Post by thread:[PIC]: Controlling two L298 Motor Drivers

James, I'm working on a similar project right now (using discrete MOSFETS rather than the L298). The current version runs fine on the breadboard, but I can't say it's a tested, finished, approach. I'm using two PWM channels and some 74HCxx glue logic to take one PWM signal for each motor and apply it to the correct high/low MOSFET combination. In the first version, I used two control lines; F (forward/!forward) and E (Enable/Disable drivers -- the PWM pulses). Looking at one channel (one motor) an H-Bridge could be labelled as follows: A B | | +-M M-+ | | C D The truth table is F E A B C D 0 0 0 1 0 0 0 1 0 1 1 0 1 0 1 0 0 0 1 1 1 0 0 1 The latest version adds an active brake control signal, "B". When B is zero, it has no effect. When it's a one, A and B are off and C and D are on. (Yes, I'm using kickback protection diodes). I am a tinkerer, not an Electrical Engineer so I don't know if this the best solution, or even if it's a good one. As always, I hope to learn from the feedback on this list. Andrew ________________________________________________ Get your own "800" number Voicemail, fax, email, and a lot more http://www.ureach.com/reg/tag ---- On Fri, 11 Nov 2005, James James (spam_OUTjames_iboxTakeThisOuTyahoo.com) wrote: > Hi everyone, > > I am trying to control two L298 motor drivers (need 4 PWM outputs) from a PIC18F2620 MCU. However, > the PIC only has 2 independent hardware PWM channels. Is there anyway to multiplex the 2 PWM channels > to 4 independent PWM channels? Is there any other PIC that has 4 (or more) independent PWM channels? > Thanks! > > --------------------------------- > Yahoo! FareChase - Search multiple travel sites in one click. > --

Alan B. Pearce wrote: >> Is there any other PIC that has 4 (or more) independent PWM channels? > > I think some of the 18F may also, but I believe you can only use these > for motor driving, and not as independent channels. There are a number of PICs with 4 and more semi-independent PWM channels. Look for numbers of (E)CCP modules. They are independent in that you can run every (E)CCP module with a different duty cycle, but they are not fully independent in the selection of PWM frequencies -- you only have one or at most two different frequencies. (The 18F6722 is one with 5 (E)CCP channels that can be driven from two different timer sets.) But as others have said before, for two motors you only need two PWM channels, a few standard digital outputs (for direction and possibly breaking) and a bit of glue logic. Gerhard

Thank you everyone for your reply. I just want to clarify that the L298 is a DUAL Full-Bridge driver. Driving 2 of them means I need 4 PWM channels. All channels can be at the same frequency, as long as I can vary the duty cyle for each channels. I think I am going to use the PIC18F4431 since it also have a Quadrature Encoder Interface (QEI). Too bad it has only one QEI. I need two! Is the frequency for the PWM matter? Is there a "rule of thump" on what frequency is good? Since I am running the PIC at 40Mhz, it looks like 2.4Khz is the slowest it will go. Regards, --- Gerhard Fiedler <.....listsKILLspam@spam@connectionbrazil.com> wrote: {Quote hidden}> Alan B. Pearce wrote: > > >> Is there any other PIC that has 4 (or more) > independent PWM channels? > > > > I think some of the 18F may also, but I believe > you can only use these > > for motor driving, and not as independent > channels. > > There are a number of PICs with 4 and more > semi-independent PWM channels. > Look for numbers of (E)CCP modules. They are > independent in that you can > run every (E)CCP module with a different duty cycle, > but they are not fully > independent in the selection of PWM frequencies -- > you only have one or at > most two different frequencies. (The 18F6722 is one > with 5 (E)CCP channels > that can be driven from two different timer sets.) > > But as others have said before, for two motors you > only need two PWM > channels, a few standard digital outputs (for > direction and possibly > breaking) and a bit of glue logic. > > Gerhard > > --

Hi, You can generate many PWM channels with any PIC that can provide a regular interruption. For example, if you set an interruption every 8us and have a variable (say T) that increments on each interrupt and resets when it gets to 63 you will have a period of about 2KHz. If you want 4 PWM channels you would need 4 extra counting variables. So, the idea is to set all the PWMs high each time T resets and set them down independently according to the desired duty cycle. The code in C would look something like this: void periodic_interrupt (void) { T++; U1++; U2++; U3++; //increment counters if (T>=63) { T=0; U1=0; U2=0; U3=0; //clear counters PWM1=1;PW2=1;PW3=1; //set pwm outputs high } if (U1>=DC1) { //DC1 is where you save the desired duty cycle PWM1=0; U1=0; } ....and so on Julio 2005/11/15, James James <james_iboxKILLspamyahoo.com>: {Quote hidden}> > Thank you everyone for your reply. I just want to > clarify that the L298 is a DUAL Full-Bridge driver. > Driving 2 of them means I need 4 PWM channels. All > channels can be at the same frequency, as long as I > can vary the duty cyle for each channels. I think I > am going to use the PIC18F4431 since it also have a > Quadrature Encoder Interface (QEI). Too bad it has > only one QEI. I need two! > > Is the frequency for the PWM matter? Is there a "rule > of thump" on what frequency is good? Since I am > running the PIC at 40Mhz, it looks like 2.4Khz is the > slowest it will go. > Regards, > > --- Gerhard Fiedler <.....listsKILLspam.....connectionbrazil.com> > wrote: > > > Alan B. Pearce wrote: > > > > >> Is there any other PIC that has 4 (or more) > > independent PWM channels? > > > > > > I think some of the 18F may also, but I believe > > you can only use these > > > for motor driving, and not as independent > > channels. > > > > There are a number of PICs with 4 and more > > semi-independent PWM channels. > > Look for numbers of (E)CCP modules. They are > > independent in that you can > > run every (E)CCP module with a different duty cycle, > > but they are not fully > > independent in the selection of PWM frequencies -- > > you only have one or at > > most two different frequencies. (The 18F6722 is one > > with 5 (E)CCP channels > > that can be driven from two different timer sets.) > > > > But as others have said before, for two motors you > > only need two PWM > > channels, a few standard digital outputs (for > > direction and possibly > > breaking) and a bit of glue logic. > > > > Gerhard > > > > --

James James wrote: > Thank you everyone for your reply. I just want to clarify that the L298 > is a DUAL Full-Bridge driver. Driving 2 of them means I need 4 PWM > channels. I'm not sure, and please excuse me if I'm wrong, but I don't think you understood the suggestions about how to reduce the number of PWM channels required. The pairs of inputs for each channel are not driven independently. In normal PWM operation, one is the inverse of the other. Look at figure 6 in http://www.st.com/stonline/products/literature/ds/1773.pdf, for example. It is easier to provide this type of encoding with hard-wired logic (or a CPLD) than with the PIC's CCP modules. It would have one PWM input, an enable and possibly a break input, and two outputs for the driver inputs, and in between a few gates. To me, it seems that providing the required signals using the PIC's CCP modules is not exactly easy: they are designed to set their outputs all high at the same time. (The, or maybe some, ECCP modules have a polarity selection, but not the CCP modules.) > Is the frequency for the PWM matter? Is there a "rule of thump" on what > frequency is good? Since I am running the PIC at 40Mhz, it looks like > 2.4Khz is the slowest it will go. The driver seems to be fast enough for that frequency. Given that, this depends probably mostly on your motor. Normal DC motors seem to run fine with this frequency. Gerhard

From: "Gerhard Fiedler" <EraseMElistsspam_OUTTakeThisOuTconnectionbrazil.com> > James James wrote: > >> Thank you everyone for your reply. I just want to clarify that the L298 >> is a DUAL Full-Bridge driver. Driving 2 of them means I need 4 PWM >> channels. I missed the preceding discussion, so don't know what was suggested. You can drive each half using just one PWM per bridge. Modulate the enable line with PWM, while holding steady levels on the two In pins. The In pins select direction, while Enable modulates speed. That's two digital output lines per bridge, plus one PWM line. I find it odd that the Fig. 6 truth table is set up that way. Enable would be more sensibly useful as a current sense limiter, except modulating the In lines would cause braking rather than free-wheel coasting during the off periods. Ick. {Quote hidden}> > I'm not sure, and please excuse me if I'm wrong, but I don't think you > understood the suggestions about how to reduce the number of PWM channels > required. > > The pairs of inputs for each channel are not driven independently. In > normal PWM operation, one is the inverse of the other. Look at figure 6 in > http://www.st.com/stonline/products/literature/ds/1773.pdf, for example. > It > is easier to provide this type of encoding with hard-wired logic (or a > CPLD) than with the PIC's CCP modules. It would have one PWM input, an > enable and possibly a break input, and two outputs for the driver inputs, > and in between a few gates. I wouldn't bother putting gates in there for that. You still need a third state for braking, so it needs a second digital line for full control anyway. > To me, it seems that providing the required signals using the PIC's CCP > modules is not exactly easy: they are designed to set their outputs all > high at the same time. (The, or maybe some, ECCP modules have a polarity > selection, but not the CCP modules.) ECCP doesn't add much for Fig. 6 control. Full-bridge mode can drive the L298 In lines for direction control, but you would need an OR or XOR gate to modulate the B-D lines on the Enable pin. The only real gain is using the auto-shutdown comparators as a current limiter. Alas, I don't believe any of any mid-range or 18F come with dual ECCPs, at most one each of the enhanced and regular. To conclude, I think all you need is two independent, plain, ordinary CCPs. Each PWM line modulates the Enable line on one channel for speed control. You'll control the In lines directly for direction and braking.

Thanks again for everyone help. I think I got many of you confused. I am sorry about that. What I meant was I want to drive two L298 ICs, so that's 4 full-bridges in total. Mike Young wrote: “I missed the preceding discussion, so don't know what was suggested. You can drive each half using just one PWM per bridge …” James: Yes, but I am using TWO L298 ICs and each IC has two full-bridges. So the total is “4 half”, which means I will need 4 PWM channels, right? Julio Jumpa wrote: “you set an interruption every 8us…” James: This would work, but the PIC will have to spend a lot of time servicing the interrupts. It might not have enough time to update the LCD, read encoder, do some math, serial comm, etc. I am thinking of a dedicated PIC just to drive the two L298 ICs… --- Mike Young <mikewhyspam_OUTsbcglobal.net> wrote: {Quote hidden}> From: "Gerhard Fiedler" <@spam@listsKILLspamconnectionbrazil.com> > > > > James James wrote: > > > >> Thank you everyone for your reply. I just want > to clarify that the L298 > >> is a DUAL Full-Bridge driver. Driving 2 of them > means I need 4 PWM > >> channels. > > I missed the preceding discussion, so don't know > what was suggested. You can > drive each half using just one PWM per bridge. > Modulate the enable line with > PWM, while holding steady levels on the two In pins. > The In pins select > direction, while Enable modulates speed. That's two > digital output lines per > bridge, plus one PWM line. > > I find it odd that the Fig. 6 truth table is set up > that way. Enable would > be more sensibly useful as a current sense limiter, > except modulating the In > lines would cause braking rather than free-wheel > coasting during the off > periods. Ick. > > > > > I'm not sure, and please excuse me if I'm wrong, > but I don't think you > > understood the suggestions about how to reduce the > number of PWM channels > > required. > > > > The pairs of inputs for each channel are not > driven independently. In > > normal PWM operation, one is the inverse of the > other. Look at figure 6 in > > > www.st.com/stonline/products/literature/ds/1773.pdf, {Quote hidden}> for example. > > It > > is easier to provide this type of encoding with > hard-wired logic (or a > > CPLD) than with the PIC's CCP modules. It would > have one PWM input, an > > enable and possibly a break input, and two outputs > for the driver inputs, > > and in between a few gates. > > I wouldn't bother putting gates in there for that. > You still need a third > state for braking, so it needs a second digital line > for full control > anyway. > > > To me, it seems that providing the required > signals using the PIC's CCP > > modules is not exactly easy: they are designed to > set their outputs all > > high at the same time. (The, or maybe some, ECCP > modules have a polarity > > selection, but not the CCP modules.) > > ECCP doesn't add much for Fig. 6 control. > Full-bridge mode can drive the > L298 In lines for direction control, but you would > need an OR or XOR gate to > modulate the B-D lines on the Enable pin. The only > real gain is using the > auto-shutdown comparators as a current limiter. > Alas, I don't believe any of > any mid-range or 18F come with dual ECCPs, at most > one each of the enhanced > and regular. > > To conclude, I think all you need is two > independent, plain, ordinary CCPs. > Each PWM line modulates the Enable line on one > channel for speed control. > You'll control the In lines directly for direction > and braking. > > -

----- Original Message ----- From: "James James" <KILLspamjames_iboxKILLspamyahoo.com> > Mike Young wrote: "I missed the preceding discussion, > so don't know what was suggested. You can drive each > half using just one PWM per bridge ." > > James: Yes, but I am using TWO L298 ICs and each IC > has two full-bridges. So the total is "4 half", which > means I will need 4 PWM channels, right? That's right, one PWM for each bridge. Four bridges, four PWM. Four independent PWM if you're controlling 4 motors. You can use fewer if you're controlling, say, two steppers instead. > Julio Jumpa wrote: "you set an interruption every > 8us." > > James: This would work, but the PIC will have to > spend a lot of time servicing the interrupts. It > might not have enough time to update the LCD, read > encoder, do some math, serial comm, etc. I am > thinking of a dedicated PIC just to drive the two L298 > ICs. I keep a small bag of 12F683 and 16F690 around, like jelly-beans. I wouldn't think twice about putting one 12F per bridge on the board. But with all that going on, I'd seriously consider an AVR or ARM. The ATmega169 has dedicated LCD control, USART, 4 PWM channels, probably enough ADC, and one instruction per clock compared to PIC's 4 Tosc. The Butterfly eval board is $20, and comes with a '169 and LCD panel. If you like C, you'll find free GCC based tools, a nice change from the pay-in-blood PIC world. The serial ISP is $30; a soon to be released USB ISP is expected to be a few dollars more. It only comes in a 64 pin TQFP, which is either a deal breaker or a likable bonus. (If you don't already etch your own boards, maybe it's time to start thinking about it.)