Add rtc docs

doc/rtc/rtc.md

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+# RTC
+
+## Overview
+
+The RTC is capable of keeping track of elapsed time as well as generating
+periodic interrupts. Its input frequency is (currently) the same as the CPU
+frequency. The architecture is shown below:
+
+![rtc.drawio.png](rtc.drawio.png)
+
+The `counter` register is a 32 bit register which increments by `increment`
+every clock tick. When this value reaches `threshold`, both `output` and
+`irq_counter` are incremented by 1. When `irq_counter` reaches `irq_threshold`,
+an interrupt is generated.
+
+The interrupt frequency can be calculated with the following equations:
+
+$$ F_{out}=\frac{F_{in}\cdot{increment}}{threshold} $$
+$$ F_{irq}=\frac{F_{out}}{irq\_threshold} $$
+
+Where $ F_{out} $ is the frequency at which the output changes and $ F_{irq} $
+is the frequency at which interrupts occur.
+
+## Interface
+
+The RTC is controlled through two 8-bit registers, `CMD` and `DAT`.
+The upepr 4 bits of `CMD` control which register is accessed, and the lower 4
+bits control the byte select. The RTC uses 32 bit registers, so only values of
+0-3 are supported.
+
+### CMD
+| [7:4]            	| [3:2     	| [1:0]       	|
+|------------------	|----------	|-------------	|
+| Register Address 	| Reserved 	| Byte Select 	|
+
+### DAT
+| [7:0]                 	|
+|-----------------------	|
+| Data[8*Byte_sel +: 8] 	|
+
+## Registers
+
+### Register Map
+
+| Address 	| Read          	| Write         	|
+|---------	|---------------	|---------------	|
+| 0       	| Threshold     	| Threshold     	|
+| 1       	| Increment     	| Increment     	|
+| 2       	| IRQ Threshold 	| IRQ Threshold 	|
+| 3       	| Output        	| Control       	|
+
+### Threshold
+
+32 bit threshold register. When `counter` reaches this value, `output` and
+`irq_counter` are incremented by 1, and `counter` is reset back to 0.
+
+### Increment
+
+32 bit increment register. Every clock cycle, `counter` is incremented by
+`increment`
+
+### IRQ Threshold
+
+32 bit IRQ threshold register. When `irq_counter` reaches this value, and IRQ
+is generated and `irq_counter` is reset back to 0.
+
+### Output
+
+32 bit output register. This value ticks up.
+
+### Control
+
+| [7:2]    	| [1]        	| [0]    	|
+|----------	|------------	|--------	|
+| Reserved 	| IRQ Enable 	| Enable 	|
+
+8 bit Control register. Regardless of Byte Select, any write to address 3 will
+write to this register. `IRQ Enable` controls interrupt generation. `Enable` is
+a global enable. When 0, all counter registers are set to 0.
+
+
+