SLVUA41E
9
2.3 Quick guide to spin with speed control
The second GUI tab operates the DRV8308 in Clock Frequency Mode, utilizing the closed-loop speed
control system. It’s important to determine the right frequency for your desired RPM; for example:
If you are using FGSEL = 00b (Hall U), and the mot
or’s rotor has 8 magnet poles, there will be 4
Hall U cycles per revolution and
RPM = f
CLKIN
/4 * 60
.
If you are using FGSEL = 10b (FG), and there are 42 FG cycles per physical revolution, then
RPM = f
CLKIN
/42 * 60
.
If you’re unsure of your motor’s number of Hall U cycles per revolution, you can test it by doing this:
disconnect the 3 phase wires, enable the DRV8308 (to apply Hall power), probe the Hall U testpoint,
and use your hand to rotate the motor 360° while counting the number of cycles.
The included TelcoMotion motor has 8 magnetic poles, and the below steps will involve a 100Hz f
CLKIN
,
so RPM will be 1500.
Step 1:
Un-click ENABLE so the motor is stopped.
Step 2:
Select the second GUI tab. Then:
Set AG_SETPT to 5 - 95Hz
Set LOOPGAIN to 200
Set MOD120 to 3970
Select CLK from MSP430
Step 3:
Click ENABLE to spin at exactly 1500 RPM. Connecting a scope probe to FGOUT allows you
to observe the Hall U frequency. If you load the motor with your fingers, you will see power supply
current increase, but Hall U frequency stay the same since speed is locked.
If you’re able to stop the
motor by squeezing hard enough, your power supply is probably set to 1A or less.
2.4 The third GUI tab
The last tab shows all the device registers, for the highest level of complexity and control. Changes
made to this tab stay in effect when switching back to the first two tabs.
By default, the “Auto Write” checkbox is enabled, so changes you make are immediately written
through SPI.
The button “Burn OTP Memory” permanently writes the DRV8308 non-volatile memory with the
registers shown. ENABLE must be active for it to work. After writing the OTP, whenever the DRV8308
is powered up with SMODE = Low, the default registers will be what was programmed.
Here’s how
you can verify the OTP was successfully written: uncheck Auto Write, cycle power to the EVM, type a
random number into one of the register fields, click the Read All button, and verify the random number
gets overwritten to the correct value.
