{"id":2961,"date":"2024-12-31T15:04:41","date_gmt":"2024-12-31T15:04:41","guid":{"rendered":"https:\/\/www.robertprice.co.uk\/robblog\/?p=2961"},"modified":"2024-12-31T15:13:07","modified_gmt":"2024-12-31T15:13:07","slug":"decoding-dtmf-tones-on-an-rc2014","status":"publish","type":"post","link":"https:\/\/www.robertprice.co.uk\/robblog\/decoding-dtmf-tones-on-an-rc2014\/","title":{"rendered":"Decoding DTMF tones on an RC2014"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">Dual-tone multi-frequency, commonly known as DTMF, is a technology used in telecoms systems that uses audible tones to represent keys on a touch-tone phone.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">You have probably heard this will navigating a menu on your phone, such as &#8220;Press 1 for &#8230;, Press 2 for&#8230;.&#8221;, etc.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">While browsing on AliExpressing for some parts for another project, it suggested I may like the <a href=\"https:\/\/s.click.aliexpress.com\/e\/_onPltSL\">MT8870 module<\/a> (affiliate link). This is a small 5V DTMF decoder module with digital outputs. I thought this would be ideal for an RC2014 project.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">We&#8217;re interested in 7 pins on this module. +5v and GND are for powering the module. STQ, D0, D1, D2, and D3 are for the decoded tones. The STQ line goes high when a decoded tone is present. The actual code is a 4 bit value stored on D0, D1, D2, and D3. <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">We can pass this output to a 74HCT245 transceiver which we can enable using a 74HCT688 to decode an input address on the RC2014, along with a 74HCT32 to check the RD and IORQ lines. <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">I&#8217;m using an RC2014 Classic 2 for this project, so inout port 1 is free, but I know this isn&#8217;t necessarily the case on more advanced RC2014s. Because of this I&#8217;ve put an 8 inline switch module on the 74HCT688 so I change the address if necessary. <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">I am also adding some LEDs on the input lines so I can see the incoming data visually. The MT8870 has some SMD LEDs on board to do this, but the built-in pin headers I want to use will mean these face the RC2014 module PCB and so won&#8217;t be visible. I&#8217;ve gone for green for the STQ line, blue for the data lines.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><a href=\"https:\/\/www.robertprice.co.uk\/robblog\/assets\/Schematic_RC2014-DTMF-Decoder_2024-12-31.png\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"724\" src=\"https:\/\/www.robertprice.co.uk\/robblog\/assets\/Schematic_RC2014-DTMF-Decoder_2024-12-31-1024x724.png\" alt=\"Circuit diagrams for the DTMF Decoder Module designed for the RC2014\" class=\"wp-image-2963\" srcset=\"https:\/\/www.robertprice.co.uk\/robblog\/assets\/Schematic_RC2014-DTMF-Decoder_2024-12-31-1024x724.png 1024w, https:\/\/www.robertprice.co.uk\/robblog\/assets\/Schematic_RC2014-DTMF-Decoder_2024-12-31-300x212.png 300w, https:\/\/www.robertprice.co.uk\/robblog\/assets\/Schematic_RC2014-DTMF-Decoder_2024-12-31-768x543.png 768w, https:\/\/www.robertprice.co.uk\/robblog\/assets\/Schematic_RC2014-DTMF-Decoder_2024-12-31.png 1169w\" sizes=\"auto, (max-width: 709px) 85vw, (max-width: 909px) 67vw, (max-width: 1362px) 62vw, 840px\" \/><\/a><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">The PCB looks like this.<\/p>\n\n\n\n<figure class=\"wp-block-image size-full\"><a href=\"https:\/\/www.robertprice.co.uk\/robblog\/assets\/PCB_PCB_RC2014-DTMF-Decoder_2024-12-31-1.png\"><img loading=\"lazy\" decoding=\"async\" width=\"419\" height=\"262\" src=\"https:\/\/www.robertprice.co.uk\/robblog\/assets\/PCB_PCB_RC2014-DTMF-Decoder_2024-12-31-1.png\" alt=\"DTMF Decoder Module PCB designed for the RC2014.\" class=\"wp-image-2967\" srcset=\"https:\/\/www.robertprice.co.uk\/robblog\/assets\/PCB_PCB_RC2014-DTMF-Decoder_2024-12-31-1.png 419w, https:\/\/www.robertprice.co.uk\/robblog\/assets\/PCB_PCB_RC2014-DTMF-Decoder_2024-12-31-1-300x188.png 300w\" sizes=\"auto, (max-width: 419px) 85vw, 419px\" \/><\/a><\/figure>\n\n\n\n<figure class=\"wp-block-embed is-type-video is-provider-youtube wp-block-embed-youtube wp-embed-aspect-16-9 wp-has-aspect-ratio\"><div class=\"wp-block-embed__wrapper\">\n<iframe loading=\"lazy\" title=\"Decoding DTMF tones on an RC2014.\" width=\"840\" height=\"473\" src=\"https:\/\/www.youtube.com\/embed\/OskO-Bx01_E?feature=oembed\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe>\n<\/div><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">Decoding the input using Z80 assembly language<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">I want to write a small Z80 assembly language program to print out any incoming DTMF tones to the terminal.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">I&#8217;m using the SCM monitor program on my RC2014 Classic 2 computer to load and execute my assembly language program. This gives a handy print character to the terminal routine I can use. I pass the ASCII character I want to print in register A, the value $02 in register C, and execute RST $30. I can wrap this into my own subroutine called printchar. This gives me the flexibility to swap to another output method such as an LCD screen in future.<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>; pass the character to print in register A\n; This uses SCM to print back to the terminal.\nprintchar:\t\n\t\tpush bc\n\t\tld c, $02 \t; SCM API function $02 - print character\n\t\trst $30\n\t\tpop bc\n\t\tret<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">I will need to debounce any input I read from the decoder module. This is because the value will be present for longer than the first read I make. To do this I will need to check if STQ is high and compare this to the last value read. If it&#8217;s the same, I know I have already handled this value. However, when STQ goes low, I need to reset the last value read to a dummy value else it won&#8217;t detect multiple presses of the same button.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Once I have the data, I can map it to the correct character. I do this by having the valid decoded characters in order, and then simply the incoming character value as an offset to this list.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">My final code looks like this.<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>\t\tOUTPUT DTMFDecoder.z80\n\t\t\n\t\tORG $9000\n\t\t\n; Input data from the DTMF encoder is\n; Bits 0-3 = number data (D0, D1, D2, D3)\n; Bit 4    = new data flag (STQ)\n; Bits 5-7 = unused.\n\t\t\n; The input port where we can find the DTMF decoder\nDTMFDecoder     EQU 1\n\t\t\n; masks for handling the input data.\nNEW_INPUT_MASK  EQU %00010000\nDATA_INPUT_MASK EQU %00001111\n\t\t\n; Flags to show if the data is new or not.\nDataRead        EQU $FF\nDataClear       EQU $00\n\t\t\n; Register C holds the data read flag. This is set to DataRead when we've\n; read a byte, and set to DataClear when we are safe to read a byte.\n\t\tld c, DataClear\n\t\t\nmainloop:\t\n; read the data from the input port DTMFDecoder and store this in register B.\n\t\tin a, (DTMFDecoder)\n\t\tld b, a\n; see if the new data flag bit has been set. If it has go to datapresent.\n\t\tand NEW_INPUT_MASK\n\t\tjr nz, datapresent\n\t\t\n; clear Register C as we have no data to read, and loop again.\n\t\tld c, DataClear\n\t\tjr mainloop\n\t\t\ndatapresent:\t\n; we have data present so check if register C is set to DataRead. If it\n; is we know we have already handled that input so can loop again.\n\t\tld a, DataRead\n\t\tcp c\n\t\tjr z, mainloop\n\t\t\n; set register C to DataRead to flag we have handled this input.\n\t\tld c, DataRead\n\t\t\n; get the input into register A and mask out the button press bit.\n\t\tld a, b\n\t\tand DATA_INPUT_MASK\n\t\t\n; decode the character from the input and find it in the charmap.\n\t\tld hl, charmap\n\t\t\n; add the value of a to hl\n\t\tadd a, l\n\t\tld l, a\n\t\tadc a, h\n\t\tsub l\n\t\tld h, a\n\t\t\n; get the character from the charmap and print it.\n\t\tld a, (hl)\n\t\tcall printchar\n\t\t\n; now loop again.\n\t\tjr mainloop\n\t\t\n\t\t\n; pass the character to print in register A\n; This uses SCM to print back to the terminal.\nprintchar:\t\n\t\tpush bc\n\t\tld c, $02 \t; SCM API function $02 - print character\n\t\trst $30\n\t\tpop bc\n\t\tret\n\t\t\n; The DTMF characters in order.\ncharmap:\t\n\t\tdb 'D1234567890*#ABC'\n<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\"><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Dual-tone multi-frequency, commonly known as DTMF, is a technology used in telecoms systems that uses audible tones to represent keys on a touch-tone phone. You have probably heard this will navigating a menu on your phone, such as &#8220;Press 1 for &#8230;, Press 2 for&#8230;.&#8221;, etc. While browsing on AliExpressing for some parts for another &hellip; <a href=\"https:\/\/www.robertprice.co.uk\/robblog\/decoding-dtmf-tones-on-an-rc2014\/\" class=\"more-link\">Continue reading<span class=\"screen-reader-text\"> &#8220;Decoding DTMF tones on an RC2014&#8221;<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":2971,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"inline_featured_image":false,"footnotes":""},"categories":[84,157],"tags":[159,117],"class_list":["post-2961","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-hardware","category-rc2014","tag-dtmf","tag-rc2014"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Decoding DTMF tones on an RC2014 - 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