hutorny / hast

Software Defined High-speed Asynchronous Serial Transmitter

HAST

Software Defined High-speed Asynchronous Serial Transmitter

HAST is a C++11 template that generates a transmitter routine for a software UART. It deploys unrolled loop with variable padding technique for generating precise timing between bits, and a short sequence (3 cycles) for driving the output. Such approach allows baud rates up to CLOCK/3.

Usage

HAST is a header-only library. Include hast.hpp and instantiate hast::avr::trasmitter as the following

#include "hast.hpp"

void hast_putc(char c) {
	hast::avr::transmitter<
		1000000,							/* 1 MHz			*/
		hast::_115200,					/* 115200 baud	*/
		ADDR_OF(PORTB), PORTB3, 0,	/* pin PORTB3	*/
		hast::stopbits::one			/* one stop bit	*/ 
	>::send(c);
}

Notes

Clock error

Built-in RC-oscillator is calibrated for certain conditions. If operating conditions differ, RC-oscillator may deviate from the expected frequency. This deviation may have negative impact on HAST operations. To improve results, you may need fine tune the RC-oscillator.

Timing table

Baudrate 921600, clock 8 MHz

#	start μs	finish μs	start cycles	finish cycles	wait cycles	error μs	err %
0	0	8680	0	9	6	320	3.7%
1	8680	17360	9	17	5	360	4.1%
2	17360	26040	17	26	6	40	0.5%
3	26040	34720	26	35	6	280	3.2%
4	34720	43400	35	43	5	400	4.6%
5	43400	52080	43	52	6	80	0.9%
6	52080	60760	52	61	6	240	2.8%
7	60760	69440	61	69	5	440	5.1%
8	69440	78120	69	78	6	120	1.4%
9	78120	86800	78	87	7	200	2.3%