Microcontroller Solutions TechZone Magazine, April 2011 - Digikey

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Peripheral Reflex System Avoids MCU Overload contributed by Energy Micro Normally, MCUs spend much of their time passing information between peripherals. That doesn’t have to be the case. The Peripheral Refl ex System (PRS) is a network which lets the different peripheral modules communicate directly with each other without involving the CPU. Peripheral modules which send out Refl ex signals are called producers. The PRS routes these refl ex signals to consumer peripherals which apply actions depending on the Refl ex signals received. The format for the Refl ex signals is not given, but edge triggers and other functionality can be applied by the PRS. Overview An overview of one channel and how four different peripherals can be connected using PRS is given in Figure 1. The PRS contains eight interconnect channels, and each of these can select between all of the output Refl ex signals offered by the producers. The consumers can then choose which PRS channel to listen to and perform actions based on the Refl ex signals routed through that channel. The Refl ex signals can be both pulse signals and level signals. Synchronous PRS pulses are one HFPERCLK cycle long, and can either be sent out by a producer (e.g. ADC conversion complete) or be generated from the edge detector in the PRS channel. Level signals can have an arbitrary waveform, but will be synchronized with the HFPERCLK. Producer Side Channel 0 Signals from producer peripherals TIMER0 Overflow Pulse ACMP Output Level . . . . . PRS x8 SIGSEL[2:0] SOURCESEL[5:0] EDSEL[1:0] SWPULSE[0] SWLEVEL[0] ADC0 Start single conv. Reg APB bus Signals to consumer perpherals TIME1 CC1 Input Consumer Side Figure 1 shows four peripherals connected to two PRS channels. On one channel is TIMER0 and ADC0 and the ACMP and TIMER1 are connected to a second channel. An overfl ow from TIMER0 can start an ADC single conversion and an ACMP output can be used as input for a Compare/Capture channel on TIMER1. General operation Channel functions Different functions can be applied to a Refl ex signal within the PRS. Each channel includes an edge detector to enable generation of pulse signals from level signals. It is also possible to generate output Refl ex signals by software writing to PRS_SWPULSE and PRS_SWLEVEL registers. PRS_SWLEVEL is a programmable level for each channel and holds the value it is programmed to. The PRS_SWPULSE will give out a one-cycle high pulse if it is written to 1, otherwise a 0 is asserted. The SWLEVEL and SWPULSE signals are then XOR’ed with the selected input from the producers to form the output signal sent to the consumers listening to the channel. This is illustrated in Figure 1. The efm32lib function void PRS_ SourceSignalSet(unsigned int ch, uint32_t source, uint32_t signal, PRS_Edge_TypeDef edge) can be used to easily confi gure the PRS channels. By specifying the PRS channel producing peripheral signal from the peripheral and the edge for pulse generation, the function confi gures the PRS accordingly. Producers Each PRS channel can choose between signals from several producers, which is configured in SOURCESEL in PRS_CHx_CTRL. Each of these producers outputs one or more signals which can be selected by setting the SIGSEL field in PRS_CHx_CTRL. Setting the SOURCESEL bits to 0 (Off) leads to a constant 0 output from the input mux. An overview of the available producers is given in Table 1. Consumers Consumer peripherals (Table 2) can be set to listen to a PRS channel and perform an action based on the signal received on that channel. Most consumers expect pulse input, while some can handle level inputs as well. Figure 1: PRS overview. 44
Table 1: Reflex producers. Module Reflex Output Output Format ACMP Comparator Output Level ADC Single Conversion Done Pulse Scan Conversion Done Pulse DAC Channel 0 Conversion Done Pulse Channel 0 Conversion Done Pulse GPIO Pin 0 Input Level Pin 1 Input Level Pin 2 Input Level Pin 3 Input Level Pin 4 Input Level Pin 5 Input Level Pin 6 Input Level Pin 7 Input Level Pin 8 Input Level Pin 9 Input Level Pin 10 Input Level Pin 11 Input Level Pin 12 Input Level Pin 13 Input Level Pin 14 Input Level Pin 15 Input Level RTC Overflow Pulse Compare Match 0 Pulse Compare Match 1 Pulse TIMER Underflow Pulse Overflow Pulse CC0 Output Level CC1 Ouput Level CC2 Output Level UART TX Complete Pulse RX Data Received Pulse USART TX Complete Pulse RX Data Received Pulse IrDA Decoder Output Level VCMP Comparator Output Level Table 2: Reflex consumers. Module Reflex Input Input Format ADC Single Mode Trigger Pulse Scan Mode Trigger Pulse DAC Channel 0 Trigger Pulse Channel 1 Trigger Pulse TIMER CC0 Input Pulse/Level CC1 Input Pulse/Level CC2 Input Pulse/Level DTI Fault Source 0 (TIMER0 only) Pulse DTI Fault Source 1 (TIMER0 only) Pulse DTI Input (TIMER0 only) Pulse/Level UART TX/RX Enable Pulse USART TX/RX Enable Pulse IrDA Encoder Input (USART0 only) Level Software examples This section describes four software examples that explore possible interactions between peripherals using the PRS: • TIMER triggered ADC conversion • Pulse Width Measurement with the ACMP and TIMER • GPIO triggered UART transmission • Software triggered DAC conversion TIMER triggered ADC conversion Figure 2 shows how to set up ADC0 to start a single conversion every time that TIMER0 overflows. TIMER0 sends a one HFPERCLK cycle high pulse sent from TIMER0 to the ADC0 through the PRS on each overflow, and the ADC does a single conversion which is displayed on the LCD of the STK/DK development boards. The ADC consumes pulse signals, which are the same signals produced by the TIMER. In this case there is no edge detection needed, so the PRS leaves the incoming signal unchanged. Producer Side Channel 5 Signals from producer peripherals TIMER0 Overflow Pulse 1 HFPERCLK Pulse SIGSEL[2:0] SOURCESEL[5:0] EDSEL[1:0] SWPULSE[5] SWLEVEL[5] ADC0 Start single conv. APB bus Signals to consumer perpherals Figure 2: TIMER0 overflow starting ADC0 single conversions using the PRS. PRS Consumer Side The ADC is configured with 8-bit resolution and Vdd as both reference and input. When the ADC finishes the conversion it generates a single conversion complete interrupt. The CPU will then fetch the result and display it on the LCD. The displayed result is a direct reading from the ADC0_SINGLEDATA register which is always 255, given that the input is the same as the reference. The DMA can also be used to fetch the conversion result and that is covered by the AN0021 Analog to Digital Converter. The software project prs_timer_adc implements this example and can be used on both STK and DK development boards. Pulse width measurement with ACMP and TIMER Figure 3 illustrates how to measure the pulse width or period of an arbitrary waveform. The ACMP is used to send a level signal through the PRS. TIMER0 consumes both pulse and level signals so the PRS leaves the incoming signal unchanged. On TIMER0 the PRS signal is used as input for CC0 channel. TIMER0 starts counting on a positive edge and captures the counter value on a negative edge. Reg 1 HFPERCLK Pulse www.digikey.ca/microcontroller 45
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Microcontroller Solutions TechZone Magazine, April 2011 - Digikey

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