Hello everyone,
It took a bit of time to messing around today to get this to work, so I thought I would post (details may be specific to the STM32F4 microcontroller I am using).
But I do have a question also for any STM32 experts at the end.
On the STM32 platform the if you pick a external crystal with frequency that is a multiple of the 96kHz then you can, unsurprisingly, get 0% error between desired and actual I2S frequency. 24.576MHz / 96kHz = 256, so 24.576MHz seems a good choice being a standard frequency with lots of options of low-cost crystals.
USB needs exactly 48MHz clock, fortunately 48MHz / 96kHz = 500. Below is a solution that gives exactly 96kHz for I2S and 48MHz for USB.
Question
What is I2S_APB1 CLK and why does it having a value of 147.456MHz make everything work so well? I mean, this is 6x the crystal frequency and 1536 x 96kHz but why is this the required value for the I2S_APB1 CLK?
It took a bit of time to messing around today to get this to work, so I thought I would post (details may be specific to the STM32F4 microcontroller I am using).
But I do have a question also for any STM32 experts at the end.
On the STM32 platform the if you pick a external crystal with frequency that is a multiple of the 96kHz then you can, unsurprisingly, get 0% error between desired and actual I2S frequency. 24.576MHz / 96kHz = 256, so 24.576MHz seems a good choice being a standard frequency with lots of options of low-cost crystals.
USB needs exactly 48MHz clock, fortunately 48MHz / 96kHz = 500. Below is a solution that gives exactly 96kHz for I2S and 48MHz for USB.
Question
What is I2S_APB1 CLK and why does it having a value of 147.456MHz make everything work so well? I mean, this is 6x the crystal frequency and 1536 x 96kHz but why is this the required value for the I2S_APB1 CLK?
What MCU are you planning to use? IMO the best approach is to feed audio clock to I2S_CKIN (PC9) and use non-audio clock for HSE (e.g. 8/16/25MHz).
What do you normally use to create the I2S_CKIN? A don’t have any other clock master on the board other than an STM32.
Anyway, the above shows how it is possible using a single 24.576MHz crystal that is fairly standard. The reason I put effort into this is to get the clocking working with one crystal.
It’s an STM32F479.
But the thing I don’t understand is the significance of I2S_APB1 CLK being 147.456MHz
Anyway, the above shows how it is possible using a single 24.576MHz crystal that is fairly standard. The reason I put effort into this is to get the clocking working with one crystal.
It’s an STM32F479.
But the thing I don’t understand is the significance of I2S_APB1 CLK being 147.456MHz
