From 1353b3da9cf94d96557cba281b381b62a8a3c510 Mon Sep 17 00:00:00 2001
From: Matt Johnston <matt@ucc.asn.au>
Date: Sat, 1 Oct 2022 19:17:35 +0800
Subject: [PATCH] starting ascii art
---
examples/rp/src/bin/usb_serial.rs | 86 +++++++++++++++++++++++--------
1 file changed, 65 insertions(+), 21 deletions(-)
diff --git a/examples/rp/src/bin/usb_serial.rs b/examples/rp/src/bin/usb_serial.rs
index 0d10d4fb..17dc04f4 100644
--- a/examples/rp/src/bin/usb_serial.rs
+++ b/examples/rp/src/bin/usb_serial.rs
@@ -2,7 +2,7 @@
#![no_main]
#![feature(type_alias_impl_trait)]
-use defmt::{info, panic};
+use defmt::{info, debug, panic};
use embassy_executor::Spawner;
use embassy_time::{Timer, Duration};
use embassy_rp::pac as pac;
@@ -112,18 +112,47 @@ async fn run<'d, P: Pin>(pin: &mut Flex<'d, P>, syst: &mut SYST) -> Result<(), D
let mut up_x = 1u32;
let mut down_x = 1u32;
- let mut up = 0u32;
- let mut down = 0u32;
+
+ /*
+ |prev | |
+ |iter | |
+ | | |
+ . |
+ . . | .
+ . . | . .
+ . . | . .
+ .---------------.---------------.-----------.------ GPIO threshold. Probably isn't really flat.
+ . . .
+ . .
+ . .
+ .
+ */
loop {
- // number of loops per interrupt-free block
+ // number of iterations per interrupt-free block
const LOOP: usize = 2000;
- // count of cycles to target
- const T: u32 = 256;
-
- // BUF iterations from the start (or end?) will be printed
- const BUF: usize = 16;
+ // Range of cycle counts for overshoot. Upper limit is the pullup/pulldown time
+ // from vcc/gnd to threshold.
+ // A large upper limit will also decrease the number of samples/second (but will
+ // perhaps have more entropy bits/sample?)
+ // Lower limit is necessary because we don't want to hit the threshold immediately
+ // on reading.
+ // Empirical values for a 1uF capacitor.
+ // TODO this could be determined from a calibration step?
+ // const UPRANGE: u32 = 50000;
+ // const LOWRANGE: u32 = 15000;
+ // nice integers
+ const LOW_OVER: u32 = 15000;
+ const HIGH_OVER: u32 = LOW_OVER + 8192;
+
+ // for debug printout
+ // BUF iterations from the start (or end?) will be printed. must be <LOOP
+ const BUF: usize = 64;
+ // for end of buffer
+ // const BUF_START: usize = LOOP - BUF;
+ // for start of buffer
+ const BUF_START: usize = 0;
let mut ups = [0u32; BUF];
let mut downs = [0u32; BUF];
let mut upx = [0u32; BUF];
@@ -135,24 +164,35 @@ async fn run<'d, P: Pin>(pin: &mut Flex<'d, P>, syst: &mut SYST) -> Result<(), D
// taken for the return to centre value (which is targetting `T`).
// eg `up_x = step(up_x, down);`
let step = |prev: u32, meas: u32| {
- if meas < T {
- (prev + meas)*2
+ // doubling step.
+ // x[n] = 2*x[n-1] + meas
+ let v = prev*2 + meas;
+
+ // modulo to sensible range
+ if v > UPRANGE {
+ LOWRANGE + (v % (UPRANGE - LOWRANGE))
} else {
- prev - prev/4 + meas % T
- }.max(1)
+ v
+ }
};
critical_section::with(|_cs| {
+ // We alternate between taking the measured down/up values as real output,
+ // and using it to compute the next overshoot.
+ // That prevents leaking "real output" values in the timing of subsequent overshoot
+ // delays.
for i in 0..LOOP {
pin.set_pull(Pull::Up);
syst.clear_current();
let t1 = SYST::get_current();
while pin.is_low() {}
let t2 = SYST::get_current();
- up = t1 - t2;
+ let t_up = t1 - t2;
if i % 2 == 0 {
- down_x = step(down_x, up);
+ down_x = step(down_x, t_up);
+ } else {
+ // TODO: keep `up` as our random output
}
cortex_m::asm::delay(up_x);
@@ -161,21 +201,25 @@ async fn run<'d, P: Pin>(pin: &mut Flex<'d, P>, syst: &mut SYST) -> Result<(), D
let t1 = SYST::get_current();
while pin.is_high() {}
let t2 = SYST::get_current();
- down = t1 - t2;
+ let t_down = t1 - t2;
if i % 2 == 0 {
- up_x = step(up_x, down);
+ up_x = step(up_x, t_down);
+ } else {
+ // TODO: keep `down` as our random output
}
cortex_m::asm::delay(down_x);
- if i >= LOOP-BUF {
- let n = i - (LOOP-BUF);
- ups[n] = up;
+ if (BUF_START..BUF_START+BUF).contains(i) {
+ let n = i - BUF_START;
+ ups[n] = t_up;
upx[n] = up_x;
- downs[n] = down;
+ downs[n] = t_down;
downx[n] = down_x;
}
}
+ // put it in idle state so it'll be closer to threshold
+ // on the next critical_section iteration.
pin.set_pull(Pull::None);
});
--
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