AdSense

If you haven't already noticed, I've decided to try enabling AdSense on the blog.  I figure I'll give it a shot; if it's not too intrusive I'll leave it there.  Send me feedback and let me know what you think-- there may be better ways to fund my hobby.

Tutorial 18: Finishing the UART Transceiver

This last tutorial in this series will be fairly short, because there's very little to be done! If we combine tutorial 16f and tutorial 17, we're nearly done building a full-duplex UART transceiver.  There are a few little details that need to be explained in order to get it to work properly, however.

• A bit counter will be needed for both transmitting and receiving, if you're to be able to do them simultaneously.  This takes another 8 bytes of memory, of course.
• While not necessary for the UART aspect, this code uses interrupts on two pins of P1. Any output on GPIO, even if interrupts are disabled for the pin, will set the corresponding bit of P1IFG on a rising or falling edge, depending on the setting of the corresponding bit in P1IES. Because the two LEDs are being used, any time the LED is turned on we have a rising edge on that GPIO, which sets the bit in P1IFG. The method used in this code to deal with P1 interrupts fails in those cases, since P1IFG would not be exactly P1RX or BTN1.  To fix this, a mask is introduced so the switch statement in the P1 ISR only considers the bits for P1RX and BTN1, ignoring any others set bits of P1IFG.  There is one disadvantage to this particular scheme: if the UART receives a start bit at the same time as a button press occurs, one or both will be ignored (depending on the order and timing of the two events).
• A new function is introduced to encapsulate sending a string of characters over UART.  This function makes use of a pointer, allowing us to write new values to the variable message. Be aware that other values in RAM could be overwritten if you're not careful.  Also, the function assumes a message length of exactly 16 characters, and will print garbage (or remnants of the last message) if you write a shorter message.  A longer message will be truncated.
• Because the DCO is executing commands at a faster rate than the transmission, a short delay of bit_time is added to tx_byte().  This delay lets the transmission start before returning control to the main() function. Without it, the code could call tx_byte() again before transmitting actually starts, changing the value of TXBuffer before it has a chance to get sent.  

The transceiver code can be found in uartTXCVRG2231.c.  I've added a few features to demonstrate the full-duplex capability of the transceiver.  The commands 'r', 'g', and 'z' are retained from the receiver tutorial. To this has been added 'c', which prints a continuous message until another command (eg. 'z' to sleep) is sent.  Note that the command can be sent in the midst of a byte transmission and still work, since the receiver is operating on a separate timer/interrupt configuration.  I verified this to be the case using my logic analyzer, and found that a command was correctly received even while starting in the middle of a transmission.

In addition to the 'c' command, another command is set from a button press (using a non-keyboard ASCII character 0xAF), and can be done at any time, showing that the code can also work with physical interface interrupts as well as the serial transmission of commands.  The button will also stop the continual printing of a 'c' command as well, as it also resets the command variable to 'z'.

Remember to use 9600 baud transmission and to configure CCS to build and link to the header file with the UART calibration, as well as not erase the information memory segment containing the UART DCO calibration.

And with that, we've reached the conclusion of what I'll term as the "basic" tutorials for the MSP430.  A big "Thank you!" to all of you; I've received numerous notes of encouragement from many of my readers, and it's been very helpful.  This project and blog have been a great deal of fun, and wer a key addition to the training I received as a Ph.D. student. This isn't the end of the blog, of course.  I'll be back with more advanced tutorials, and, more importantly, examples of scientific measurements and experiments done with the MSP430.  In fact, my next post will start describing a project I've had in mind for a while.  In the mean time, I will start recompiling these first 18 tutorials into a single volume.  I have as-yet to decide just how it would be distributed, and exactly what form it would take. In any case, I hope all of you stay with me and enjoy exploring the MSP430.

Tutorial 17: The Receiver

Once you've built a UART transmitter, the receiver is not much different!  The way we'll build the receiver here will be done in anticipation of putting both the transmitter and receiver on the same device, and we'll look at a few new things.

What's the Same

We'll use Timer_A to work with the timings, using the same bit_time value as the transmitter.  The timer will need an accurate clock, and we'll use the calibrated UART frequency from last time as well.  The interrupt service routine for the timer will handle all of the work for reading the serial data as it's transmitted to the device.

What's Different

There are a number of differences in how the receiver is handled, but most of them are quite minor.  We'll use interrupts on the GPIO to trigger receiving, for one.  Once we've seen a falling edge on the GPIO (signalling a start bit being received), we need to wait one half bit_time so that we're sampling in more or less the center of each bit, thus we need a definition for a half bit_time as well.  (We could just divide bit_time by 2, but that's actually kind of slow-- it's probably more conducive to just define another variable with the exact value we want.)  All of this will be handled by an interrupt service routine for the GPIO port.

In addition, we want to use TA1 instead of TA0 for the timing-- this will allow us to send and receive simultaneously, as each has its own independent timer.  TA1 has a slightly different way of handling the ISR, however.  It's worth noting that there's nothing special about TA0 for transmitting nor TA1 for receiving-- we could just as easily have reversed the two.  Which one you use depends on which pins you connect for transmit and receive.  On the LaunchPad (the earlier versions, anyway), Tx is connected to P1.1 and Rx to P1.2.  Looking at the datasheet for the devices that connect to the LaunchPad, P1.1 is connected to TA0.0, letting us use the output feature of Timer_A to change the bit automatically.  P1.5 can also be connected to the TA0.0 output, so that is another option.  P1.2, P1.6, and P2.6 can all be connected to the TA0.1 output, and are all possibilities for the transmit pin if you change from CCR0 to CCR1 for the transmitter's timer.  The receiver will be reading a GPIO, and could in principle be used with any GPIO pin.  (We'll look in a short while to how we can do transmitting on any GPIO as well.)

The Specifics

Alright, let's dig into the code.  First, we need to define our bit_time again, as well as a half_bit_time.  For 9600 baud at the UART calibrated frequency (7.3728 MHz), these correspond to 768 and 384 respectively.  We'll make use of the UART_FG flag variable again, leaving BIT0 for a transmitting flag and using BIT1 for a receiving flag.  The data will be loaded into a buffer again before being saved, using the int value RXBuffer.  The bit_count variable comes into play again, and will be used to count down the bits as they are received.  When we combine the transmitter with the receiver, we'll need one of these for each of the parts so they can count independently.  For now, I've just used the same variable name bit_count.

The DCO and Timer are initialized identically to the transmitter-- the differences in the code happen in the interrupt routines.  P1 is configured to read from our chosen receive pin (P1.2), enabling interrupts on a falling edge.  When the interrupt occurs (presumably at the start bit of a transmission), CCR1 is initialized to a half bit-time later, CCR interrupts are enabled, and P1 interrupts temporarily disabled until the data has been received.  This method precludes having to wait for the receive flag to clear before the next P1 interrupt, so no while loop is needed here.

The CCR1 interrupt is handled differently than the CCR0 interrupt.  For one, CCR0 triggers the interrupt flag TAIFG in the TACTL register.  All other capture/compare registers in a device will trigger flags in the TAIV register.  There is a single interrupt for this register, and so TAIV must be read to know which CCR caused the interrupt.  (Though in the LaunchPad devices there is only CCR1, the interrupt must be handled the same way as though there were further CCR's in the device.)  I've done this by using a C switch statement, looking specifically for a CCR1 flag, which is marked by bit 1 in TAIV.  TAIFG also appears in TAIV, but as the value 0xA rather than 0x2, so a case 2: is sufficient for identifying a CCR1 interrupt flag.  The flag is automatically cleared with the interrupt is serviced.  The bits are then read in bit_time intervals from the GPIO input.  When all 10 bits have been read, the timer interrupts are disabled and P1 interrupts re-enabled.

Now that we've covered essentially how the code works, you can test it out with uartRXG2231.c.  (Don't forget to configure it to not erase the information segments!)  Look at the main function here-- it's set up to look for single-character commands, recognizing the characters 'r' to toggle the red LED, 'g' to toggle the green LED, and 'z' to do nothing but go into a low power mode and wait for a command.  If an invalid command is sent, the code holds on to the current state of P1OUT and flashes the LEDs to signal an error and returns to the saved state.  After each command, the device returns to a sleep mode.  To use the code, be sure you have at least exited the debugger-- you may find that serial terminals don't like trying to communicate through the same port that is opened in CCS in debug mode; exiting debug mode releases the serial port for use. When connected to the proper port (at 9600 baud, of course), you should be able to send single character commands to toggle the red and green LEDs. Try sending an invalid character to see the error signal.

This is the simplest way of sending commands to the MSP430.  Obviously there are uses for multiple-character commands, or sending data directly.  We'll look at those techniques at some point in the future, but next time we'll combine the two parts into a full-duplex transceiver.

Reader Exercise: Add a command to clear both LEDs. See if you can come up with another function to add a command for.

Tutorial 16f: The Transmitter

We now have all the tools to build a UART transmitter on the MSP430. We have an accurate clock, which operates at a frequency that gives us the best accuracy on standard transmission rates with the DCO. The best way to control the timing of our serial transmission will be to use the Timer_A peripheral, so let's look at how to configure it.

First, we need to ensure we have our DCO set to our custom calibration. The way I've chosen to do this is to use definitions in the header:
   #define CALDCO_UART  *(char *)0x10BE
   #define CALBC1_UART  *(char *)0x10BF
Then in our code, we can set the DCO just as we would for the 1 MHz calibration:
   BCSCTL1 = CALBC1_UART;
   DCOCTL = CALDCO_UART;
To make this even easier, you can put the #defines in a header file (I've called mine calibrations.h), and then include it in any program where you need to use the custom DCO calibration.

Now to set up our timer. First, we need to choose our transmission rate. The commonly selected rate is 9600 baud, and if you use only the LaunchPad's USB connection it may be the only rate that works.  (It's a little unclear still how the USB connection is set up-- the MSP430 UART connects to the MSP430 in the emulator, which has separate pins to forward the data to the TUSB chip interfacing with the USB port.)  This DCO frequency can reliably do transmission up to a rate of 921,600 baud. Any faster, and there may not be enough time to run the transmission code between bits.  The code you can download at the end of the tutorial is configured for 9600 baud.

The next thing we need is to know how many clock cycles occur during the time to transmit one bit. We'll transmit at 9600 bits per second, and our clock oscillates at 7,372,800 times per second, so there are 7,372,800/9600 = 768 clock cycles in each bit. (Note that this is exact; there's no fractional part being truncated in this division, which is why we chose this DCO frequency to start with.) This period is often called the bit time.
   bit_time = 24576;  // bit time for 300 baud
   bit_time = 768;    // bit time for 9600 baud
   bit_time = 64;     // bit time for 115,200 baud

The LaunchPad is set up to send the transmission over P1.1. Looking in the datasheet, we see that this pin can be used as the TA0.0 output–using Timer_A, we can toggle this output when the TAR counter reaches the value stored in TACCR0. We have two options for this: we can use the timer's up-mode so that the counter resets after reaching TACCR0 of bit_time, or we can use continuous mode and reset TACCR0 to be bit_time cycles beyond the current value of TAR when it triggers an interrupt. (Doing this automatically accounts for roll-over.) The first would be an easy solution for our transmitter, but as we're working towards a full-duplex receiver/transmitter, and the receiver looks at P1.2 (using TA0.1, based on TACCR1 for timing, of course), it makes more sense to use the second method in anticipation of setting up the receiver next time so that we don't have conflicts in using the timer for both tasks.

The idle state for UART is logic high. When we configure the timer, we'll ensure that the TA0.0 defaults to setting P1.1. There's no need to start the timer until we're ready to transmit, but on the other hand there's no need to stop it unless you're concerned about power consumption. We'll go ahead and let the timer run for now, since as a peripheral it runs independently and won't interfere with any code running. We will, however, wait to enable interrupts until we're ready to transmit.

Once we're ready to transmit a character, we need to enable interrupts, and configure Timer_A to reset the TA0.0 output to signal the start of a character. TACCR0 is incremented to bit_time counts later, so the next interrupt occurs at the timing necessary for our chosen transmission rate. We then step through the bits being sent and tell the MSP430 whether the next bit should be set or reset, depending on our encoding. We'll use the standard 8N1, little-endian encoding, so it is determined by the lsb of the character we're sending. When all 8 bits have been sent, we then set TA0.0 to mark a stop bit, making a total of 10 bits for each character. Interrupts are then disabled until we have another character to send.

Take a look at the code in uartTXG2231.c. Note first of all the routine DCO_init(). This includes a trap–since we're accessing the flash memory directly, we run the risk of setting the DCO to its highest setting if we use a device that hasn't had the DCO calibration written to that memory address! This type of check will prevent that, and is not a bad thing to include even when using the factory calibrations. I've created an LED signal specifically to indicate a DCO calibration error: three flashes of LED1, repeated with a pause between sets. If you try this code with an uncalibrated G2231, it should immediately indicate this error. Note that it cannot tell if a written code is correct or not, only if the addresses we're accessing have been erased.

If you've jumped right in and tried to run the code after calibrating your DCO, you may still encounter the error.  CCS by default erases both the main memory and the information memory segments (except for INFO_A) when you program the chip; so your calibrations in INFO_B have been erased and the code won't run.  Rerun the UART calibration code, and then open the project properties.  (When uartTXG2231 is the [Active -Debug] project, you can find the properties in the File menu, or right-clicking the project name in the Project Explorer tab.)  In the menu on the left, select Debug.  In the window that comes up, select MSP430 Properties.  Here you'll see a list of Download Options; be sure to choose "Erase Main Memory Only" and close the properties window.  Now when you load this project into your device, it will only erase the Main Memory and leave the information memory segments alone.

This code is set up to demonstrate a few ways to transmit data.  Each loop is initiated by pressing the button on P1.3.  First, a string is stepped through and sent over UART character by character.  Individual characters are also sent, as is the value of a variable count, which is set to repeatedly count from 1 through 9.  Single digits can be sent as characters easily as they appear in order in ASCII-- the digit 0 is ascii code 48 = 0 + 48, 1 is 49 = 1 + 48, and so on.  A multiple digit number could be pushed through using code similar to what we used for the LCM.  These are some simple ways of putting data and messages through UART that take very little code space in your device.

The real magic happens in the tx_byte() and CCR0_ISR() routines.  When tx_byte is called, it first polls the UART_FG flag and waits for the TXbit to clear.  The byte being sent is loaded into a buffer and formatted to include the stop bit. The TXbit is set in UART_FG, an LED turned on to indicate transmission, and the timer is configured to reset at the next interrupt.  A character cannot be transmitted until at least bit_time after the previous transmission; since the MSP430 clock is so much faster than the data rate, you could potentially run into a problem of initiating a new character send microseconds after the last, rather than leaving at least one bit_time space between.  To prevent this, any present interrupt flag is cleared and the next interrupt is set for one bit_time after the current time.  This configuration leaves just slightly more than one bit time between characters.  The interrupts run the CCR0_ISR routine, which sets the next interrupt to be bit_time later, configures TA0.0 to the next bit state for the next interrupt, and shifts the transmission buffer down to pull the next bit to the front.

To see all of this happen, you'll need a serial terminal, such as RealTerm or PuTTY.  When your terminal is directed to watch the port your LaunchPad is on at the 9600 baud rate, you should see the message display every time you push the button.

Next time we'll use Timer_A to receive data, getting ready for a complete UART transceiver.

Addendum to Tutorial 16c

As I've been reviewing what I've done so far, I started thinking about some way to test my claims in the tutorial discussing the need for accurate clocks in UART.  I recently acquired a logic analyzer, which is fast proving to be an extremely useful tool for me! I'll probably use it quite a bit in the coming months and years trying to understand and debug some of the designs I'm working through.

In any case, I thought it might be interesting to use it to show what happens when clocks are off slightly.  The four channels shown here are all measuring the same signal, with the Red channel clocking at the correct rate (9600 baud), Yellow at 3% Error (effectively 9888 baud), Green at 6% Error (the theoretical error from using two +/- 3% clocks, effectively 10,176 baud), and Blue at 10% Error (effectively 10,560 baud).

The second image zooms in to the first characters so you can see the frame errors more clearly.  The dots placed on each plot show where each channel is sampling the data stream.  At 6% error, the software is smart enough to decode the characters, but the stop bit on each is missed, making communication unreliable.  In practice, you need better than 5% combined error to have reliable communication in UART.  (Using identical systems, each needs a clock that is accurate to better than 2.5%.)

Note that this isn't really true in all cases; I'm using 8N1 encoding here.  If I were to use larger frames than the 10 bits used for 8N1, the smaller clock errors may eventually cause a frame error as well.  If you use a protocol that sends more than 10 bits, you'll need even more accuracy in your clocks.

Just an interesting little test I put together to try out my fancy logic analyzer.

Dr. Olson

Boy, it's been a long time since I've worked on this blog. The good news is that, as of this month, I have finished my doctoral degree and will now (hopefully) have more time to dedicate to working on it! There are still many, many more ideas and projects I have in mind, so I hope those who have been here before will stick around, and those looking for new help will find it.

Thanks, everyone, for the patience and kind words that have consistently rolled in over the past few months. There are still a number of comments I have yet to moderate, mostly because I need to give them some thought before I can respond. I don't want to just post them, because I'm afraid I'll never respond if I do. =)