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| 1 | +* Circuit |
| 2 | + |
| 3 | +: /-----------------\ |
| 4 | +: | | |
| 5 | +: | /-------------|---\ |
| 6 | +: | | | | |
| 7 | +: | | |-ATmega--| | | |
| 8 | +: | \-| 1 28 | | | |
| 9 | +: R1 | | 2 27 | | | |
| 10 | +: /---\/\/\/\/---+ | 3 26 | | | |
| 11 | +: | | | 4 25 | | | / |--RPi GPIO--| |
| 12 | +: | |-Osc-----| | | 5 24 | +---|---/ ---| 3.3V | 5V | |
| 13 | +: \-| E Vcc |--+ | 6 23 | | | | 2 | 5V | |
| 14 | +: | | \-----| 7 22 |-|-\ | | 3 | GND | |
| 15 | +: | | /-| 8 21 |-+ | \---------| 4 | 14 | |
| 16 | +: /-| GND OUT |------|-| 9 20 |-/ | | GND | 15 | |
| 17 | +: | | | 10 19 |---|-----------| 17 | 18 | |
| 18 | +: | | | 11 18 |---|-----------| 27 | GND | |
| 19 | +: | | | 12 17 |---|-----------| 22 | 23 | |
| 20 | +: | | | 13 16 | | | 3.3V | 24 | |
| 21 | +: | | | 14 15 | | | 10 | GND | |
| 22 | +: | | | | 9 | 25 | |
| 23 | +: | | | | 11 | 8 | |
| 24 | +: \------------------+---------------+-----------| GND | 7 | |
| 25 | +: |------+-----| |
| 26 | +: | 0 | 1 | |
| 27 | +: | 5 | GND | |
| 28 | +: | 6 | 12 | |
| 29 | +: | 13 | GND | |
| 30 | +: | 19 | 16 | |
| 31 | +: | 26 | 20 | |
| 32 | +: | GND | 21 | |
| 33 | + |
| 34 | + |
| 35 | +* Operation |
| 36 | + |
| 37 | + - The circuit operates at 3.3V, provided by the Raspberry Pi. |
| 38 | + |
| 39 | + - Power is provided by one of the Raspberry Pi's 3.3V pins. There |
| 40 | + is a switch which allows cutting off the power to the ATmega and |
| 41 | + the oscillator. |
| 42 | + |
| 43 | + - The oscillator should have a frequency of 10MHz or less. This |
| 44 | + allows the ATmega to operate at 3.3V. |
| 45 | + |
| 46 | + - Check your oscillator's datasheet for the correct wiring. Common |
| 47 | + variations are needing pin 1 to be high or needing pin 1 to be left |
| 48 | + unconnected. The circuit diagram here shows pin 1 pulled high with |
| 49 | + pull-up resistor R1. The value of R1 is not very critical; I like |
| 50 | + to use 10K ohm, but any lower value should work, too, including |
| 51 | + wiring to Vcc directly - provided your oscillator actually needs the |
| 52 | + pin to be high. |
| 53 | + |
| 54 | + - The oscillator's output is connected to ATmega pin 9 (XTAL1). |
| 55 | + This allows the programming circuit to function even when the |
| 56 | + ATmega is configured to use an external clock source. |
| 57 | + |
| 58 | + - The ATmega is programmed using a serial protocol. It uses three |
| 59 | + signals: SCK (clock), MISO (data from the ATmega to the programmer), |
| 60 | + and MOSI (data from the programmer to the ATmega). |
| 61 | + |
| 62 | + - SCK is provided by GPIO17 of the Raspberry Pi and connected to |
| 63 | + pin 19 of the ATmega. |
| 64 | + |
| 65 | + - MISO is provided by GPIO27 of the Raspberry Pi and connected to |
| 66 | + pin 18 of the ATmega. |
| 67 | + |
| 68 | + - MOSI is provided by GPIO22 of the Raspberry Pi and connected to |
| 69 | + pin 17 of the ATmega. |
| 70 | + |
| 71 | + - The ATmega's reset# pin (pin 1) is held low during programming. This |
| 72 | + is controlled by GPIO4 of the Raspberry Pi. |
| 73 | + |
| 74 | +* Alternatives to 3.3V Oscillator |
| 75 | + |
| 76 | +The circuit runs at 3.3V, but many oscillator ICs want 5V. If you don't |
| 77 | +have an oscillator that will work at 3.3V, there are several options. |
| 78 | +Note that I have not tested any of these. |
| 79 | + |
| 80 | +If you have an oscillator that runs at 5V, one simple option is to |
| 81 | +power the oscillator using one of the 5V pins on the Raspberry Pi and |
| 82 | +run the oscillator's output through a voltage divider before feeding |
| 83 | +it to the ATmega. Something like |
| 84 | + |
| 85 | +: |
| 86 | +: 5V |
| 87 | +: | |
| 88 | +: +---------------\ |
| 89 | +: | | |
| 90 | +: > | |
| 91 | +: < R | |
| 92 | +: > 1 | |
| 93 | +: < | |
| 94 | +: | | |
| 95 | +: \---| E Vcc |-/ |
| 96 | +: | | |
| 97 | +: | | |
| 98 | +: /-| GND OUT |----\ |
| 99 | +: | | |
| 100 | +: | > |
| 101 | +: | < R |
| 102 | +: | > 2 |
| 103 | +: | < |
| 104 | +: | | to ATmega XTAL1 pin |
| 105 | +: | +--------- |
| 106 | +: | | |
| 107 | +: | > |
| 108 | +: | < R |
| 109 | +: | > 3 |
| 110 | +: | < |
| 111 | +: | | |
| 112 | +: +----------------/ |
| 113 | +: | |
| 114 | +: ------- |
| 115 | +: --- |
| 116 | +: - |
| 117 | + |
| 118 | +To get from 5V to 3.3V, you need R3 to have twice the resistance of |
| 119 | +R2. Of course, resistors do not always have exactly the resistance |
| 120 | +they are rated for. A 10% tolerance is common. At the same time, we |
| 121 | +do not need to get to exactly 3.3V. Let's do some calculations. |
| 122 | + |
| 123 | +From the ATmega328 datasheet, we get that the absolute maximum voltage |
| 124 | +on the XTAL1 pin is Vcc+0.5V. Assuming a 10% tolerance for the supply, |
| 125 | +that means we want to go no higher than 3.3V * 90% + 0.5V = 3.47V. To |
| 126 | +get that from a 5V supply with 10% tolerance, we need R3 / (R2 + R3) |
| 127 | +to be at most 3.47 / 5.5, which means actual R2 >= (5.5 - 3.47) * R3 / |
| 128 | +3.47. To account for resistor tolerances, we want rated R2 >= 1.21 |
| 129 | +nominal R2. This works out to rated R2/R3 >= 0.71. |
| 130 | + |
| 131 | +On the low side, we need a high signal to be at least 0.6Vcc to be |
| 132 | +recognized as high by an ATmega running at 3.3V. This means we want at |
| 133 | +least 3.3V * 110% * 0.6 = 2.18V. To get that from a 5V signal with 10% |
| 134 | +tolerance, we need R3 / (R2 + R3) to be at least 2.18 / 4.5. Again |
| 135 | +accounting for resistor tolerances, this means we need rated R2/R3 <= |
| 136 | +0.86. |
| 137 | + |
| 138 | +For example, if we pick R3 rated 1.5K ohm and R2 rated 1.2K ohm, we |
| 139 | +can expect an actual voltage between (5V * 0.9) * (1.5 * 0.9) / (1.5 * |
| 140 | +0.9 + 1.2 * 1.1) = 2.28V and (5V * 1.1) * (1.5 * 1.1) / (1.5 * 1.1 + |
| 141 | +1.2 * 0.9) = 3.32V, which is within the tolerances for XTAL1. |
| 142 | + |
| 143 | +(The above calculations ignore the impedance of XTAL1 itself, assuming |
| 144 | +it is much higher than the resistor values being used here.) |
| 145 | + |
| 146 | +Another option would be to use a diode so that current can only flow from |
| 147 | +the ATmega's XTAL1 pin to the oscillator's output and a pull-up resistor |
| 148 | +to 3.3V on the XTAL1 pin. This way, when the oscillator output is low, it |
| 149 | +drives the XTAL1 pin low, and when the output from the oscillator is high, |
| 150 | +the pull-up resistor pulls XTAL1 to 3.3V. |
| 151 | + |
| 152 | +Yet another possibility is to use a 74LVC245 at 3.3V to drive XTAL1, with |
| 153 | +the oscillator's output connected to the '245's 5V-tolerant inputs. |
| 154 | + |
| 155 | +Alternatively, you could build your own oscillator circuit that runs |
| 156 | +at 3.3V. Possibilities here include using a crystal, a resonator, or |
| 157 | +an LC circuit. The frequency on XTAL1 does not have to be very |
| 158 | +precise, so this should work fine. |
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docs/design/atmega328-programmer.txt