AN2591 Application note Microcontroller input / output expander Introduction In certain applications, the number of input and/or output lines available in the microcontroller you have chosen may be too small and this may lead you to select another microcontroller in order to have enough I/Os to meet your application needs. In some cases this choice means you have to use a more expensive part. However it may often be better to look for a solution that allows you to add more I/Os and still use the microcontroller you selected originally. This application note shows how to use a simple technique to increase the number of inputs and/or outputs. August 2007 Rev 1 1/19 www.st.com Contents AN2591 Contents 1 Expanding the number of outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 1.2 2 2.2 4 5 2/19 1.1.1 Expanding the number of outputs to 8 lines . . . . . . . . . . . . . . . . . . . . . . 3 1.1.2 Expanding the number of outputs to 16 lines . . . . . . . . . . . . . . . . . . . . . 3 Using a shift register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.1 Expanding the number of outputs to 8 lines . . . . . . . . . . . . . . . . . . . . . . 4 1.2.2 Expanding the number of outputs to 16 lines . . . . . . . . . . . . . . . . . . . . . 5 Expanding the number of inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1 3 Using a decoder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Using a multiplexer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1.1 Expanding the number of inputs to 8 lines . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1.2 Expanding the number of inputs to 16 lines . . . . . . . . . . . . . . . . . . . . . . . 8 Using shift registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2.1 Expanding the number of inputs to 8 lines . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2.2 Expanding the number of inputs to 16 lines . . . . . . . . . . . . . . . . . . . . . . 10 Using a STMPE801 (I²C bus) 8-bit I/O port expander . . . . . . . . . . . . . . 11 3.1 Expanding the number of inputs/outputs to 8 lines . . . . . . . . . . . . . . . . . 11 3.2 Expanding the number of I/Os to 16 lines . . . . . . . . . . . . . . . . . . . . . . . . 12 Using the External Memory Interface for I/O expansion . . . . . . . . . . . 14 4.1 Expanding the number of inputs and/or outputs to 8 lines . . . . . . . . . . . . 14 4.2 Expanding the number of inputs and/or outputs to 16 lines . . . . . . . . . . . 16 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 AN2591 1 Expanding the number of outputs Expanding the number of outputs This section describes how to increase the number of output lines using two techniques: by using a decoder or by using shift registers. 1.1 Using a decoder 1.1.1 Expanding the number of outputs to 8 lines In certain applications, you may need to use several microcontroller outputs but only one of these outputs is activated at any given time. For example when you control a multiplexed display (7-segment or LCD display), only one digit at a given time is selected. This is also the case with matrix keypad management. In order to generate row signals to poll a keypad to determine which key is pressed, you have to use several microcontroller outputs. Figure 1 shows how to reduce the number of outputs used in this type of application. This solution uses a 3-to-8 line decoder (M74HC138) to determine which one of the outputs goes low (Y0 to Y7) by applying an address value on the three inputs A, B and C. The number of lines saved is 8 - 3 = 5 lines. Figure 1. Expander for 8 outputs using a M74HC138 decoder M74HC138 3.3v 16 6 µC GPIO0.0 GPIO0.1 GPIO0.2 G1 Y0 Y1 1 2 3 4 5 8 1.1.2 Vcc A Y2 B Y3 C Y4 G2A G2B GND Y5 Y6 Y7 15 14 13 12 11 10 9 8 Expanding the number of outputs to 16 lines Sometimes, more than 8 outputs are used although only one output needs to be active at a time (for example for a multiplexed display with more than 8 digits, a matrix keypad with more than 7 rows or a row of 16 LEDs for a bargraph level display, and so on). 3/19 Expanding the number of outputs AN2591 This example uses a 4-to-16 line decoder (M74HC4514 or M74HC4515) to expand the number of outputs to 16. The number of lines saved is 16 - 5 = 11 lines. Contrary to the solution described in Section 1.1.1, the microcontroller GPIOs connected to the decoder address lines can also be used for other application purposes. This is made possible by the decoder strobe input which stores the address of the decoder output. The decoder output values remain unchanged while ST = 0, so in this state you can change the GPIO values and you are free to use the lines (GPIO0.0 to GPIO0.3) for other application purposes. In this case, the number of lines saved is 16 - 1 (for strobe line) = 15 lines. Figure 2. Expander for 16 outputs using a M74HC4514 or M74HC4515 decoder Can also be used for other application purposes M74HC4514 / M74HC4515 Strobe 3.3v 24 1 GPIO0.4 Vcc ST Y0 Y1 µC Y2 GPIO0.0 GPIO0.1 GPIO0.2 GPIO0.3 2 A 3 B 21 22 23 12 Y3 Y4 11 9 10 8 7 16 outputs C D 16 INH Y14 GND Y15 15 Note: M74HC4514 and M74HC4515 are similar and pin to pin compatible except that the latter has inverted outputs. 1.2 Using a shift register 1.2.1 Expanding the number of outputs to 8 lines This is another technique you can use to free up some microcontroller lines. This example uses a 74HC595 SIPO (Serial In / Parallel Out) shift register that requires only 3 microcontroller GPIOs to generate one byte of data on its outputs (Q0 to Q7) (see M74HC595 datasheet). 4/19 AN2591 Expanding the number of outputs The shift register receives data serially from the microcontroller via the Sin input (pin 14) using synchronous transmission. The M74HC595 includes two registers. The first one is a shift register to shift in the data, the second one is a latch register to enable the data on the outputs. This register prevents the output values being disturbed while data is being shifted into the shift register. You need to perform two steps to make the data available on the M74HC595 outputs: 1. 2. Shift the data from the microcontroller to the shift register via the Sin line: – Shift each bit of data via the microcontroller (GPIO0.0) and – Apply a clock pulse (via SCK line) to shift this bit into the register. You need 8 clock times to load all bits of the data byte into the shift register. At this stage the data is not yet available on the outputs (Q0 to Q7). To make it available on the outputs (Q0 to Q7), you must apply a latch pulse via the RCK line to load the data into the second register. The number of lines saved is 8 - 3 = 5. This is same amount of saving as with the M74HC138 decoder but the difference that you can activate or deactivate any of the outputs independently, unlike with the decoder when only one output can be activated at a time. Figure 3. Expander for 8 outputs using a shift register: M74HC595 M74HC595 3.3v 16 10 µC Vcc SCLR Q0 Q1 GPIO0.0 GPIO0.1 GPIO0.2 14 Sin 12 RCK 11 SCK Q2 Q3 Q4 Q5 Q6 13 8 1.2.2 G GND Q7 SDO 15 1 2 3 4 5 8 outputs 6 7 9 Expanding the number of outputs to 16 lines In this section uses the same technique as described in Section 1.2.1, but the number of outputs is expanded to 16 lines. This is done by cascading two shift registers (the same IC reference used in Section 1.2.1). The M74HC595 SDO output is used to connect the two shift registers to allow the data flow to pass from the first register into the second one. This creates a 16-bit shift register. 5/19 Expanding the number of outputs AN2591 The availability of data on the outputs of the two M74HC595 components is controlled by the same sequence described in Section 1.2.1 but to make 16 bits of data, the process is done for 16 clock times. You have to apply the latch pulse to the two ICs at the same time to enable the data in the 16 output lines. You can expand the number of outputs even further without using any additional microcontroller GPIO lines because there are two common signals for all registers (RCK and SCK). The number of lines saved is 16 - 3 = 13. Figure 4. Expander for 16 outputs using two shift registers: M74HC595 M74HC595 3.3v 16 10 µC Vcc SCLR Q0 Q1 GPIO0.0 GPIO0.1 GPIO0.2 14 Sin 12 RCK 11 SCK 13 G 8 GND Q2 Q3 15 1 2 3 4 Q4 5 Q5 6 Q6 Q7 7 9 SDO 16 outputs M74HC595 3.3v 16 10 Vcc Q0 SCLR Q1 Q2 14 12 11 Q3 RCK Q4 SCK 13 8 6/19 Sin Q5 Q6 G GND Q7 SDO 15 1 2 3 4 5 6 7 9 To other registers if needed to expand to more than 16 outputs AN2591 2 Expanding the number of inputs Expanding the number of inputs To expand the number input lines, for example to get the state of several bi-state sensors or to scan switch states, two techniques can be used: using multiplexers or shift registers. 2.1 Using a multiplexer 2.1.1 Expanding the number of inputs to 8 lines Figure 5 shows an example of how a multiplexer can be connected to the microcontroller. To get a bit value from a multiplexer input, apply the address value of the input via address lines (A, B and C) and read it in via the multiplexer OUT output (pin 5). To get all 8 bits of data available on the inputs (D0 to D7), you must read all bit states sequentially from the MSB (address value = 0x7) to LSB (address value = 0x0) by decrementing the bit address and applying it on GPIO0.0 to GPIO0.2. For each bit address you read the bit value and shift it into the microcontroller on GPIO0.3 by software to build a data byte. The multiplexer used is the M74HC151. This allows you to save 8 - 4 = 4 lines. Figure 5. Expander for 8 inputs using a multiplexer: M74HC151 3.3V 16 M74HC151 Vcc D0 µC GPIO0.0 GPIO0.1 GPIO0.2 GPIO0.3 D1 11 10 9 5 A D2 B D3 C D4 Out D5 D6 7 8 ST D7 4 3 2 1 15 14 8 inputs 13 12 GND 7/19 Expanding the number of inputs 2.1.2 AN2591 Expanding the number of inputs to 16 lines This section uses the same technique as described in Section 2.1.1. The M74HC4067 multiplexer 16 to 1 is used to expand the input lines to 16. In this case the number of lines saved is 16 - 5 = 11. To get 16 bits of data from the multiplexer inputs you must scan all inputs (as described in Section 2.1.1) by applying a 4 bit address to the A,B,C and D lines sequentially from MSB (address value = 0xF) to LSB (address value = 0x0) and shift each bit into the microcontroller to build a 16 bit data word. Figure 6. Expander for 16 inputs using a multiplexer: M74HC4067 M74HC4067 3.3v 24 Vcc Y0 9 Y1 8 Y2 Y3 µC Y4 GPIO0.0 GPIO0.1 GPIO0.2 GPIO0.3 GPIO0.4 10 11 14 13 1 A Y5 B Y6 C D Out Y7 3 2 Y9 22 E Y12 GND Y13 Y14 Y15 8/19 5 4 23 Y11 12 6 Y8 Y10 15 7 21 20 19 18 17 16 16 inputs AN2591 Expanding the number of inputs 2.2 Using shift registers 2.2.1 Expanding the number of inputs to 8 lines A PISO (Parallel In Serial Out) shift register can also be used to increase the number of inputs. Figure 7 shows how to connect a shift register M74HC165 to a microcontroller using only 3 GPIOs. The shift register sends data serially to the microcontroller via its QH output (pin 9) using synchronous communication. First, you load the 8 bit data available on the inputs (A to H) into the register by applying a load pulse (set it low and then high again), the 8 bits of data are loaded in parallel into the register. The logic level that was present at the H input appears immediately on the QH output when the latch pulse is applied. Then, on each subsequent rising edge of the clock (pin 2), the logic level of the next bit appears on the QH output and can be read via the GPIO0.2 input and shifted by software to build a byte of data. 8 bits of data take 7 clock pulses (generated by the microcontroller) to shift out one byte and not 8 clocks because initially the higher bit was present on QH. The number of lines saved is 8 - 3 = 5. Figure 7. Expander for 8 inputs using a shift register: M74HC165 M74HC165 3.3v 16 µC Vcc A GPIO0.0 GPIO0.1 GPIO0.2 1 2 9 15 10 8 S/L B CK C QH D QH E F CK INH G SI H 11 12 13 14 3 4 8 inputs 5 6 GND 9/19 Expanding the number of inputs 2.2.2 AN2591 Expanding the number of inputs to 16 lines To expand the number of inputs to more than 8 using shift registers, you can cascade two or more shift registers by connecting the QH output of one shift register, that receives the inputs for the low order data byte, to the SI input of the next register that receives the inputs for the high order data byte. The register for the high order data byte is the one that must have its QH output directly connected to the microcontroller. The same technique as described in Section 2.2.1 is used to read all 16 inputs and 15 clock pulses generated by the microcontroller to get all 16 bits of data. In this case, when two registers are used, the number of lines saved is 16 - 3 = 13. By cascading more than two registers, in order to expand more than 16 inputs, the number of GPIOs used for this technique remains the same: always 3 lines from the microcontroller. Figure 8. Expander for 16 inputs using two shift registers: 74HC165 M74HC165 3.3v 16 µC Vcc A GPIO0.0 GPIO0.1 GPIO0.2 1 2 9 7 10 S/L B CK C QH D QH E SI 15 8 CK INH F G H 11 12 13 I8 I9 I10 14 I11 3 I12 4 I13 5 I14 6 I15 GND 16 inputs M74HC165 3.3v 16 Vcc A 9 7 1 2 15 From previous register QH output if needed to expand more than 16 inputs if not used it must be tied to ground 10/19 10 8 QH B QH S/L C D CK E F CK INH SI GND G H 11 I0 12 I1 13 I2 14 I3 3 4 I4 I5 5 I6 6 I7 AN2591 3 Using a STMPE801 (I²C bus) 8-bit I/O port expander Using a STMPE801 (I²C bus) 8-bit I/O port expander The STMPE801 is a General Purpose Input / Output (GPIO) expander specially designed for expanding the input/output lines and solving the common problem of not having enough GPIOs available for the application. This IC offers several features: ● 8 available GPIOs. ● Each GPIO can be individually configured as input or output in the data direction register GPDR and can be a source of an external interrupt configurable in the IEGPIOR register. ● This device interfaces to the microcontroller using an I2C bus. ● Up to 2 devices (2 x STMPE801) can share the same I2C bus. Note: For more details refer to the STMPE801 datasheet. 3.1 Expanding the number of inputs/outputs to 8 lines Figure 9 shows how the STMPE801 is connected via I2C bus to the microcontroller. When ADDR0 pin is connected to ground, the I2C device address will be 0x82, otherwise (ADDR0 = Vcc) the device address will be 0x88. Figure 9. Eight channel expander STMPE801MTR 3.3v 16 1 R1 4.7K R2 4.7K µC 13 SCL XTI GPIOX.Y 11 If needed VIO 3 GPIO0 4 5 GPIO1 15 SDA VCC 12 SDAT GPIO2 SCLK GPIO3 6 7 GPIO4 INT 8 GPIO5 9 RESET GPIO6 14 2 ADDR0 GPIO7 8 inputs/ outputs 10 GND (I2C device address=0x82) Each GPIO (0 to 7) can be configured independently as input or output (see details in the GPDR section in STMPE801 datasheet). You can read the state of each GPIO in the GPMR register. To set a GPIO state, you need to set its value in the GPSR register. The INT (interrupt) output can be optionally used to increase the number of external interrupt sources of the microcontroller by connecting it to one of external interrupt lines: XTI microcontroller interrupt input. To use this option, you must first configure the IEGPIOR register to enable the interrupt from a particular GPIO interrupt source to the host. After that, 11/19 Using a STMPE801 (I²C bus) 8-bit I/O port expander AN2591 you must configure the INTPolarity bit and the INT_Enable bit in the SystemControl register (address = 0x04). You must read the ISGPIOR register in interrupt routine to determine which GPIO has generated the interrupt. Each bit of this register, represents the interrupt status of each GPIO pin. The VIO pin is provided to fix the voltage used by the GPIOs. The VIO pin level, must be greater than or equal to the Vcc level. The RESET input pin is connected to any microcontroller GPIO to initialize the STMPE801 when it is needed. 3.2 Expanding the number of I/Os to 16 lines The STMPE801 design supports connecting up to two devices in the same I2C bus to obtain 16 additional input/output lines. Figure 10 shows how two STMPE801 share the two lines of the I2C interface with different addresses. The first device the has its ADDR0 input connected to ground will have the address 0x82, whereas the ADDR0 input of the second device is connected to Vcc to fix the I2C device address to 0x88. It is possible to use the two interrupt outputs of the two devices to expand the interrupt lines to 16. Each interrupt output (INT: pin 11) is connected to an interrupt input (two interrupt inputs are used, see Figure 10). Note: 12/19 When only one interrupt is available in the microcontroller you are using, you can add an appropriate logic gate to connect the two INT outputs to its inputs and connect the gate output to the interrupt input of the microcontroller. In this case, to determine which GPIO has generated the interrupt, you must read the ISGPIOR register of each device). AN2591 Using a STMPE801 (I²C bus) 8-bit I/O port expander Figure 10. 16-channel expander STMPE801MTR 3.3v 16 1 R1 4.7K STR710 13 SCL 11 If needed 12 XTI1 VIO GPIO0 GPIO1 15 SDA XTI0 GPIOX.Y R2 4.7K VCC SDAT GPIO2 SCLK GPIO3 GPIO4 INT GPIO5 RESET GPIO6 14 2 ADDR0 GPIO7 3 4 5 6 7 8 9 10 GND 16 inputs/ outputs (I2C device address=0x82) STMPE801MTR 3.3V 16 1 14 15 13 12 11 If needed 2 VCC GPIO0 VIO ADDR0 GPIO1 GPIO2 SDAT GPIO3 SCLK GPIO4 RESET GPIO5 INT GPIO6 GPIO7 3 4 5 6 7 8 9 10 GND (I2C device address=0x88) 13/19 Using the External Memory Interface for I/O expansion 4 AN2591 Using the External Memory Interface for I/O expansion The EMI can be used not only to interface external memories, but also to create additional microcontroller ports. The EMI can be used to expand the number of inputs and/or outputs lines when all GPIOs are used and you need more GPIOs. In this section we give a solution using latches and buffers that allows you to expand the number of inputs and/or outputs using the EMI interface of the STR710 microcontroller. 4.1 Expanding the number of inputs and/or outputs to 8 lines Figure 11 shows how the input/output expander is implemented using 74LCX573 (8-bit latch) to create an 8-line output port and 74LCX244 (8-bit buffer) to create an 8-line input port (see 74LCX573 and 74LCX244 datasheets). The latch inputs and the buffer outputs are connected to the LSB data byte: D0 to D7. Figure 11. Eight-channel expander EMI data bus 16 8 74LCX573 VDD D[0..7] 1 16 8 D[0..7] 11 STR710 3.3v 20 Q[0..7] 8 outputs LE OE GND 10 D[0..15] 74V1G02 WRL CSn.x RD 74V1G08 3.3v 74LCX244 20 1 1G VDD 19 8 2G 1A-2A 8 D[0..7] 1Y-2Y 10 GND 8 inputs To put a byte of data on the latch outputs, you must write the data in an address located in address range corresponding to the CSn.x bank. For example, if the CSn.x used is CSn.0, and the EMI address is not remapped at address 0 (EMI address range = 0x60000000 to 14/19 AN2591 Using the External Memory Interface for I/O expansion 0x6C00000C), the bank0 address range will be 0x60000000 to 0x60FFFFFF. So the register and the buffer addresses must be included in this range for example: Register address: 0x60000001. Buffer address: 0x60000002. Note: You can use the same address for the register and the buffer. The bank used for the buffer and the register is reserved only for these devices and does not use any memory or other device in this bank. EMI configuration: Note: ● Configure the EMI effective external bus size as 8-bit data bus wide to read from the external 8-bit buffer and to write in the external 8-bit register. ● Configure the number of wait states to obtain a WRL pulse width delay greater than 15ns (15ns is the greatest propagation time value of the latch, buffer, NOR and AND gates. Refer to the 74LCX573, 74LCX244, 74V1G02 and 74V1G08 datasheets). The WRL and RD pulse widths must be greater than the propagation delay of the circuit which has the highest propagation delay (in this case 74V1G08, refer to its datasheet). So you must configure the corresponding number of wait states to write to the register and to read from the buffer properly. Example of software that gets the value of the 8-bit buffer inputs and writes it in the 8-bit latch outputs. The two devices are available in bank0 (CSn.x = CSn.0): int main(void) { u8 a; u8 *Buffer_Data8_in = (u8*)0x60000002; /* Initialize the buffer address to 0x60000002 */ u8 *Latch_Data8_out = (u8*)0x60000001; /* Initialize the latch address to 0x60000001 */ *(u8*)0x6C000000 = 0x8004; /* Configure EMI_BCON0 register: - 8 bit data bus wide - 1 wait state - Enable bank 0 */ while(1) { a = *Buffer_Data8_in; /* Get the value available on inputs of the external buffer */ *Latch_Data8_out = a; /* Write this value in the external latch */ } } 15/19 Using the External Memory Interface for I/O expansion 4.2 AN2591 Expanding the number of inputs and/or outputs to 16 lines In this section, the same hardware structure is implemented as described in Section 4.1, except that a 16-bit latch (74LCX16373) and 16-bit buffer (74LCX16244) are used. It is important to know the propagation delay times of the devices you are using in order to configure the corresponding number of wait states (Refer to the 74LCX16373, 74LCX16244, 74V1G02 and 74V1G08 datasheets). Figure 12. 16-channel expander EMI data bus 74LCX16373 16 D[0..15] 1D-2D[0..16] VDD* 1 1LE 1Q-2Q[0..16] 25 2LE 24 1OE 48 2OE GND* 16 STR710 16 D[0..15] 74LCX16244 1 1G 48 2G 25 3G 24 4G 74V1G02 WRL CSn.x RD 74V1G08 D[0..15] 16 16 outputs 3.3v VDD* 16 1A-2A-3A-4A 16 3.3v 16 inputs 1Y-2Y-3Y-4Y GND* Note (*): The VDD and GND power inputs of 74LCX16373 and 74LCX16244 are not all represented in this figure: VDD pins of the two devices are: 7, 18, 31 and 42, must connected to 3.3V. All GND pins of the two devices are: 4, 10, 15, 21, 28, 34, 39 and 45, must connected to ground. For more details refer to the IC datasheets. 16/19 AN2591 Using the External Memory Interface for I/O expansion Example of software that gets the value on the 16-bit buffer inputs and writes it in the 16-bit latch outputs. The two devices are available in bank0 (CSn.x = CSn.0): int main(void) { u16 a; u16 *Buffer_Data16_in = (u16*)0x60000002; /* Initialize the buffer address to 0x60000002 */ u16 *Latch_Data16_out = (u16*)0x60000001; /* Initialize the latch address to 0x60000001 */ *(u16*)0x6C000000 = 0x8005; /* Configure EMI_BCON0 register: - 16 bit data bus wide - 1 wait state - Enable bank 0 */ while(1) { a = *Buffer_Data16_in; /* Get the value available on inputs of the external buffer */ *Latch_Data16_out = a; /* Write this value in the external latch */ } } 17/19 Revision history 5 AN2591 Revision history Table 1. 18/19 Document revision history Date Revision 06-Aug-2007 1 Changes Initial release. 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