{"id":243,"date":"2013-06-19T10:28:19","date_gmt":"2013-06-19T10:28:19","guid":{"rendered":"http:\/\/ermine-electronics.co.uk\/?p=243"},"modified":"2018-02-24T21:13:15","modified_gmt":"2018-02-24T21:13:15","slug":"16f628-pic-ad-using-jal","status":"publish","type":"post","link":"https:\/\/www.richardmudhar.com\/blog\/2013\/06\/16f628-pic-ad-using-jal\/","title":{"rendered":"16F628 PIC A\/D using JAL"},"content":{"rendered":"

The 16F628 pic doesn’t actually have a A\/D converter. It has a couple of comparators and a software controllable voltage reference which does pretty well for most applications. I’m looking at using it for a low-battery cutoff device to protect 12V lead-acid batteries.The two comparators would work for that – one for the low-water mark, and one set a little bit above for a level the charge needs to reach before the load is reapplied, with some time delay probably too.<\/p>\n

However, I came across Microchip’s Application note AN700 Make a Delta-Sigma Converter Using a Microcontroller’s Analog Comparator Module <\/a>which shows how to use the comparator as an A\/D\u00a0 converter. There are plenty of PICs that already have an A\/D converter – the 16F88 has several analogue channels, but the 16F628 is cheaper. A lot of applications need just one analogue value sampled, and my application also wants an A\/D that includes 12V. If I used a chip with an onboard A\/D I’m going to need two resistors anyway to pad 12V down to the 5V range, and AN700 fig 7 shows how to take advantage of the external parts to shift the A\/D range up to about 12V.<\/p>\n

Trouble is, I want to use JAL, to make life easier when I write the rest of the code, and the assembler mode of JAL didn’t work. I swiped this from here<\/a> probably derived from here<\/a> but it didn’t work for me. I took a look at the code and suspected the assembler part, because what was happening was as soon as I called the DeltaSigA2D() the whole program seemed to sit down and start again from the top, resulting in endless printing of “STARTED” and little else.<\/p>\n

Thinking about what JAL is probably doing with assembler, I suspected some of the clever space-saving constructs like the bit in eat5cycles that goes<\/p>\n

goto +1<\/p>\n

which was supposed to be<\/p>\n

goto $+1<\/p>\n

could be asking for trouble unless I could really assume that next instruction is the next one in the program space. In the end if you’re going to use something like JAL you aren’t trying to eke out every last drop of code space, so I figured I would nut the cleverness and use stacked nops. And it worked.<\/p>\n

 <\/p>\n

-- JAL 2.4i\n-- Just the A\/D 15 June 2013\n-- modded by Richard\n--\n-- adc_test_16f628_v03\n-- after DeltaSig.asm v1.00 from Dan Butler, Microchip Technology Inc. 02 December 1998\n-- after http:\/\/den-nekonoko.blog.so-net.ne.jp\/2010-11-25 and AN700\n-- continually spews 10-bit value on TXD at 9600 baud, 5V=1024 0V=0\n-- AN700 Fig 2\ninclude 16f628_inc_all\n-- We'll use internal oscillator. It work @ 4MHz\npragma target clock\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 4_000_000\nconst word _fuses\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 = 0x3fff ; default value\npragma target osc\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 INTRC_NOCLKOUT\npragma target WDT\u00a0\u00a0\u00a0 off\npragma target MCLRE\u00a0 off\npragma target PWRTE\u00a0 on\npragma target BODEN\u00a0\u00a0\u00a0 on\npragma target LVP\u00a0\u00a0\u00a0 off\n-- HS, -BOD, ,-LVP, -WDT, -CP\u00a0 = 0x3F22\n; pragma target fuses\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 0x3D02\n--enable_digital_io()\n\n; const bit enhanced_flash_eeprom = false\ninclude pic_general\ninclude delay\n\nvar word result\nvar byte result_l\nvar byte result_h\nvar byte counter1\nvar byte counter2\n\nconst byte booted[] = \"STARTED\"\n\npin_a0_direction = input\npin_a1_direction = input\npin_a2_direction = input ; comparator in \/ vref out\npin_a3_direction = output ; comp1 out\npin_a4_direction = output ; comp2 out (OD)\npin_b2_direction=output ; RS232 TX\n\nconst bit usart_hw_serial = TRUE\nconst serial_hw_baudrate = 9600\ninclude serial_hardware\nserial_hw_init()\n\ninclude print\ninclude format\n\nProcedure InitDeltaSigA2D() is\nVRCON=0b11101100 --\u00a0\u00a0\u00a0 0xEC\nCMCON=0b00000110\u00a0\u00a0 -- 0x06 two common ref comps with outputs\nend procedure\n\n;\n; Delta Sigma A2D\n; The below contains a lot of nops and goto next instruction. These\n; are necessary to ensure each pass through the Elop1 takes the same\n; amount of , no the path through the code.\n;\nProcedure DeltaSigA2D() is\nassembler\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 LOCAL\u00a0\u00a0\u00a0 elop1,comphigh,complow,eat2cycles,endelop1,eat5cycles,exit1\n--DeltaSigA2D\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 clrf counter1\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 clrf counter2\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 clrf result_l\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 clrf result_h\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 movlw 0x03 ; set up for 2 analog comparators with common reference\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 movwf cmcon\nelop1:\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 btfsc c1out ; is comparator high or low?\n--\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 btfsc cmcon,c1out ; is comparator high or low?\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 goto complow ; go the low route\ncomphigh:\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 nop ; necessary to timing even\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 bcf porta,3 ; porta.3 = 0\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 incfsz result_l,f ; bump counter\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 goto eat2cycles ;\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 incf result_h,f ;\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 goto endelop1 ;\ncomplow:\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 bsf porta,3 ; comparator is low\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 nop ; necessary to keep timing even\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 goto eat2cycles ; same here\neat2cycles:\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 goto endelop1 ; eat 2 more cycles\nendelop1:\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 incfsz counter1,f ; count this lap through the elop1.\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 goto eat5cycles ;\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 incf counter2,f ;\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 movf counter2,w ;\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 andlw 0x04 ; are we done? (we're done when bit2 of\n--\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 btfsc status,_z ; the high order byte overflows to 1).\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 btfsc Z --status,z ; the high order byte overflows to 1).\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 goto elop1 ;\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 goto exit1\neat5cycles:\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 nop\u00a0 -- this really seems to hate the goto $+1, keeps resetting. I am not so short of code space, case to be made to turn everything into nops\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 nop\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 --goto +1 ; more wasted time to keep the elop1s even\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 nop ;\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 goto elop1 ;\nexit1:\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 movlw 0x06 ; set up for 2 analog comparators with common reference\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 movwf cmcon\nend assembler\nend procedure\n\nfunction rescale(WORD IN value) RETURN WORD is\n-- 1024 corresponds to 5V\n-- multiply by 50000\/1024 (~49)\nvar word temp =value*49\ntemp=temp\/100 -- now lose excess precision\nreturn temp\n\nend function\n\nInitDeltaSigA2D()\n\nprint_string(serial_hw_data,booted)\n\nforever loop\nDeltaSigA2D()\nresult=word(result_h)*256 + word(result_l) -- you must explicity make result_h a word as described on p11 of JAL manual\nresult=rescale(result)\nformat_word_dec(serial_hw_data, result, 3, 2)\nserial_hw_data = \" \"\nserial_hw_data = 13 -- CR\nserial_hw_data = 10 -- LF\ndelay_1ms(500)\u00a0\u00a0 -- this is purely to allow the terminal display to catch up and not overload buffer of the slow device\nend loop<\/pre>\n
Files<\/p>\n
adc_test_16f628_v03.jal<\/a><\/p>\n
and the hex file<\/p>\n
adc_test_16f628_v03.hex <\/a><\/p>\n
There’s a case to be made for using a voltage reference and setting it to be some convenient power of 2 like 2.05 V to save all the messy integer calculation in rescale<\/p>\n
This is a video of the display on the serial port compared with a multimeter on the input, connected to a 5k pot between 0V and +5V. I should have moved the control a bit slower to allow the 0.5s sampling to catch up at times, but it shows it is reasonably accurate.<\/p>\n