Saturday, March 28, 2015

USB HID based IO utils

This project was started with the idea of turning a USB device into a kind of IO Swiss-knife, so that many things could be done with a single program. The use-case was to implement a remote-control, first to read the durations, and next to write them back. I had success in the read, by adjusting for code-delays. (The writes require more accurate timings, and i wrote a separate app for that. See ) This project is based on Microchips Custom HID MLA project. Not all files are provided, only the main ones that were changed. The app_device_hid_custom.* files replaced with app_device_hid_io.*. Actually, i wanted to abstract the io routines into pin_io module, which would be independent of protocol, i.e USB/Serial/Other. And to create it as a standalone project with as few files as possible. Some such items are TODO, i may not get time to finish them. i have used the 18F4550 for this project. Probably, the project is quite primitive, considering the level of expertise on these forums, but i hope it may help somebody to begin.
The project files are available here :

Firing the IO commands :

This is done using the pyusb framework. The file has the code to send the commands and receive the results. Just as a POC, the results can be saved to a .JS file, and viewed as a chart in mychart.html, using the Chart.js framework. That needs to be downloaded separately. The script also has a facility to adjust results, based on expected timing inaccuracies. For the sampling commands, num_samples are specified, and the script will calculate the number of packets and loop to receive them.

Common features

The framework is generic, and the commands accept input like which pin to use, what delay intervals, etc. (As a result, accurate timing is an issue. ). There is a debug feature that can return you the actual time taken for an interval. (not very exact) Routines are provided to measure and execute 2^24 cycle delays. Timings can be specified in uS or mS. There are flags for each command, to provide additional control. All sampling commands also specify an IDLE state/value, and sampling does to start until the input changes from the IDLE state/value. This is useful for manually-triggered sampling, like reading from a remote.
The commands are

WP : Write Pin :

This allows us to write a pattern to the <pin_num>. A pattern is an array of 2-byte durations, starting from HI. Currently, upto ~ 60 durations are supported. They end with a zero duration marker. There is a facility to make the HIGHs pulsed at a carrier-freq, as needed for IR codes. A reset flag, mS/uS flag, and a repeat flag are also provided.

RP : Read Pin :

This allows us to read <num_samples> of HIGH/LOW states from <pin_num> at <sampleInterval> intervals. Not well tested. TODO : store the result in bits instead of bytes. A debug flag and mS/uS flag are also provided.

RPD : Read Pin durations :

This allows us to read <num_samples> durations of HIGH/LOWs by sampling <pin_num> at intervals of <sampleInterval>. durations upto FFFF cycles are currently supported. A debug flag, idle-state flag and mS/uS flag are also provided. This was used to read durations of IR-code from tata-sky remote, but i needed to adjust the values for code-exec delays. i have blogged about it here :

RA : Read ADC :

This allows us to read <num_samples> samples from ADC at intervals of <sampleInterval>. A debug flag, idle-value and mS/uS flag are also provided. The results can be viewed as a chart.

Friday, March 13, 2015

PIC micro remote for tata sky


My entry into the world of micro-controllers was out of my desire to create a USB device. I wanted to create a general purpose IO framework that could be used to do many useful things, and one of my use cases was to add infra-red capability to my mobile which had none, and use it like a TV remote. While that framework is under way, and i may blog about it later, what i realised was that the runtime overheads of generic code might prevent me from achieving the exact timings that are required for remote protocols like RC6. Hence i fell back to writing a more basic/specific code for this purpose.

I have tested this code on the PIC18F4550.


My work would be in two phases, first find the codes, and second, transmit them. Since i did not find any reference for codes for the tata sky settop box, i had to find out the codes myself. i.e. what was the tata-sky remote transmitting. My starting idea was that it would be a series of ON & OFFs, with specific durations. To capture them, i would need an infra red sensor.  i searched the net and found the following links : 

This is a project for arduino, and describes how to capture the IR codes for a Nikon camera, and how to play them back. Since i was programming with PICs, this project would not work as is for me and i had to start on my own code. is also a good reference for knowledge of IR protocols. 

One basic thing to understand, is that the HIGHs in the code are usually transmitted as a series of pulses at high freq, called the carrier frequency. This is so that ambient light/disturbances are not mistaken for a code. Also, it lets the IR diode cool, since it takes rather high current, 50- 100mA. 

There are 2 kinds of sensors, a simple IR sensitive transistor/diode that would capture the signal as is, i.e. even the carrier pulses. This is available for 4-5 Rs. The other filters out the carrier pulses, so we directly get the highs and lows of the code, it costs slightly more, around 25 Rs. i got both, and experimented with them. The plain receiver has low sensitivity, and also, i did not have a oscilloscope to measure the waveform exactly. The one with the filter had good sensitivity so i decided to go with it. 
The one i got was TSOP1730( carrier freq 30Khz). Note that its output is inverted, i.e. it is usually HIGH and when it receives IR input, it goes low.
i also got an IR diode for transmitting for 4-5 Rs.

Trial and error

i added a facility to the read code to define an IDLE state and wait until it changes. so we can run the program, it will wait till input is in IDLE state, in our case, HIGH. when we press the remote, the state changes and now it starts taking the reading, i.e. measuring the durations of ones and zeroes. 

Why measure durations, why not just measure the sequence of 1s and 0s ? The reason being that in protocols like RC5/6, both 0s and 1s are represented using both the HIGH and LOW states.  HIGH first, LOW next could mean 1 , and LOW first, HIGH next could mean 0 etc. The durations are important and need to match the protocol.

One of the problems with the TSOP1730 sensor is that it can get randomly triggered by surrounding disturbances, so sometimes the program would start taking readings without my pressing the remote. Its suggested to tie the TSOP output pin to VDD using a > 10K resistor, and putting a capacitor betw the TSOP's VDD and GND to reduce power-supply interference. i also tried to tie the output to GND instead, thru a 175K resistor, and used a .1uF capacitor betw the TSOP's VDD and GND. None of the methods proved totally fool-proof however, and the random triggering still happens sometimes. 

i started to look at the readings i was getting on pressing the remote. then i would try to transmit those durations, inverted now, (since out sensor was inverted ), and see if it worked, but no luck. i did not even know what protocol my remote used, so i had no idea if my readings where right or not.

i searched the net again, came up with link link :, that somebody else had built a remote for tata-sky and the protocol was like RC6, tho no details were provided. The guy had built it from the arduino link i have given at the start, so i started looking at it again. one thing i realised was that timing need to be accurate. 

at this time, i was using a generic io framework, where one could specify in the request which pin to use, what carrier-freq, the durations etc. i started measuring the timings of my transmit and read using the PIC's timer, and realised that the generic nature of the framework was adding a lot of overhead over direct hardcoding. So i changed the generic code to bare and specific code for the task in hand. One more problem was that my readings did not fit the RC6 protocol exactly; they extended some bits longer. i decided to try with both options, i.e. once stopping at RC6 length, and once with my readings as is.
i started to test again, however, still no success. The settop box links red when it receives a valid code, and this was not happening. i went back to the article to check what else i was missing, and there it was. The article said that we need to send the code twice, not once. i added the repeat with a gap of 65ms, and the set-top box finally responded by changing the channel. yay ! It turned out that the readings that i had got were correct, even tho longer than the RC6 length.
The strange part was that my readings did not show the code being repeated, so how did the remote work ? maybe the remote encodes some more info that i have missed.

The timings of my readings were another issue, they did not match RC6 timings, where one unit is of 444 uS. So i wrote a python script to change my readings to RC6.

def fit_to_rc6( durations) :
    rc6durations = []
    for loop in range( 0,len(durations) ):
        duration = durations[loop]
        newduration = duration
        if  duration < 600 :
            newduration = 444
        elif duration < 1000 :
            newduration = 889
        elif duration > 1800 and duration < 3500 :
            newduration = 2666
        rc6durations.append( newduration)
    return rc6durations

arr_durations = fit_to_rc6(
Here is a sample of the readings i got :
the program has a max wait of  65535 us. after the signal ends,we get all max-durations i.e. 65535. Also the sensor gives inverted output,so S=0 is actually HIGH input to the sensor
Starting..                      S:0,D:2662,S:1,D:656,S:0,D:495,S:1,D:265,S:0,D:528,S:1,D:224,S:0,D:535,S:1,D:663,S:0,D:522,S:1,D:656,S:0,D:948,S:1,D:218,S:0,D:542,S:1,D

we can see that the reading vary quite a bit, from 224 to 542 are all actually 444 us, and from 656 to 948 are all 889 us ? how do i know ? by getting a bunch of readings, and seeing which part repeats most, and comparing with the Rc6 protocol. The remote_codes.txt file contains codes for some common keys.
Most of them have been implemented in the sample code too.

about the transmitter, its a simple transistor circuit, with the base driven from the PIC, and the IR diode with a 100R resistor in series with the collector. emitter to ground.


The code is available at The read_pin_durations.c is for the reading of the remote codes. The tata_sky_remote_serial.c is for transmitting the codes. it uses a uart to accept a code, and looks it up from an array and transmits it. The array contains the durations of ON/OFFs. Ideally, since we know the protocol and timings, we could use a more compact format to store the codes, rather than actual durations. e.g, we could keep the start-bit of 2666 and 889 separate, and represent the rest with 444 as 0 and 889 as one, and hold the entire code in just an int. Also, no keyboard interfacing is done. One could use a multiplexer, to get 16 lines from 4 pins, or implement it in the PIC. a good link to understand a keyboard is

Other references

Ken sheriff has a IR library for Arduino, which can capture and play-back remote code on the go ! cool, no ?

Another project for PICs is

Monday, March 2, 2015

Generic UART for any microcontroller

Communicating with a micro-controller at runtime, is a very useful facility. It can be used for debugging, or to send commands etc. One of the easiest available protocol to communicate is UART. It requires a pin for transmit and a pin for receive, and the GROUND connection. There are many available programs that allow us to communicate using UART on the serial port, or using a usb-serial adapter. e.g. hyperterminal, minicom, etc.

However, not all microcontrollers have UART ports in built.

i was taking a look at the UART protocol, and it seemed pretty simple to implement.  So i gave it a go, and was zapped when it just worked the first time :).

Below is a little app that accepts 2 byte commands. Toggles LATD1 when it receives the command "TG", and echoes back the command after its done. Invert flag is provided. ( Inversion may be needed if we are not working directly with a real serial port). Tested with minicom @9600 baud, with hardware-flow-control OFF, so that it sends the chars we type.

It is tested on the PIC18F4550, will need modifications to run on other microcontrollers.

Its also available as a module to include at

 * File:   main.c
 * Author: manoj
 * Writing to uart
 * Created on September 27, 2014, 6:01 PM


// PLL with pre-scaler and post-scale options is a way to derive multiple
// freq from the same source, e.g. for low=6Mhz/high=48Mhz speed USB and for MCU clock
// Input to PLL has to be 4 Mhz and its output is 96 MHz
// So, for e.g. if we are using exernal 20 MHz osc, its o/p has to be
// divided by 5 to get 4 Mhz input for PLL
// PLLDIV is prescaler, o/p has to be 4MHz
// CPUDIV and USBDIV are postscalers, input is 96Mhz

#pragma config PLLDIV   = 5         // (20 MHz crystal on PICDEM FS USB board)
#pragma config CPUDIV   = OSC1_PLL2
#pragma config USBDIV   = 2         // Clock source from 96MHz PLL/2
#pragma config FOSC     = HSPLL_HS

#pragma config IESO = OFF
#pragma config WDT = OFF
#pragma config STVREN = ON
#pragma config LVP = ON
#pragma config BOR = ON
#pragma config MCLRE = ON
#pragma config PWRT = OFF
#pragma config PBADEN = OFF

// Effective CPU Freq, considering PLL and CPUDIV. Needed for the delay routines
#define _XTAL_FREQ 48000000

#define UART_BIT_TIME 104 // us. inverse gives the bit-rate
#define PIN_UART_TX LATD4 // output
#define PIN_UART_RX PORTDbits.RD5 // input

uint8_t UART_INVERTED = 0; // If inverted, low and high states will be inverted
uint8_t HIGH_STATE = 1;
uint8_t LOW_STATE = 0;

// flexible way to set some params.
void uart_init( uint8_t is_inverted){
    UART_INVERTED = is_inverted;
    if( is_inverted){
        HIGH_STATE = 0;
        LOW_STATE = 1;

// start of transmission
void uart_start_tx(){

// end of transmission
void uart_end_tx(){

// write a char
void uart_write( char c){

    // start bit
    __delay_us( UART_BIT_TIME);

    uint8_t bit_pos = 0;
    // transmit bits, LSB first
    while (bit_pos < 8) {
        uint8_t currbit =  c & 0x01;
        PIN_UART_TX = UART_INVERTED ? ! currbit : currbit;
        __delay_us( UART_BIT_TIME);
        c = c >> 1;
    // stop bit
    __delay_us( UART_BIT_TIME);

// Read specified number of chars and put them into holder array
void uart_read( uint8_t len, char holder[] ){
    uint8_t i;
    uint8_t bit_pos;

    // loop and read len number of chars.
    for( i=0; i< len; i++){

        // Wait for idle state to change, i.e. start bit
        while( PIN_UART_RX == HIGH_STATE);

        // start bit
        __delay_us( UART_BIT_TIME);

        // read bits, LSB first
        bit_pos = 0;
        char c = 0;
        while (bit_pos < 8) {
            uint8_t currbit = UART_INVERTED ? ! PIN_UART_RX : PIN_UART_RX;
            c = c | (currbit << bit_pos);
            __delay_us( UART_BIT_TIME);
        holder[i] = c;

        // stop bit
        __delay_us( UART_BIT_TIME);

// write a string, optionally of specified len.
// if null terminated, len can be -1
void uart_writestr( char str[], int len ){
    int i;
    for( i=0; i< len || len == -1; i++){
        char curr = str[i];
        if( curr == '\0'){
        uart_write( curr);

// process the command we received
void process_cmd(char cmd[] ){
    if( strncmp( cmd, "TG", 2) == 0){
        LATD1 ^= 1;

int main(int argc, char** argv)
    TRISDbits.RD4 = 0; //TX pin set as output
    TRISDbits.RD5 = 1; //RX pin set as input
    TRISDbits.RD1 = 0; // command output

    uart_init(1); // invert
    char cmd[2]; // array to hold received command.
       uart_read( 2, cmd); // read a command, specified bytes long
       process_cmd( cmd);
       uart_writestr( cmd, 2); // echo it back

Monday, January 19, 2015

Controlling Microchips 18F4550 as HID Custom USB device using python pyUSB

The MLA( Microchip Libraries for Applications ) ships with a MPLABX project for 18F4550 that makes the chip behave like a HID Custom device. i built the project, burnt the hex file, and was also able to test it using the plug_and_play executable provided for Linux

As per this project, there is 1 interface with 2 endpoints.
One is IN, other is OUT. Both are of type INTERRUPT.
( See usb_descriptors.c )

For the commands, see app_device_custom_hid.c.
The commands are :
  • COMMAND_TOGGLE_LED = 0x80 -- LED on RD0
  • COMMAND_GET_BUTTON_STATUS = 0x81 -- Button on RB4, pressing should bring it to Ground

Commands should be sent on the OUT endpoint, and data received on the IN.

i wanted to write my own code to control the device. came across the pyusb framework, which looked easy to use.

Here is working code for the first 2 commands :
import usb.core
import usb.util
import sys

# find our device
dev = usb.core.find(idVendor=0x04d8, idProduct=0x003f)

# was it found?
if dev is None:
    raise ValueError('Device not found')

interface = 0

if dev.is_kernel_driver_active(interface) is True:
            print "but we need to detach kernel driver"

# set the active configuration. With no arguments, the first
# configuration will be the active one

# get an endpoint instance
cfg = dev.get_active_configuration()
intf = cfg[(0,0)]

for cfg in dev:
    sys.stdout.write("ConfigurationValue:" + str(cfg.bConfigurationValue) + '\n')
    for intf in cfg:
        #print vars(intf)
        sys.stdout.write('\t' + \
                         "InterfaceNumber:" + str(intf.bInterfaceNumber) + \
                         ',' + \
                         "AlternateSetting:" + str(intf.bAlternateSetting) + \
        for ep in intf:
            #print vars(ep)
            sys.stdout.write('\t\t' + \
                             "EndpointAddress:" + str(ep.bEndpointAddress) + \

assert ep is not None

# toggle the led. COMMAND_TOGGLE_LED
# args : ep, value, timeout
dev.write( 1, chr(128), 100)

# read the button state. COMMAND_GET_BUTTON_STATUS
# first send the command on EP_OUT
dev.write( 1, chr(129), 100)
# read on EP_IN
ret = 129, 100)
print "Received: %d,%d", ret[0], ret[1]
print "Button Status : " + "Released" if ret[1] == 1 else "Pressed"

Thursday, January 15, 2015

PIC Programmer using a USB to Serial Converter - HVP


This is a follow up to my earlier post by the same name, for LVP. Please read that first, since it discusses the basics. In this one, i will discuss HVP.

Current Status

I am able to program the PIC12F675 in HVP mode. As expected its quite slow, takes around 2-3 mins to burn a 500 byte program.


Obtaining a High voltage > 9V from USB

i wanted to investigate whether a high voltage could be obtained from the USB port. There are voltage-pump/ charge-pump ICs like LM2623, MAX660,
mc34063a etc. I could obtain ADM660 for Rs 175. Later i also found mc34063a which is cheaper, but i don't know if it is locally available. With just two 1uF capacitors, ADM660 delivers double the input voltage, and i was able to get 10V from the 5V of the USB port.

The problem with Txd and flip-flop at 10V

i planned to supply the 10V to the flip-flop as supply, and use the flip-flops output to drive the VPP. However, i ran into an issue here. Txd, which drove the CLK of the flip-flop, was still at 5V, and while it could reset the flip-flop, it could not set it. Hence i kept the flip-flop at 5V, and added a NPN transistor to provide the 10V.
The VCC of the transistor was 10V from the ADM660. Output of the flip-flop was fed as input to base. emitter to ground. The Output from collector now provided the high voltage to VPP. Rbase=Rload=10k.

The working circuit

The circuit is powered by the usb port on the right. The Flip-flop with the LED indicating its output state is on the left. Next comes the transistor which changes the fip-flop output from 5V to 10V. Next comes the ADM660 voltage-doubler which provides the VCC of 10V for the transistor. Last is the PIC12F675, which gets its VPP from the transistor output.


This time, i tried out a new software : pickle( old version is called k8048) from, written in C. (This site also has a nice list of PICs that support LVP.) It supports a lot of PICs and modes, and is simple and easy to modify. I had to modify it, since it depends on the usb-serial-port to support the BREAK functionality, which my adapter does not.
The change is to introduce a new config param( bit-rule) VPP_TOGGLE_ON_PULSE, and alter the set_vpp() method so that it when VPP_TOGGLE_ON_PULSE is true, it sends a single 0 instead of the BREAK functionality.

The .k8048 file should be created in the home directory, and has the config.

the ktest command helps to check whether the pins have correct voltages.
For serial-port option, the connections are fixed in the code, not in the .k8048 file. So DTR->PGD, PGD->CTS,RTS->CLK and Txd->Vpp. You need to follow these exactly. 
The common reason for "device not detected" is that Vpp is not properly applied. Confirm that you have a Vpp > VDD + 3.5 V. Check whether it turns on and off correctly. Put an LED with a 10k resistor as an indicator on it. It should be low before programming, and will go high/low during programming, and go low after its finished.
The command to use for programming depends on the instruction-set of the chip. For 12f675, i used k14, since it has a 14-bit instructions set.
For validating device id, i use "k14 id".

For programming, k14 program <hexfile>

Do not set any flags(bitrules), unless you understand them.
The zip with the modified files is available as under

More to come...