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I use the one that is down and it works fine, but since I don't understand the difference between these circuits I decided to ask. Both circuits I found online, without explaining what is the difference between them. DVR8825

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  • \$\begingroup\$ Use your eyes. What do you see that is different? \$\endgroup\$
    – Andy aka
    Jun 25, 2020 at 9:47
  • \$\begingroup\$ I can describe the difference: reset and sleep pins are not connected to 2.5-5.25V, the same difference with the fault pin. I said: "but since I don't understand the difference between these circuits I decided to ask" I did not say that I don't see, I said I don't understand. \$\endgroup\$ Jun 25, 2020 at 9:55
  • \$\begingroup\$ OK, next... what does the data sheet for the module tell you about those pins? \$\endgroup\$
    – Andy aka
    Jun 25, 2020 at 10:16
  • \$\begingroup\$ First of all, the datasheet is about IC not about this module (I don't have the circuit for this module.) The datasheet describes what the pins do, it says nothing about connecting reset and sleep pins together (or maybe my keyword search was done poorly). Both pins seem to be input, why they connected together? maybe because they use the same pull up resistor on the module... I don't know. \$\endgroup\$ Jun 25, 2020 at 11:14
  • \$\begingroup\$ What to check for when buying an electronic component or module?. I'm done. \$\endgroup\$
    – Andy aka
    Jun 25, 2020 at 11:39

3 Answers 3

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Top one is minimal wiring - it will work, but that's all. Bottom one have more options - you can reset device, put it into sleep mode, and monitor fault condition (overheat or excessive current). And you can go deeper - configuring microsteps via M0-M2 pins, monitor current in bridges, etc. Just read the datasheet.

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  • \$\begingroup\$ I still don't get it, the datasheet for drv8825 says "Fault conditions are indicated via the nFAULT pin" if nFAULT is the same as FAULT then I don't understand why it is connected to 2.5-5.25V (bottom circuit). The datasheet is more about IC, and this module has other components, which makes this a bit confusing because I don't have the curcuit for this module. \$\endgroup\$ Jun 25, 2020 at 10:57
  • \$\begingroup\$ nFAULT it the same as F̅A̅U̅L̅T̅. Most text engines don't support overline, so it is often replaced by n. nFAULT connected to power because it is open-drain output, so it needs a pullup to voltage used in your scheme. This property is often used in multi-voltage systems. Other components are described in the datasheet too - look at section 8.2 for block diagram and section 6 for components' values. \$\endgroup\$
    – Morris
    Jun 25, 2020 at 11:19
  • \$\begingroup\$ It makes more sense to leave nFAULT unconnected if not used. Just tested my module, seems like F̅A̅U̅L̅T̅ connected not directly to pin on IC but via 1.5K resistor, which makes sense since it is connected to VDD 2.5-5.25V, so 1.5K resistor works as a pullup resistor. Also, it seems that there is 10K resistor between F̅A̅U̅L̅T̅ pin on the module and nSLEEP pin of IC. So when connecting F̅A̅U̅L̅T̅ (module pin) to VDD it pulls up nFAULT IC pin and nSLEEP IC pins. I think the module design is confusing. Thanks for help it is much clear now. \$\endgroup\$ Jun 25, 2020 at 12:07
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The bottom diagram from this section: Key differences between the DRV8825 and A4988 from the DRV8825's page.

If you look at the A4988's page, it is wired like so:

enter image description here

Unlike Morris' answer, "Bottom one have more options"--it doesn't. It's just a way to use the DRV8825 driver dev board with applications that had previously used the A4988.

The A4988 does not have an nFAULT output. Yet, for old applications that used the A4988, the "DRV8825 module can be used in systems designed for the A4988 that route logic power to this pin."

Since the DRV8825 module has more functionality, the question becomes what ways might it be useful to utilize nFAULT?

Connecting nFAULT to an input pin of the microcontroller would allow one to know if the H-bridge were disabled (LOW). Monitoring that pin would give data for any instances of overtemp/overcurrent.

As for the reset and sleep functions in both modules, they can be utilized for different power conservation. Setting the nRESET pin low sets the translator to the home position, and turns off all of the FET outputs.

Setting the nSLEEP pin low disables much of the internal circuitry including the output FETs, current regulator, and charge pump (used to generate a gate supply greater than that of VBB for driving the source-side FET gates).

8.3.6 nRESET, nENBL, and nSLEEP Operation

The nRESET pin, when driven active low, resets internal logic, and resets the step table to the home position. It also disables the H-bridge drivers. The STEP input is ignored while nRESET is active. The nENBL pin is used to control the output drivers and enable/disable operation of the indexer. When nENBL is low, the output H-bridges are enabled, and rising edges on the STEP pin are recognized. When nENBL is high, the H-bridges are disabled, the outputs are in a high-impedance state, and the STEP input is ignored. Driving nSLEEP low will put the device into a low power sleep state. In this state, the H-bridges are disabled, the gate drive charge pump is stopped, the V3P3OUT regulator is disabled, and all internal clocks are stopped. In this state all inputs are ignored until nSLEEP returns inactive high. When returning from sleep mode, some time (approximately 1 ms) needs to pass before applying a STEP input, to allow the internal circuitry to stabilize. Note that nRESET and nENABL have internal pulldown resistors of approximately 100 kΩ. The nSLEEP pin has an internal pulldown resistor of 1 MΩ. nSLEEP and nRESET signals need to be driven to logic high for device operation.

8.3.7.1 Overcurrent Protection (OCP)

An analog current limit circuit on each FET limits the current through the FET by removing the gate drive. If this analog current limit persists for longer than the OCP time, all FETs in the H-bridge will be disabled and the nFAULT pin will be driven low. The device remains disabled until either nRESET pin is applied, or VM is removed and reapplied. Overcurrent conditions on both high-side and low-side devices; that is, a short to ground, supply, or across the motor winding all result in an overcurrent shutdown. Note that overcurrent protection does not use the current sense circuitry used for PWM current control, and is independent of the SENSE resistor value or xVREF voltage.

https://www.ti.com/lit/ds/symlink/drv8825.pdf

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The short answer: The top picture is using the board the way it was designed to be used; the bottom picture is using the board in an unusual way which happens to work some of the time (but could destroy the board, depending on what kind of board it is).

When you're using a DRV8825, the most straightforward thing is to connect a logic HIGH signal to nSLEEP and nRESET, and you don't need to connect anything to nFAULT. So, the top diagram has the board connected in exactly the most straightforward way: it has you connecting VDD to nSLEEP and nRESET, and leaving nFAULT. This will work with pretty much any DRV8825 breakout board.

Given the above information, the bottom diagram doesn't seem to make a lot of sense, right?

Normally, it doesn't make any sense to connect a logic HIGH signal to nFAULT, because nFAULT is an output. However, it turns out that Pololu's #2133 DRV8825 board is designed to tolerate having nFAULT connected to logic HIGH. If you do that with a different DRV8825 board, that may cause damage (check the datasheet for the DRV8825 chip and the board to see whether or not that's the case).

Also, normally, it doesn't make any sense to connect nSLEEP and nRESET directly to each other without connecting them to anything else. However, it turns out that Pololu's #2133 DRV8825 board has the nSLEEP and nFAULT pins connected together through a 10 kΩ resistor. As a result, on this particular board, if nFAULT is connected to logic HIGH, and nSLEEP and nFAULT are connected together, then both nSLEEP and nFAULT will be held HIGH, too.

So, the top diagram is the normal way to do things and will work with pretty much any DRV8825 board. The bottom diagram is an unusual way which happens to work with this Pololu board, but probably won't work with other boards.

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