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Relax us ticker frequency requirement. #1123

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Aug 15, 2019
Merged

Relax us ticker frequency requirement. #1123

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1350e0b
Relax us ticker frequency requirement.
mprse Aug 5, 2019
7bcac4d
Edit us_ticker.md
Aug 6, 2019
39d4085
Merge branch 'development' into us_ticker_freq_fix
Aug 6, 2019
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13 changes: 7 additions & 6 deletions docs/porting/target/us_ticker.md
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Expand Up @@ -6,7 +6,8 @@ Implementing the microsecond ticker enables Mbed OS to perform operations that r

### Defined behavior

- Has a reported frequency between 250KHz and 8MHz.
- Has a reported frequency between 250 KHz and 8 MHz for counters that are less than 32 bits wide.
- Has a reported frequency up to 100 MHz for counters that are 32 bits wide.
- Has a counter that is at least 16 bits wide.
- The function `ticker_init` is safe to call repeatedly.
- The function `ticker_init` allows the ticker to keep counting and disables the ticker interrupt.
Expand All @@ -17,7 +18,7 @@ Implementing the microsecond ticker enables Mbed OS to perform operations that r
- The ticker interrupt fires only when the ticker time increments to or past the value set by `ticker_set_interrupt`.
- It is safe to call `ticker_set_interrupt` repeatedly before the handler is called.
- The function `ticker_fire_interrupt` causes `ticker_irq_handler` to be called immediately from interrupt context.
- The ticker operations `ticker_read`, `ticker_clear_interrupt`, `ticker_set_interrupt` and `ticker_fire_interrupt` take less than 20us to complete.
- The ticker operations `ticker_read`, `ticker_clear_interrupt`, `ticker_set_interrupt` and `ticker_fire_interrupt` take less than 20 us to complete.
- The ticker operations `ticker_init` and `ticker_read` are atomic.

### Undefined behavior
Expand All @@ -31,13 +32,13 @@ Implementing the microsecond ticker enables Mbed OS to perform operations that r

Be careful around these common trouble areas when implementing this API:

- The ticker cannot drift when rescheduled repeatedly
- The ticker keeps counting when it rolls over
- The ticker interrupt fires when the compare value is set to 0 and overflow occurs
- The ticker cannot drift when rescheduled repeatedly.
- The ticker keeps counting when it rolls over.
- The ticker interrupt fires when the compare value is set to 0 and overflow occurs.

## Dependencies

To implement this API, the device must have a hardware counter that has a count value at least 16 bits wide and can operate between 250KHz and 8MHz.
To implement this API, the device must have a hardware counter that has a count value at least 16 bits wide and can operate between 250 KHz and 8 MHz.

## Implementing the microsecond ticker API

Expand Down