GD32F1x0_Firmware_Library_v3.1.0 this library is from GigaDevice. Download here:; For toolchain, please check, I have a link there.

The library is for Keil originally. I make some patches, all changes are in project/core folder.

  • startup_gd32f1x0.s this mainly change the interrupt vector, we need the interrupt callback function name same.
  • gd32f150g8.ld this is used to create bin or hex file which is used to load into flash.

For VoCore2 Ultimate, we use internal 8M clock, so comment the line 46 #define __SYSTEM_CLOCK_72M_PLL_HXTAL and uncomment line 47 #define __SYSTEM_CLOCK_72M_PLL_IRC8M_DIV2 in GD32F1x0_Firmware_Library_v3.1.0/Firmware/CMSIS/GD/GD32F1x0/Source/system_gd32f1x0.c

Now we are ready to make. in adc1 folder, call make, it will create adc1.hex.

Connect BOOT pin on the dock to 3.3V pin on VoCore2, then power on them from microUSB.

Once VoCore2 ready, run gd32up to load adc1.hex. After upload done, power off it and disconnect BOOT from 3.3V (We do not export its RST pin, so have to power off to reset).

After you connect it back to power, the GD32F150 will in ADC mode, UART2 on VoCore2 is not for USB2TTL anymore, it will output ADC value to VoCore’s UART2. We can use minicom to view it at VoCore2 console, 115200, 8n1.

VoCore2: run GCC in it

This is a tutorial for using C/C++ compile application in VoCore2.

1. Prepare a USB disk or SD card, at least 256MB, because GCC takes around 110MB. USB disk or SD card must be EXT4 format.

For macOS or Linux, call mkfs.ext4 /dev/disk2 to do this. /dev/disk2 is the USB disk name on my computer, please change to the name on your computer. Or another way, directly format it in VoCore2, need to install e2fsprogs.

opkg update
opkg install e2fsprogs
mkfs.ext4 /dev/sda

NOTE: opkg updaterequires internet, check, AP+Client mode for wireless connection. Or modify network switch settings, set ethernet port0 to VLAN 2(wan).

This is an example, directly patch /etc/config/network. Another way, you can change virtual network switch in LuCI, Network->Switch.

 config switch_vlan     
         option device 'switch0'
-        option vlan '1'  
+        option vlan '2'    
         option ports '0 6t'

2. Mount SD/USB storage to /overlay

Patch /etc/config/fstab, add target /overlay to make sure once storage is detected, system will auto mount it to /overlay.

config 'global'
        option  anon_swap       '0'
        option  anon_mount      '0'
        option  auto_swap       '1'
        option  auto_mount      '1'
        option  delay_root      '5'
        option  check_fs        '0'

config 'mount'
        option  device  '/dev/sda'
        option  target  '/overlay'
        option  enabled '1'

config 'mount'
        option  device  '/dev/mmcblk0'
        option  target  '/overlay'
        option  enabled '1'

/dev/sda is for USB disk, /dev/mmcblk0 is for SD card.

3. Reboot

Make sure /overlay is valid. You can check by command df.

root@OpenWrt:~# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/root                12800     12800         0 100% /rom
tmpfs                    62492       660     61832   1% /tmp
/dev/mmcblk0          30363504    148636  28649428   1% /overlay
overlayfs:/overlay    30363504    148636  28649428   1% /
tmpfs                      512         0       512   0% /dev

Or another way, directly manually mount /dev/sda or /dev/mmcblk0 to /overlay folder by command mount /dev/sda /overlay.

4. Now we can install GCC

This part is easy, just call

opkg update
opkg install gcc

It will install gcc, ar, binutils,libbfd, objdump, libopcodes packages from openwrt server.

Then we can compile C source code, try gcc yourcode.c -o out. Speed is not very fast, but works.

NOTE: compile better in /tmp folder(it is memory virtual disk) or in /overlay(it is the SD card we inserted). Rest path will store in NOR flash who has very limited write times and very little free space.

VoCore2: touch screen for HMI ready

Finally I make this:


It works very smooth, just like smart phone. And color is very well too. 🙂 I really like this small device, I guess it should be the lowest cost WVGA(800×480) UI solution or HMI solution. 1K units should be less than 10USD, so with VoCore2, the full solution cost can be less than 20USD.

Now I am busy on doing the final debug and fix. Hopefully in next month, I can get first batch of the production and put on

Because currently I am updating the fully screen every frame, so its FPS is pretty low, only 30fps, but I find a way to update the screen partially, so once I finish the next patch, the screen will reach 60fps, can be run on VoCore, Android smart phone and Windows computer as an extend screen.

Qt recently public MCU1.0, running on a very expansive platform over 30USD with low speed CPU and only 1MB~4MB memory. I guess it will be perfect with VoCore and this screen. I will be the first guy port it. :p

VoCore2: Temperature and Humidity Sensor(I2C)

i2c-tools is not that easy to use in some situation. Recently I need to make sure my room is wet enough to kill virus, so I develop a simple tool based on SHT20 — a temperature and humidity sensor, but I find I can not use i2c-tools to test SHT20 I2C bus, it missed some key features.

I have to spend couple of hours write another one.

usage: i2ctest [dev path] [address] [r/w] [length/data]


show device on i2c-0:
	i2ctest /dev/i2c-0

read 3 byte from i2c-0, address 0x40:
	i2ctest /dev/i2c-0 0x40 r 3

write 2 byte(0xe2 0x88) to i2c-0, address 0x21:
	i2ctest /dev/i2c-0 0x21 w '0xe2 0x88'

For my usage, query i2c devices:

root@OpenWrt:/tmp# i2ctest /dev/i2c-0 
0x10: BUSY
0x40: READY

0x10 is BUSY because the ES8388 driver already taken the address.

0x40 is the SHT20 address. From SHT20 datasheet, write 0xF3 first then read 3 bytes from it, we will get temperature.

root@OpenWrt:/tmp# i2ctest /dev/i2c-0 0x40 w 0xf3
write: 1 byte(s)
root@OpenWrt:/tmp# i2ctest /dev/i2c-0 0x40 r 3
00000000: 64 98 56                                            d.V

Calculate from the data: T = 0x6498 * 175.72 / 65536 – 46.85 = 22.198C

Here is the source code for i2ctest.c

#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <sys/ioctl.h>

#include <linux/i2c.h>
#include <linux/i2c-dev.h>

#define min(a, b)	((a) < (b) ? (a) : (b))

void hex_print(const unsigned char* s, int size, FILE *fp)
	int i = 0, n = 0, c = 0, p = 0;

	// default print to stdout.
	if (fp == NULL)
		fp = stdout;

	while(1) {
		n = size > 0x10 ? 0x10 : size;
		size -= n;

		p += fprintf(fp, "%08X: ", c);
		for(i = 0; i < n; i++) {
			if(i == 8) {
				p += fprintf(fp, " ");
			p += fprintf(fp, "%02X ", *(s + c + i));
		for(i = n; i < 0x10; i++) {
			if(i == 8) {
				p += fprintf(fp, " ");
			p += fprintf(fp, "   ");
		p += fprintf(fp, "   ");
		for(i = 0; i < n; i++) {
			if(*(s + c + i) >= 0x20 && *(s + c + i) <= 0x7E) {
				p += fprintf(fp, "%c", *(s + c + i));
			} else {
				p += fprintf(fp, ".");
		p += fprintf(fp, "\n");
		if(size <= 0) {
		c += n;

// convert string to unsigned int.
// hex: 0x, oct: 0, bin: ...b, dec: normal.
unsigned int atox(char* s)
	char *p = s, *e = s;
	unsigned int r = 0;

	while(*e != '\0')

	if(*p == '0' && (*(p + 1) == 'x' || *(p + 1) == 'X')) {
		// hex: skip first 0x or 0X header.
		p += 2;

		while(p != e) {
			r <<= 4;
			r += ((*p >= '0' && *p <= '9') ? (*p - '0') : ((*p >= 'A' && *p <= 'F') ?
			    *p - 'A' + 0xA : ((*p >= 'a' && *p <= 'f') ? *p - 'a' + 0xA : 0)));
	} else if((e - p) >= 2 && *(e - 1) == 'b') {
		// bin: ignore last b.

		while(p != e) {
			r <<= 1;
			r += (*p == '0' ? 0 : 1);
	} else if(*p == '0') {
		// oct: skip first 0 header.
		p += 1;

		while(p != e) {
			r <<= 3;
			r += ((*p >= '0' && *p <= '7') ? (*p - '0') : 0);
	} else {
		// dec: we do not need to deal the number.
		while(p != e) {
			r *= 10;
			r += ((*p >= '0' && *p <= '9') ? (*p - '0') : 0);

	return r;

int decode_data(unsigned char *out, int max, const char *in)
	int used = 0;
	const char *p = in;
	char num[5] = {0};

	while (*p) {
		if (*p == ' ') {

		if (*p != '0')
			return 0;

		if (used >= max)
			return used;

		memcpy(num, p, 4);
		out[used++] = atox(num);

		p += 4;

	return used;

int main(int argc, char *argv[])
	unsigned char buf[0x100] = {0};
	int i2c, code, size;

	if (argc != 2 && argc != 5) {
		printf("usage: i2ctest [dev path] [address] [r/w] [length/data]\n");
		printf("\nshow device on i2c-0:\n"
		       "\ti2ctest /dev/i2c-0\n");
		printf("\nread 3 byte from i2c-0, address 0x40:\n"
		       "\ti2ctest /dev/i2c-0 0x40 r 3\n");
		printf("\nwrite 2 byte(0xe2 0x88) to i2c-0, address 0x21:\n"
		       "\ti2ctest /dev/i2c-0 0x21 w '0xe2 0x88'\n\n");
		return 0;

	i2c = open(argv[1], O_RDWR);
	if (i2c < 0) {
		printf("error: can not open device %s\n", argv[1]);
		return -1;

	if (argc == 2) {
		for (code = 0; code < 0x7f; code++) {
			ioctl(i2c, I2C_SLAVE, (unsigned char)code);
			if (errno == EBUSY)
				printf("0x%02x: BUSY\n", code);
			size = write(i2c, buf + code, 1);
			if (size > 0)
				printf("0x%02x: READY\n", code);
		return 0;

	code = ioctl(i2c, I2C_SLAVE, (unsigned char)atox(argv[2]));
	if (code < 0) {
		printf("error: can not set address to %s\n", argv[2]);
		return -1;

	switch (argv[3][0]) {
	case 'r': {
		size = min(sizeof(buf), atox(argv[4]));
		size = read(i2c, buf, size);
		hex_print(buf, size, stdout);
		break; }

	case 'w': {
		size = decode_data(buf, sizeof(buf), argv[4]);
		size = write(i2c, buf, size);
		printf("write: %d byte(s)\n", size);
		break; }

	default: {
		printf("error: unsupport mode %c.\n", argv[3][0]);
		return -1; }

	return 0;

compile it: mipsel-openwrt-linux-gcc i2ctest.c -o i2ctest