apb_target_rv_timer

Files
file	apb_target_rv_timer.cdl
	RISC-V compatible timer target for an APB bus.

Detailed Description

Modules

module apb_target_rv_timer::apb_target_rv_timer	(	clock	clk,
		input bit	reset_n,
		input t_timer_control	timer_control,
		input t_apb_request	apb_request,
		output t_apb_response	apb_response,
		output t_timer_value	timer_value
	)

Parameters

[in]	clk	System clock
[in]	reset_n	Active low reset
[in]	timer_control	Control of the timer
[in]	apb_request	APB request
[out]	apb_response	APB response
[out]	timer_value	RISC-V compatible timer with an APB interface.

This is a monotonically increasing 64-bit timer with a 64-bit comparator.

The timer has a fractional component to permit, for example, a 'nanosecond' timer that is clocked at, say, 600MHz; in this case the timer is ticked every 1.666ns, and so an addition in each cycle of 0xa to a 4-bit fractional component and a 1 integer component. The timer_control has, therfore, a fixed-point adder value with a 4-digit fractional component.

However, this would actually lead to a timer that would be only 99.61% accurate.

Hence a further subfraction capability is supported; this permits a further 1/16th of a nanosecond (or whatever the timer unit is) to be added for M out of every N cycles.

In the case of 600MHz a bonus 1/16th should be added for 2 out of every 3 cycles. This is set using a bonus_subfraction_numer of 0 and a bonus_subfraction_denom of 2 (meaning for 2 out of every 3 cycles add a further 1/16th).

Hence every three cycles the timer will have 0x1.b, 0x1.b, 0x1.a added to it - hence the timer will have gone up by an integer value of 5ns, which is correct for 3 600MHz clock cycles.

If the control values are tied off to zero then the extra fractional logic will be optimized out.

Some example values for 1ns timer values:

Clock	Period	Adder (Int/fraction)	Subfraction Numer/Denom
1GHz	1ns	1 / 0x0	0 / 0
800MHz	1.25ns	1 / 0x4	0 / 0
600MHz	1.66ns	1 / 0xa	0 / 2

The period should be:

subfraction numer/denum=0/x: (Int + fraction/16)

subfraction numer/denum=M/N: (Int + (fraction+(N-M-1)/N)/16)