AVR Embedded Tutorial - SPI MCP4922: what changed

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Old	New		Line
1	1		# Control an MCP4922 12 bit DAC via SPI
2	2
3	3		This code is for an Atmega328 / Arduino with a 16MHz clock and an SPI compatible [MCP4922](https://www.microchip.com/wwwproducts/en/MCP4922) Digital/Analog Converter (DAC).
4	4
5	5		The following functions are described in other tutorials:
6	6
7	7		- [[AVR Embedded Tutorial - Simple GPIO on and off output\|GPIO]] - Output/Input - How do I access the GPIO on the AVR.
8	8		- [[AVR Embedded Tutorial - SPI\|SPI]] - use of the hardware SPI interface.
9	9
10	10		This example shows how to control the MCP4922 DAC via SPI.
11	11
12	12		## Functions for the 74HC595 shift register
13	13
14	14		### Constants
15	15
16	16		Pins which are required at PORTB for SPI.
17	17
18	18		``` pascal
19	19		type
20	20		TSPIGPIO = bitpacked record
21	21		p0, p1, SlaveSelect, MOSI, MISO, Clock, p6, p7 : boolean;
22	22		end;
23	23
24	24		var
25	25		SPI_PORT : TSPIGPIO absolute PORTB;
26	26		SPI_DDR : TSPIGPIO absolute DDRB;
27	27		```
28	28
29	29		### Initialize SPI
30	30
31	31		``` pascal
32	32		procedure SPIInit;
33	33		begin
34	34		// Setup SPI port for output
35	35		SPI_DDR.MOSI := True;
36	36		SPI_DDR.Clock := True;
37	37		SPI_DDR.SlaveSelect := True;
38	38
39	39		SPCR := (1 shl SPE) or (0 shl SPIE) or (0 shl DORD) or (1 shl MSTR) or (0 shl CPOL) or (0 shl CPHA) or (%01 shl SPR);
40	40		SPSR := 1 shl SPI2X; // SCK x 2 on 1 MHz
41	41		end;
42	42		```
43	43
44	44		### Write SPI
45	45
46	46		``` pascal
47	47		procedure SPIWrite(p : PByte; len : byte);
48	48		var
49	49		i : byte;
50	50		begin
51	51		SPI_Port.SlaveSelect := False;
52	52
53	53		for i := len - 1 downto 0 do
54	54		begin
55	55		SPDR := p[i];
56	56
57	57		while(SPSR and (1 shl SPIF)) = 0 do
58	58		begin
59	59		end;
60	60		end;
61	61
62	62		SPI_Port.SlaveSelect := True;
63	63		end;
64	64		```
65	65
66	66		## MCP4922 functions
67	67
68	68		Bit assignments for the MCP4922:
69	69
70		-	~~<table>~~
71	70		<thead>
72		-	~~<tr>~~
73		-	~~<th>Bit</th>~~
74		-	~~<th>Name</th>~~
75		-	~~<th>Description</th>~~
76		-	~~</tr>~~
	71	+
77	72		</thead>
78	73		<tbody>
79		-	~~<tr>~~
80		-	~~<td>15</td>~~
81		-	~~<td><strong>A/B:</strong> DACA or DACB Selection bit</td>~~
82		-	~~<td><strong>0</strong> = Write to DACA<br />~~
	74	+
83	75		<strong>1</strong> = Write to DACB</td>
84		-	~~</tr>~~
85		-	~~<tr>~~
86		-	~~<td>14</td>~~
87		-	~~<td><strong>BUF:</strong> Vref Input Buffer Controlbit</td>~~
88		-	~~<td><strong>0</strong> = Unbuffered<br />~~
	76	+
89	77		<strong>1</strong> = Buffered</td>
90		-	~~</tr>~~
91		-	~~<tr>~~
92		-	~~<td>13</td>~~
93		-	~~<td><strong>GA:</strong>Output Gain Selection bit</td>~~
94		-	~~<td><strong>0</strong> = 2x (VOUT = 2 * VREF * D / 4096)<br />~~
	78	+
95	79		<strong>1</strong> = 1x (VOUT = VREF * D / 4096)</td>
96		-	~~</tr>~~
97		-	~~<tr>~~
98		-	~~<td>12</td>~~
99		-	~~<td><strong>SHDN:</strong> Output Shutdown Control bit</td>~~
100		-	~~<td><strong>0</strong> = Shutdown the selected DAC channel. Analog output is not available at the channel that was shut down. VOUT pin is connected to 500 kOhm(typical)<br />~~
	80	+
101	81		<strong>1</strong> = Active mode operation. VOUT is available.</td>
102		-	~~</tr>~~
103		-	~~<tr>~~
104		-	~~<td>11:0</td>~~
105		-	~~<td><strong>D11:D0:</strong> DAC Input Data bits. Bit x is ignored.</td>~~
106		-	~~<td></td>~~
107		-	~~</tr>~~
	82	+
108	83		</tbody>
109		-	~~</table>~~
110	84
111	85		### Send to DAC
112	86
113	87		``` pascal
114	88		procedure MCP4922sendValue(Value : UInt16; Channel : Byte);
115	89		type
116	90		TADC = bitpacked record
117	91		Value : 0..4095; // bits 0-11
118	92		ShutDown, // bit 12
119	93		Gain, // bit 13
120	94		VRef, // bit 14
121	95		DAC : 0..1; // bit 15
122	96		end;
123	97
124	98		begin
125	99		with TADC(Value) do
126	100		begin
127	101		ShutDown := 1;
128	102		Gain := 1;
129	103		VRef := 0;
130	104		Dac := Channel;
131	105		end;
132	106
133	107		SPIWrite(@Value, 2);
134	108		end;
135	109		```
136	110
137	111		## Example program
138	112
139	113		This example shows the output of two analog values from the DAC. Two LEDs connected to the DAC flash alternately with a very smooth transition.
140	114
141	115		### Constants and variables
142	116
143	117		A counting variable that counts up to 4095.
144	118
145	119		``` pascal
146	120		var
147	121		z : Int16 ;
148	122		```
149	123
150	124		### Main program
151	125
152	126		Alternately write a value in DAC 0 & 1.
153	127
154	128		``` pascal
155	129		begin
156	130		// initialize SPI.
157	131		SPIInit;
158	132
159	133		repeat
160	134		for z := 0 to 4095 do
161	135		begin
162	136		MCP4922sendValue(4095 - z, 0);
163	137		MCP4922sendValue(z, 1);
164	138		end;
165	139		until 1 = 2;
166	140		end.
167	141		```
168	142
169	143		# See also
170	144
171	145		- Overview page - [[AVR Embedded Tutorials]].
172	146		- Use of the hardware SPI interface with an ATmega328 / Arduino - [[AVR Embedded Tutorial - SPI\|SPI]].
173	147
174	148		---
175	149		From the Free Pascal wiki, page "AVR Embedded Tutorial - SPI MCP4922", as it stood on 2020-03-08 after a change by Trev, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).

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