UM0851 datasheet(28/245 Pages) STMICROELECTRONICS

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Platform section

UM0851

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Doc ID 16604 Rev 2

Setting the source clock

There are two possible clock sources for each timer block. One source can be directly from

PLL3 (constant 48 MHz) and the other can be from PLL1 through synthesizer. Either of

these source clocks can be selected by the following API.

Note:

The selection of the source clock applies to the whole timer hardware block thus affecting

both channels.

When the PLL1 (system) clock is selected as the timer clock, any change in the system

clock for power saving, etc. has an impact on the frequency of the GPT. In such cases, you

must always get the current clock rate for programming next GPT interrupt. Please refer to

clock framework chapter for information on clock framework usage.

The use of 48 MHz (PLL3) is discouraged as it leads to unpredictable results in reading

counter value. Please refer to the GPT application note for details.

/* set the appropriate clock src

source can be:

SPEAR_TIMER_SRC_SYS_CLK,from PLL1

SPEAR_TIMER_SRC_PLL3_CLK,from PLL3

void spear_timer_set_source(struct spear_timer *timer, int source);

Programming the prescaler

After configuring the appropriate clock source, you can program the prescaler. The

prescaler can be different for each timer channel (within a hardware block) and can range

from 1 to 256 in 8 levels.

/* set the appropriate prescaler

prescaler can be

GPT_CTRL_PRESCALER1 0x0

GPT_CTRL_PRESCALER2 0x1

GPT_CTRL_PRESCALER4 0x2

GPT_CTRL_PRESCALER8 0x3

GPT_CTRL_PRESCALER16 0x4

GPT_CTRL_PRESCALER32 0x5

GPT_CTRL_PRESCALER64 0x6

GPT_CTRL_PRESCALER128 0x7

GPT_CTRL_PRESCALER256 0x8

spear_timer_set_prescaler( struct *spear_timer, int prescaler);

GPT interrupt management

Each timer channel of a hardware block of GPT has separate independent interrupt lines.

You can use the following API to know which interrupt line is associated with the timer. Then

using Linux calls, you can attach an interrupt handler to these irq lines. You can use

separate APIs to enable and clear the interrupts for a timer.

/* This shall return the irq associated with timer */

int spear_timer_get_irq(struct spear_timer *timer);

/* Enable/Disable the timer match interrupt */

void spear_timer_set_match(struct spear_timer *timer, int enable);

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