1 Overview
This document describes an hardware event timer component suitable for use in real-time applications. The component provides a single free-running counter and some deadline value registers which are compared to this absolute time base.
Depending on implementations the main counter and deadline values can be up to 64 bits wide while memory mapped register accesses are 32 bits wide.
Some operations are available to atomically update deadline values relative to the main counter using a single write operation from the host processor. The component has been designed to avoid race conditions checking in software by using less or equal comparators for deadlines.
2 Operation
The component contains a 16 bits prescaler stage. The prescaler counter register is decreased by one on rising edges of the input clock if the counter enabled bit of the control register is set. Each time the prescaler counter reaches zero, it is reloaded with the value of the prescaler reload register and the main counter register is increased by one.
The 64 bits main counter register value is compared on to all deadline register values on all cycles where no other operations are requested. When a deadline value is less or equal than the main counter value, the deadline is reached and appropriate action is taken depending on associated configuration:
- When the associated bit in the period enable register is set, the value of the deadline period register is added to the current deadline value. An interrupt is raised if the associated bit in the interrupts enable register is set.
- When the associated bit in the period enable register is cleared, An interrupt is raised if the associated bit in the interrupts enable register is set and the interrupt is then disabled by clearing this bit.
Several operations can be performed to update deadlines:
- The 64 bits deadline registers can be written using 32 bits accesses. Use of absolute deadline values enables timer reprogramming to handle earlier events without timing inaccuracy.
- The deadline set registers can be used to atomically set a deadline with specified 32 bits delay. This allows scheduling a delayed interrupt with a single 32 bits write from host the processor without even disabling interrupts.
- The deadline add registers can be used to atomically add a 32 bits value to a deadline. This allows modifying the delay of an active deadline without entering a critical code section or conditionally rescheduling an event from the interrupt handler with the exact same delay for instances.
- The deadline copy register can be used to copy or shift deadline values between deadline registers.
Whether deadline interrupts are enabled automatically when one of these operations is performed is globally controlled by bits in the control register.
3 Timer registers map
Real-time timer registers table
| Name | Address | Direction | Long name | Description |
|---|---|---|---|---|
sccnt | 0x0 | rw | Prescaler counter register | Prescaler counter value is decremented on rising edge of the input clock and reloaded with a configurable value when it reaches zero. |
scrld | 0x4 | rw | Prescaler reload register | This register holds the reload value for the prescaler counter. The maximum value is 0xffff. |
cfg | 0x8 | r | Device configuration register | This read only register holds timer component informations |
ctrl | 0xc | rw | Control register | This register contains bits to configure various device behaviors |
rtcl | 0x10 | r | Main counter lower 32 bits value | This register contains the lower 32 bits of the current value of the timer and latches its higher 32 bits in the rtctmp register on read. |
rtch | 0x14 | r | Main counter higher 32 bits value | This register contains the higher 32 bits of the current value of the timer and latches its lower 32 bits in the rtctmp register on read. |
rtctmp | 0x18 | rw | 32 bits values scratch register | This register is used to store lower or higher 32 bits of 64 bits values when accessing main counter or deadlines values. |
pe | 0x20 | rw | Period enable register | If a bit is set in this register, the corresponding deadline register is updated with addition of the associated period register each time the deadline is reached. |
ie | 0x24 | rw | Interrupts enable register | If a bit is set in this register, an interrupt will be generated when the corresponding deadline is reached. |
ip | 0x28 | rw | Interrupts pending register | A bit is set in this register when an interrupt has been generated by the corresponding deadline. Setting a bit to one in this register clears the interrupt pending flag. |
copy | 0x2c | w | Deadline copy command register | When bit N is written to one in this register, the associated deadline register value is overwritten with the value of the N+1 deadline register. The last deadline register can not be overwritten. |
cancel | 0x30 | w | Deadline cancel register | When bit N is written to one in this register, the corresponding irq enable and pending flags are both cleared. |
Real-time timer registers table (deadline registers)
dln1 | 0x80 | rw | Deadline1 value register | This register contains lower 32 bits of the deadline value. When read, the higher 32 bits of the deadline value is latched in the rtctmp register. When written to, the deadline register higher 32 bits are set to the value of the rtctmp register. |
dln1s | 0x84 | w | Deadline1 set register | When writting to this register, the associated 64 bits deadline register is set to the sum of the timer current value and the written unsigned value. |
dln1a | 0x88 | w | Deadline1 add register | When writting to this register, the written value is sign-extended to 64 bits and added to the associated deadline value. |
dln1p | 0x8c | rw | Deadline1 period register | The value of this register is added to the deadline value when the associated bit in the pe register is set and the deadline is reached. |
dln2 | 0x90 | rw | Deadline2 value register | |
dln2s | 0x94 | w | Deadline2 set register | |
dln2a | 0x98 | w | Deadline2 add register | |
dln2p | 0x9c | rw | Deadline2 period register | |
dln3 | 0xa0 | rw | Deadline3 value register | |
dln3s | 0xa4 | w | Deadline3 set register | |
dln3a | 0xa8 | w | Deadline3 add register | |
dln3p | 0xac | rw | Deadline3 period register | |
dln4 | 0xb0 | rw | Deadline4 value register | |
dln4s | 0xb4 | w | Deadline4 set register | |
dln4a | 0xb8 | w | Deadline4 add register | |
dln4p | 0xbc | rw | Deadline4 period register | |
dln5 | 0xc0 | rw | Deadline5 value register | |
dln5s | 0xc4 | w | Deadline5 set register | |
dln5a | 0xc8 | w | Deadline5 add register | |
dln5p | 0xcc | rw | Deadline5 period register | |
dln6 | 0xd0 | rw | Deadline6 value register | |
dln6s | 0xd4 | w | Deadline6 set register | |
dln6a | 0xd8 | w | Deadline6 add register | |
dln6p | 0xdc | rw | Deadline6 period register | |
dln7 | 0xe0 | rw | Deadline7 value register | |
dln7s | 0xe4 | w | Deadline7 set register | |
dln7a | 0xe8 | w | Deadline7 add register | |
dln7p | 0xec | rw | Deadline7 period register |
Real-time timer cfg register fields
| Name | Width | Direction | Long name | Description |
|---|---|---|---|---|
dl_cnt | 3 bits | r | Deadline registers count | |
irq | 5 bits | r | Irq vector number | |
si | 1 bits | r | Separate interrupts |
Real-time timer ctrl register fields
| Name | Width | Direction | Long name | Description |
|---|---|---|---|---|
ce | 1 bits | rw | Chip enabled | When cleared, this bit prevents the timer from running |
iew | 1 bits | rw | Interrupt enable on deadline write | When set, deadline interrupt is automatically enabled when its value is updated by writting to the associated dln register |
ies | 1 bits | rw | Interrupt enable on deadline set | When set, deadline interrupt is automatically enabled when its value is updated by writting to the associated dln set register |
iea | 1 bits | rw | Interrupt enable on deadline add | When set, deadline interrupt is automatically enabled when its value is updated by writting to the associated dln add register |
iec | 1 bits | rw | Interrupt enable on deadline copy | When set, deadline interrupt is automatically enabled when its value is updated by writting to copy register |
