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MSP430 Ultra-Low Power 16-bit Microcontroller Forum
MSP430G2553 PWM and UART clock issue
Hello All -Disclaimer: I am new to this, so please be gentle.
Object of my code: To drive two independent PWM signals, that will each drive a different LED light. AND be able to received RS323 UART commands from a PC. The commands from the PC will change the values of TA1CCR1 and TA1CCR2, thus changing the brightness of the LEDs.
The code for the LED PWM driver and the UART tx/rx are from sample code provided by TI documentation. Independantly, the LED PWM code works fine. And I have been successful in sending a single CHAR via RS232 UART to the uC and have seen it respond (TX) back on my oscilliscope.
However, when I combine the two pieces of code into one program, I can not get the uC to respond (TX) to the sent (RX) CHAR from the PC. After some trial and error, I think I narrowed it down to that the PWMs are being driven by the SMCLK, and the UART by the ACLK, and the uC doesn't like this.??? Any pointers on how to configure the clocks correctly, to enable this to work? (note: I do have a crystal on xin/xout pins). Help is much appriciated. (Code embedded below)-Tommy
if you look at the following table which can be found in the 2xx UG:
Using 32 kHz ACLK for 9600 baud rate will gives you around 20% maximum TX and RX error. My guess is that by combining the two codes you mentioned above, the CPU becomes more busy, and therefore increases even the error rate.
I would go with sourcing the USCI module with 1MHz calibrated value of SMCLK which according to the table will give you very low TX, RX error probability. You can easily use the calibrated value of the clock registers by applygin the following small code at the beginning of your code:
BCSCTL1 = CALBC1_1MHZ;
DCOCTL = CALDCO_1MHZ;FYI, the register value above is not really hard to be calculated manually. I wrote a small guide before about calculating the USCI baud rate at the following wiki page:
I hope this helps.
Let me make sure I understand your answer. Which I'll try later today when i get back into the lab.
I should add these two lines to my main():
BCSCTL1 = CALBC1_1MHZ; DCOCTL = CALDCO_1MHZ;
AND, change the following lines too, based on your wiki page;
UCA0CTL1 |= UCSSEL_2; // Chagned from CLK = ACLK, to CLK = SMCLK UCA0BR0 = 0x68; // changed from 0x03 -> 32kHz/9600 = 3 :: to 0x68 1MHz/9600 = 104 UCA0BR1 = 0x01; // changed from 0x00 to 0x01
if that is true, do I still need the 32khz crystal osc. on xin/xout pins? I wouldn't think i do.
actually, it should be like this:
UCA0CTL1 |= UCWRST;
UCA0CTL1 |= UCSSEL__SMCLK; // CLK = ACLK UCA0BR0 = 104; // 1MHz9600 = 3.41 UCA0BR1 = 0x00; UCA0MCTL = UCBRS0; // Modulation UCBRSx = 1 UCA0CTL1 &= ~UCSWRST; //**Initialize USCI state machine** IE2 |= UCA0RXIE; // Enable USCI_A0 RX interrupt
Hope this helps.
lhendMy guess is that by combining the two codes you mentioned above, the CPU becomes more busy, and therefore increases even the error rate.
What I can imagine is that the additonal PWM output causes ripples on VCC (if there is a significant load on this pin) or crosstalk. If the UART connection is already shakey (due to the high clock error rate on 32kHz crystal), this can make things worse.It is even possibel that the PWM signal influences the crystal.
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Leo & Jens-Michael -
Thank you for the assistance. I was successful in getting the UART and the PWM control working at the same time. By changing the code as Leo suggested, (found some typos in his code, see corrected syntax in green). I also removed the crystal completely and used only the SMCLK (for both the UART and PWM) from the internal Master clock and DCO set to 1MHz..
UCA0CTL1 |= UCSWRST; UCA0CTL1 |= UCSSEL_2; // CLK = SMCLK
It still wasn't performing correctly, so I rechecked my circuit and found I had some issues with my wiring of the MAX232. Which incidently operates best at 5V, and I am running the MSP430 at 3.3V. I found the MAX3232, which does the same as the MAX232 only at 3.3V. Once I fixed this, I was able to send messages from my PC, through the USB/Serial port to the uC and get a response echo back from it, and change the PWM value via this route.
Another thing to note. The sample code for the UART puts the uC into LPM3. which disables the SMCLK, MCLK and DCO, so I changed it to LMP1 since I changed from using the ACLK to the SMCLK.
I am off and running.Thank you.
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