REM   SBC2000-062 PWM/BEEP support routines
REM
REM   Author: Steven R. Wheeler
REM
REM   Date:   17 September 1998
REM
REM   Copyright 1998-2000 Vesta Technology, Inc.
REM   All rights reserved.
REM
REM   Revision history:
REM     Feb 10, 1999     SRW
REM     Removed unused 'channel' parameter from PWM argument list.
REM     Apr  7, 2000     SRW
REM     Cleaned up comments slightly.


REM   Timer 2 control register
CONSTANT VTI_T2CON   AS INTEGER = 0x12

REM   Duty cycle register
CONSTANT VTI_CCPR1L  AS INTEGER = 0x15

REM   Control register
CONSTANT VTI_CCP1CON AS INTEGER = 0x17

REM   Period register
CONSTANT VTI_PR2     AS INTEGER = 0x92

REM   Port C direction register
CONSTANT VTI_TRISC   AS INTEGER = 0x87


REM   PWM on the SBC2000-062 is subject to certain limitations. 10-bit precision
REM   in specifying the PWM percentage is only available at 3 specific frequencies:
REM   305 Hz, 1221 Hz, and 4883 Hz. The lowest frequency available is 305 Hz.
REM   As the frequency increases from one of the three frequencies with the most
REM   precision, the precision available drops steadily until the next "best"
REM   frequency is reached.

REM   Set up a new PWM hightime proportion and frequency. The hightime parameter
REM   is expressed in 10ths of percents. In other words, 500 specifies 50%. The
REM   freq argument is specified in Hertz, and has a legal range from 305 to 32767.

VITAL SUBROUTINE PWM(hightime AS INTEGER, freq AS INTEGER)
LOCAL range AS INTEGER		: REM   Frequency prescaler selection
LOCAL portion AS INTEGER	: REM   Working variable

	IF hightime > 1000
		REM if over 100% specified, turn it off
		hightime = 0
	ENDIF

	IF freq < 305
		REM   Unable to set this frequency; it's too low.
		REM   Turn it off
		POKE(VTI_CCP1CON, 0)
		POKE(VTI_T2CON, PEEK(VTI_T2CON) AND 0xFB)
		RETURN
	ENDIF

	range = 0

	REM   Determine which frequency range we need to select. Note that a
	REM   range value of 3 has the same effect as a range of 2. We will
	REM   scale all frequencies down so that we can get by with one
	REM   simplified equation for calculating the period register value.

	IF freq < 1221
		range = 2
	ENDIF

	IF freq < 4883
		IF range = 0
			range = 1
			freq = freq / 4
		ENDIF
	ELSE
		range = 0
		freq = freq / 16
	ENDIF

	REM   Set the range
	POKE(VTI_T2CON, (PEEK(VTI_T2CON) AND 0xFC) OR range)

	REM   The PR2 value is calculated using an equation which depends upon
	REM   the crystal frequency, the prescaler, and a scaling constant. To
	REM   simplify things, we'll precalculate as much as possible.

	REM   This test is necessary because of the limitations of signed
	REM   16-bit integer arithmetic. For frequencies in the range
	REM   specified, the "more precise" portion of the calculation
	REM   results in a negative number due to overflow. Therefore,
	REM   we check for the problematic range and specify the return
	REM   value manually.

	IF (freq > 9765) AND (freq < 11340)
		range = 7
	ELSE
		IF freq > 19531
			range = 2
		ELSE
			range = (19531 / freq) * 4		: REM 78125 / freq
			range = range + (19531 \ freq) * 4 / freq	: REM more precise
		ENDIF
	ENDIF
	range = MIN(255, range - 1)			: REM adjust and limit
	POKE(VTI_PR2, range)

	REM   Now we have to program the high time. This is messy, because the
	REM   maximum value you can place into the hightime registers depends
	REM   upon the value you've put into the PR2 register.

	REM   Convert from 0.1% units to machine increments.

	portion =            (hightime         \ 10) * 1024 / 1000
	portion = portion + ((hightime /   10) \ 10) * 1024 /  100
	portion = portion + ((hightime /  100) \ 10) * 1024 /   10
	portion = portion + ((hightime / 1000) \ 10) * 1024
	portion = MIN(portion, 1023)

	REM   These next equations try to maintain as much precision as is
	REM   possible. The high time value must be split between two registers,
	REM   but the most significant portion of the value cannot exceed the
	REM   value in the PR2 register.

	POKE(VTI_CCPR1L, portion * 31 / (31744/range))
	POKE(VTI_CCP1CON, 0x0C + ((portion * 31) / (7936/range) \ 4)*0x10)

	REM   Now make certain that the PWM pin is an output, and turn the PWM
	REM   output on if it isn't already on

	POKE(VTI_TRISC, PEEK(VTI_TRISC) AND 0xFD)
	POKE(VTI_T2CON, PEEK(VTI_T2CON) OR 4)
END


REM   The BEEP subroutine is implemented in terms of the PWM subroutine. It is
REM   subject to the same limitations, except that since BEEP produces a 50%
REM   duty cycle, it isn't affected by limitations on precision.

REM SUBROUTINE BEEP(frequency AS INTEGER)
REM 	PWM(500, frequency)
REM END

REM 8 bit PWM at fixed freq - 5 kHz (-062)
REM PWM control word
REM SUBROUTINE set_duty_cycle(arg AS INTEGER)
rem	CONSTANT VTI_T2CON   AS INTEGER = 0x12
rem	CONSTANT VTI_CCPR1L  AS INTEGER = 0x15
rem	CONSTANT VTI_CCP1CON AS INTEGER = 0x17
rem	CONSTANT VTI_PR2     AS INTEGER = 0x92
rem	CONSTANT VTI_TRISC   AS INTEGER = 0x95
REM	POKE(VTI_T2CON, (PEEK(VTI_T2CON) AND 0xFC) OR 0)
REM	POKE(VTI_PR2,255)
REM	POKE(VTI_CCPR1L, arg):REM Time hi
REM	POKE(VTI_CCP1CON, 0x0C + (1 * 0x10)):REM 2 lsb time hi
REM	POKE(VTI_TRISC, PEEK(VTI_TRISC) AND 0xFD)
REM	POKE(VTI_T2CON, PEEK(VTI_T2CON) OR 4)
REM END

REM FUNCTION read_PWM() AS INTEGER
REM 	read_PWM = PEEK(VTI_CCPR1L)
REM END

REM SUBROUTINE adjust_PWM(arg AS INTEGER)
REM 	LOCAL temp AS INTEGER
REM 	temp = read_PWM()
REM 	temp = temp + arg
REM 	temp = MAX(MIN(temp,255),0)
REM 	set_duty_cycle(temp)
REM END