{"id":278,"date":"2016-05-03T10:58:09","date_gmt":"2016-05-03T17:58:09","guid":{"rendered":"https:\/\/www.crystalfontz.com\/blog\/?post_type=encyclopedia&#038;p=278"},"modified":"2025-08-27T08:41:02","modified_gmt":"2025-08-27T15:41:02","slug":"spi","status":"publish","type":"encyclopedia","link":"https:\/\/www.crystalfontz.com\/blog\/glossary\/spi\/","title":{"rendered":"SPI"},"content":{"rendered":"<p>SPI stands for Serial Peripheral Interface. SPI is a simple serial bus that is often used by <a href=\"https:\/\/www.crystalfontz.com\/blog\/glossary\/lcd\/\" target=\"_self\" title=\"LCD stands for Liquid Crystal Display. An LCD is composed of two pieces of glass with a thin layer of liquid crystals between the glass layers. When a voltage is applied to the glass, the orientation of the crystals can be changed. This change in the crystal's orientation (called polarization) will make either a dark&hellip;\" class=\"encyclopedia\">LCD<\/a> or <a href=\"https:\/\/www.crystalfontz.com\/blog\/glossary\/oled\/\" target=\"_self\" title=\"OLED stands for Organic Light Emitting Diode. OLEDs are used to make OLED Displays, which can either be PMOLED (Passive Matrix OLED) or AMOLED (Active Matrix OLED). Crystalfontz carries a full line of OLED modules. Detailed Definition What makes OLEDs useful for display construction is that they can be fabricated in bulk. For instance, to&hellip;\" class=\"encyclopedia\">OLED<\/a> controllers. SPI as implemented for OLED and LCD controllers typically uses a &ldquo;3-wire SPI&rdquo; or &ldquo;4-wire SPI&rdquo; scheme.<\/p>\n<h3>Detailed Definition<\/h3>\n<p>SPI was originally championed by Motorola (now Freescale). In its original &ldquo;pure&rdquo; form SPI uses four signals:<\/p>\n<ul>\n<li>SCK: Serial ClocK<\/li>\n<li>MOSI: Master Out \/ Slave In<\/li>\n<li>MISO: Master In \/ Slave Out<\/li>\n<li>SS: Slave Select (Also called CS: Chip Select)<\/li>\n<\/ul>\n<h3>Data Transfer and Master \/ Slave<\/h3>\n<p>SPI allows for simultaneous bidirectional data transfer; all transactions are initiated by the master single master.<\/p>\n<p>SPI is not addressed, so each slave must have its own SS line. For example, in order for one master to talk to 6 slaves, the master will need 1 input (MISO) and 8 outputs (MOSI, SCK, and the 6 slave selects).<\/p>\n<p>Typically, SPI transfers are multiples of 8 bits, though this is not a hard requirement.<\/p>\n<p>Traditionally, we are urged to think of&nbsp;SPI&nbsp;as a 16-bit shift register, with 8 bits in the master, and 8 bits in the slave. The master loads its message destined for the slave, the slave loads its data for the master, the master gives 8 clocks, and the data shifts simultaneously from the master to the slave (over MOSI) and from the slave to the master (over MISO).<\/p>\n<p>In practical situations, this model is not very useful, since the master typically needs to write a command to the slave before the slave can load the correct data, and the slave usually ignores the MOSI line while the master is reading from the slave.<\/p>\n<p>A SPI master can be implemented by GPIO under software control, or it may be implemented in hardware. SPI slaves are almost always implemented in hardware.<\/p>\n<p>When SPI is used with the controllers on LCD and OLED modules, there are two common varieties: &ldquo;3-wire SPI&rdquo; and &ldquo;4-wire SPI.&rdquo;<\/p>\n<h3>4-Wire SPI<\/h3>\n<p>In a typical &ldquo;4-wire SPI&rdquo; LCD\/OLED application the connections will be:<\/p>\n<ul>\n<li>SCK: the SPI ClocK, as usual<\/li>\n<li>&#8232;SS: the Slave Select, often labeled as CS &ndash; &ndash; Chip Select<\/li>\n<li>MOSI: the data from the master to the display,&nbsp; usually labeled as SDA (Serial DAta)<\/li>\n<li>C\/D: Command \/Data<\/li>\n<\/ul>\n<p>Now you should have been expecting the fourth line to be MISO, so you could read data back from the display&ndash;but with LCD controllers, there is a twist.<\/p>\n<p>We need some history to understand the 4th line as used in SPI displays. Traditionally, parallel interface LCD controllers had two registers, a Command register and a Data register. You would write the command to the Command register, then write the appropriate data to the Data register. To select between them, there was a C\/D (or A0) line. Set the line to 1=Command or 0=Data then the write the 8 bits of data.<\/p>\n<p>Now back to &ldquo;4-wire SPI,&rdquo; the 4th wire is &ldquo;C\/D.&rdquo; So you can either make an 8-bit SPI write to the display controller&rsquo;s Command register or&nbsp;an 8-bit SPI write&nbsp;its Data register depending on the state of the C\/D line.<\/p>\n<figure id=\"attachment_279\" aria-describedby=\"caption-attachment-279\" style=\"width: 1053px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-279 size-full\" src=\"https:\/\/www.crystalfontz.com\/blog\/wp-content\/uploads\/4-wire-SPI.jpg\" alt=\"4-wire SPI Connection - www.crystalfontz.com\" width=\"1053\" height=\"600\" srcset=\"https:\/\/www.crystalfontz.com\/blog\/wp-content\/uploads\/4-wire-SPI.jpg 800w, https:\/\/www.crystalfontz.com\/blog\/wp-content\/uploads\/4-wire-SPI-250x142.jpg 250w, https:\/\/www.crystalfontz.com\/blog\/wp-content\/uploads\/4-wire-SPI-768x438.jpg 768w, https:\/\/www.crystalfontz.com\/blog\/wp-content\/uploads\/4-wire-SPI-700x399.jpg 700w, https:\/\/www.crystalfontz.com\/blog\/wp-content\/uploads\/4-wire-SPI-120x68.jpg 120w\" sizes=\"auto, (max-width: 1053px) 100vw, 1053px\"><figcaption id=\"caption-attachment-279\" class=\"wp-caption-text\">4-wire SPI Connection<\/figcaption><\/figure>\n<h3>3-Wire SPI<\/h3>\n<p>In a typical &ldquo;3-wire SPI&rdquo; LCD\/OLED application the connections will be:<\/p>\n<ul>\n<li>SCK: the SPI clock, as usual<\/li>\n<li>SS: the Slave Select, often labeled as CS &ndash; &ndash; Chip Select<\/li>\n<li>&#8232;MOSI: the data from the master to the display,&nbsp; usually labeled as SDA (Serial DAta)<\/li>\n<\/ul>\n<figure id=\"attachment_280\" aria-describedby=\"caption-attachment-280\" style=\"width: 1053px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-280 size-full\" src=\"https:\/\/www.crystalfontz.com\/blog\/wp-content\/uploads\/3-wire-SPI.jpg\" alt=\"3-wire SPI Connection - www.crystalfontz.com\" width=\"1053\" height=\"600\" srcset=\"https:\/\/www.crystalfontz.com\/blog\/wp-content\/uploads\/3-wire-SPI.jpg 800w, https:\/\/www.crystalfontz.com\/blog\/wp-content\/uploads\/3-wire-SPI-250x142.jpg 250w, https:\/\/www.crystalfontz.com\/blog\/wp-content\/uploads\/3-wire-SPI-768x438.jpg 768w, https:\/\/www.crystalfontz.com\/blog\/wp-content\/uploads\/3-wire-SPI-700x399.jpg 700w, https:\/\/www.crystalfontz.com\/blog\/wp-content\/uploads\/3-wire-SPI-120x68.jpg 120w\" sizes=\"auto, (max-width: 1053px) 100vw, 1053px\"><figcaption id=\"caption-attachment-280\" class=\"wp-caption-text\">3-wire SPI Connection &ndash; www.crystalfontz.com<\/figcaption><\/figure>\n<h3>When does a &ldquo;3-wire SPI&rdquo; actually use 4 wires?<\/h3>\n<p>There are some controllers that use the &ldquo;3-wire SPI&rdquo; transfer format (encoding C\/D and R\/W as additional bits in the SPI transfer), but also have a MISO pin that can be used for reading data back from the display. So these displays use 4 physical wires, but have the a data encoding similar to the &ldquo;3-wire SPI&rdquo;.<\/p>\n<h3>Choosing Between a 4-Wire and 3-Wire SPI<\/h3>\n<p>Initially, &ldquo;3-wire SPI&rdquo; seems like a better solution, less wires, same function, what is there not to like? Well, the same history that gave us the extra C\/D line now gives us an extra C\/D bit&ndash;instead of using 8-bit transfers, &ldquo;3-wire SPI&rdquo; uses 9-bit (!) transfers.<\/p>\n<p>If you are using software and GPIO to implement the SPI master, then it is no problem to modify the code to make it transfer 9 bits at a time. However, making 9-bit transfer with SPI hardware implementations may be difficult or impossible. Some of the newer and more innovative processors have more flexible hardware SPI implementations that may allow 9-bit transfers.<\/p>\n<p>SPI (Serial Peripheral Interface) is <strong data-start=\"57\" data-end=\"95\">popular for daisy-chaining devices<\/strong> because of its <strong data-start=\"111\" data-end=\"155\">simple, shift-register-like architecture<\/strong> and high-speed synchronous signaling. Here&rsquo;s why it works well:<\/p>\n<h3>Shift Register Architecture<\/h3>\n<ul data-start=\"260\" data-end=\"629\">\n<li data-start=\"260\" data-end=\"360\">\n<p data-start=\"262\" data-end=\"360\">Many SPI devices (like DACs, LED drivers, or shift registers) act like cascaded shift registers.<\/p>\n<\/li>\n<li data-start=\"361\" data-end=\"464\">\n<p data-start=\"363\" data-end=\"464\">Each device passes along incoming data to the next in the chain via its <strong data-start=\"435\" data-end=\"450\">MISO &rarr; MOSI<\/strong> connection.<\/p>\n<\/li>\n<li data-start=\"465\" data-end=\"629\">\n<p data-start=\"467\" data-end=\"629\">After the master clocks in enough bits (one device&rsquo;s worth &times; number of devices), each device latches the data relevant to it when the chip-select (CS) is toggled.<\/p>\n<\/li>\n<\/ul>\n<p data-start=\"631\" data-end=\"727\">This makes it possible to address <em data-start=\"665\" data-end=\"726\">all devices in the chain with one continuous stream of bits<\/em>. Making &ldquo;Daisy-Chaining&rdquo; devices trivial.<\/p>\n<h3 data-start=\"1805\" data-end=\"1836\">Trade-offs \/ Limitations<\/h3>\n<ul data-start=\"1837\" data-end=\"2198\">\n<li data-start=\"1837\" data-end=\"1933\">\n<p data-start=\"1839\" data-end=\"1933\"><strong data-start=\"1839\" data-end=\"1874\">Latency grows with chain length<\/strong>, since you must clock through all devices to update one.<\/p>\n<\/li>\n<li data-start=\"1934\" data-end=\"2041\">\n<p data-start=\"1936\" data-end=\"2041\"><strong data-start=\"1936\" data-end=\"1982\">Not all SPI devices support daisy-chaining<\/strong>; the device must forward unused bits to the next device.<\/p>\n<\/li>\n<li data-start=\"2042\" data-end=\"2111\">\n<p data-start=\"2044\" data-end=\"2111\">Debugging chains can be harder if one device in the middle fails.<\/p>\n<\/li>\n<li data-start=\"2112\" data-end=\"2198\">\n<p data-start=\"2114\" data-end=\"2198\">Some systems may still prefer separate CS lines if random-access updates are needed.<\/p>\n<\/li>\n<\/ul>\n<h3>One More Thing About SPI LCD and OLED Display Controllers<\/h3>\n<p>There is another non-SPI-standard thing that some SPI display controllers do, and that is they use their single data line bidirectionally&ndash;for both writing and reading. Typically, you would write the &ldquo;read command&rdquo; bits, then make the data line an input, then read back some number of bits from the display. Some controllers go a bit (sorry) further and encode the traditional Read\/Write line from the old parallel interface into a 10th R\/W bit. These use a 10-bit SPI transfer, with a bi-directional data line.<\/p>\n<h3>In Summary<\/h3>\n<p>Each <a href=\"https:\/\/www.crystalfontz.com\/controllers\/\">LCD\/OLED controller IC<\/a> that supports SPI will include its specific SPI transfer format and list the formats for its read and write commands. You will want to look at the format of the controller&rsquo;s SPI interface and evaluate whether you can use the hardware SPI Master that is part of your microcontroller, or if you need to write your own software SPI master.<\/p>\n<h3>Questions?<\/h3>\n<p>If you have any questions, please&nbsp;<a href=\"https:\/\/www.crystalfontz.com\/contact.php\">contact our knowledgeable and friendly support staff<\/a> by email, phone, or chat.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>SPI stands for Serial Peripheral Interface. SPI is a simple serial bus that is often used by LCD or OLED controllers. SPI as implemented for OLED and LCD controllers typically uses a &#8220;3-wire SPI&#8221; or &#8220;4-wire SPI&#8221; scheme. Detailed Definition SPI was originally championed by Motorola (now Freescale). In its original &#8220;pure&#8221; form SPI uses [&hellip;]<\/p>\n","protected":false},"author":6,"featured_media":0,"template":"","meta":{"site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"default","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"set","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}}},"class_list":["post-278","encyclopedia","type-encyclopedia","status-publish","hentry"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.3 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>SPI - Crystalfontz LCD Glossary<\/title>\n<meta name=\"description\" content=\"SPI : SPI stands for Serial Peripheral Interface. 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