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(fpga) Update README with info on the free running mode.

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Signed-off-by: Joachim Strömbergson <joachim@assured.se>
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Joachim Strömbergson 2024-08-15 12:43:30 +02:00
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hw/application_fpga/core/timer/README.md
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# timer
A simple timer with prescaler.
Timer with prescaler and support for free running mode.
## Introduction
This core implements a simple timer with a prescaler. The prescaler
allows measurement of time durations rather than cycles. If for
example setting the prescaler to the clock frequency in Hertz, the
timer will count seconds.
This core implements a simple timer with a prescaler and support for a
free running mode.
The prescaler allows measurement of time durations rather than
cycles. If for example setting the prescaler to the clock frequency in
Hertz, the timer will count seconds. After (prescaler * timer) number
of cycles the timer will stop. Checking status of the timer can be
done by reading the STATUS_RUNNING_BIT. If set to zero, the timer has
completed.
If the free running mode is set (default off), the counter will not
stup when the number of cycles defined by (prescaler * timer) has been
reached. Instead the timer continues until the CTRL_STOP_BIT is
asserted. Note that in free running mode, the ADDR_PRESCALER shall be
set to one (1).
## API
The following addresses define the API for the timer:
```
ADDR_CTRL: 0x08
CTRL_START_BIT: 0
CTRL_STOP_BIT: 1
ADDR_CTRL: 0x08
CTRL_START_BIT: 0
CTRL_STOP_BIT: 1
ADDR_STATUS: 0x09
ADDR_STATUS: 0x09
STATUS_RUNNING_BIT: 0
ADDR_PRESCALER: 0x0a
ADDR_TIMER: 0x0b
ADDR_PRESCALER: 0x0a
ADDR_TIMER: 0x0b
ADDR_FREE_RUNNING: 0x0c
FREE_RUNNING_BIT 0
```
## Details
The core consists of the timer_core module (in timer_core.v) and a top
level wrapper, timer (in timer.v). The top level wrapper implements
the API, while the timer_core implements the actual timer
functionality.
The timer counter and the prescaler counter are both 32 bits.
When enabled the counter counts down one integer value per cycle.
The timer will stop when reaching final zero (given by prescaler times the initial value of the timer)
and the running flag will be lowered.