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4 years, 12 months ago.
Copy 1 PWM signal to 20
I need to copy 1 PWM signal to 20. 100KHz Frequency(Period = 10us, Duty=5%) So MBED's PWM output is not enough. Is there any solution? Thanks a lot...
(I'm using MBED NXP LPC1768. But due to some reasons, can not change to other MCU.)
1 Answer
4 years, 11 months ago.
Hello Jazz 0529,
If you need to connect the same PWM signal (generated with MBED LPC1768 at one of the PWM outputs) to 20 inputs then you will need some external components. One solution is to connect the same PWM signal to 20 OpAmps configured as buffer. Because OpAmps have very high input impendance they will not overload the Mbed chip. To reduce the number of external components try to select a type where more OpAmps are built into one package.
A digital buffer IC would probably be a better choice than an analog amplifier. As long as you use a CMOS logic family the input impedance will still be very high, the pin capacitance will be more of an issue than the input impedance. Typical currents per input are in the uA region, a good op-amp will be less but cheap ones will be more.
Plus digital buffers will drop the component count a lot. e.g. A SN74AHC16244 from TI will give you 16 buffers in one device. Since you have 16 buffers in a part you will have spares. So feed the mbed to drive 4 buffers. Use each of those 4 buffers to drive 5 more buffers. That gives you 20 outputs with a maximum fanout of 5. Total component count is 2 ICs and 2 capacitors (a bypass cap in each IC power pin).
You could instead use 1 16 way buffer and an 8 way buffer but it's the same component count and minimal cost saving, it's often easier just to use the same part multiple times and have the unused buffers available if you need them. Just don't forget to tie un-used inputs to a known state.
posted by 02 May 2019