[U-Boot] [PATCH] davinci: omapl138_lcdk: fix PLL0 frequency

[David Lechner]

On 03/16/2018 01:26 AM, Mike Looijmans wrote:
> 
> On 15-03-18 02:36, David Lechner wrote:
>> commit 1601dd97edc6 ("davinci: omapl138_lcdk: increase PLL0 frequency")
>> changed the PLL0 frequency to 456MHz, which is needed for the LCDC IP
>> block. However, in doing so, it caused the PLLOUT clock to be outside
>> of the allowable specifications given in the OMAP-L138 data sheet. (It
>> says PLLOUT must be 600MHz max). It also uses a PLLM value outside of
>> the range given in the TRM (it says PLLM must in the range 0 to 0x1f).
>>
>> So here is what we have currently:
>>
>> PLLOUT = 24 / (0 + 1) * (37 + 1) = 912MHz (out of spec)
>>           ^     ^         ^
>>         CLKIN PREDIV    PLLM (out of spec)
>>
>> input to PLLDIVn = 912 / (1 + 1) = 456MHz (desired result)
>>                      ^     ^
>>                   PLLOUT POSTDIV
>>
>> This changes the PLLM value to 18 and the POSTDIV value to 0 so that
>> PLLOUT is now within specification but we still get the desired
>> result.
>>
>> PLLOUT = 24 / (0 + 1) * (18 + 1) = 456MHz (within spec)
>>           ^     ^         ^
>>         CLKIN PREDIV     PLLM
>>
>> input to PLLDIVn = 456 / (0 + 1) = 456MHz (desired result)
>>                      ^     ^
>>                   PLLOUT POSTDIV
>>
> 
> These are indeed the correct settings for 456MHz operation, I've used them in an L138 device a few years ago.
> 
> Did you also increase the CPU voltage to 1.3V? This can be done writing to the I2C power management chip, in my stone age setup I had this piece of code to do that:
> 
> #define TPS6507X_I2C_ADDR    0x48
> #define TPS6507X_REG_DEFLDO2    0x17
> u8 buf = 0x17;
> i2c_write(TPS6507X_I2C_ADDR, TPS6507X_REG_DEFLDO2, 1, &buf, 1)
> 
> Without the voltage change, some batches of CPUs will fail to boot properly at 456MHz.


I was wondering about this, but it looks to me like the LCDK board (at least the A7E
revision) has a fixed regulator at 1.3V (TPS650250RHBR). Perhaps you are using a
different board?