TinyTapeout is an educational project that aims to make it easier and cheaper than ever to get your digital designs manufactured on a real chip! This is my submission for TinyTapeout 5 in early November 2023.
This design is a programmable interrupt timer that allows you to specify when a digital line is pulled high after a given number of clock ticks, the timer can either repeat or be one-shot.
This started as a reimplementation of the Intel 8253 programmable interval timer which I stripped down to the bare minimal configuration and wires required to be functional within the wire and gate constraints of a TinyTapeout project.
The following is pseudocode for configuring an interrupt pin to go high for one cycle after 10 cycles (plus 2 for setup).
uio_in = 0xA0
ui_in = 0xA
This will set the interrupt timer to be one-shot and set a 8-bit counter of 0xA (integer 10). Wait 12 cycles and then the last pin in uio_out will be high.
There are three configuration registers, written to the data held in ui_in and selected with the high 3 bits from uio_in:
uio_in[7] sets write enable (WE), which says the timer is being configured when pulled high.
The concatenation of {uio_in[5], uio_in[6]} selects which register is being written.
As previously stated, {uio_in[5], uio_in[6]} are the two bits used to select how to configure the timer. The meanings of the four registers are given below:
00sets the options: divider on and repeating01sets the high bit10sets the low bit11is unused
When you select the 00 register, there are only two options available:
divider_on <= ui_in[7];
This bit selects whether a 10x clock divider is used.
repeating <= ui_in[6];
This bit selects if you want the interrupt to repeat. The default value is for the interrupt to be one-shot.
The registers selected with 01 and 10 each specify which 8-bits of the 16-bit register to set.
The bits of 01 followed by the bits of 10 determine the full 16-bits of the timer value. 01 does not need to be set if you don't need more than 8-bits but 10 must be set, if only to zero, for the timer to start.
Configuring the third register (10) is what starts the timer as the other two registers are optional. The default settings if the first register isn't set are to be a one-shot timer with no clock divider.
Programmed IO setup is annoying and I spent a lot of time on it. I think I would prefer to use an SPI ROM or RAM and utilize more of a memory-mapped approach in the future.
I don't have a lot of tools in my toolbox for squeezing cycles out of a design that leans on registers which leads me to the RTL errata section.
No matter how long the timer is set for, there are a few extra cycles due to setup or latching.
Go to https://tinytapeout.com for instructions!