Multi Programmer: One Size Fits All （2）

Published:2011/8/1 1:28:00 Author:Li xiao na From:SeekIC

By Andreas Oyrer

Programming voltages

ICl controls all the programming signals and voltages over its 32 I/O pins. In order to generate the programming voltages required by various microcontrollers the 5 V supply from the USB is converted to approximately 13 V using a step-up regulator. Normally the output voltage of the switching regulator would be a constant 12 V, but a diode in the feedback path of IC3 raises the output voltage by the forward voltage drop of the diode: this higher voltage allows PIC microcontrollers to be programmed. The programming voltages are switched as required using p-channel and n-channel FETs. A voltage of 0 V, 5 V or 12 V can be made to appear on pin 1 or pin 31 of the programming socket. A voltage of 13 V is available for the MCLR signal on ISP connector K3, which is used for programming PICs. Diodes D5 and D7 reduce this voltage back down to 12 V: this lower voltage is used when programming Atmel microcontrollers.

TTL gates with open-collector outputs (type 74LS07) are used to drive the FETs. This allows a voltage of 0 V to appear between gate and source, ensuring that the transistors can be fully turned off. If used directly, the voltage on the I/O port pins could only rise to about 3.3 V, giving a gate-source voltage of 10 V: the transistors would then still conduct.

Some microcontrollers require a programming voltage on the reset or crystal inputs. In the case of the 90S1200, for example, a minimum voltage of 0.85VCC = 4.25 V (assuming a 5 V supply) is required on the reset input. Since the TUSB3210 runs from a 3.3 V supply it can only deliver a logic high level of 3.3 V; the remaining gates in IC4 are used to produce a high level of over 4 V.

Programming socket

Most devices can be programmed directly in socket IC5. Crystal X2 provides a clock source for Atmel 89Cxx and 89Sxx type microcontrollers. Because of the limited number of I/O pins offered by the TUSB3210 only a certain range of microcontrollers can be programmed. Other (larger) microcontrollers can, however, be programmed using the ISP connectors.

On closer inspection, you will notice that there is no ground connection on pin 10, as would be required, for example, by an 89Cx051. Since the supply current is not particularly high during programming, it is sufficient to provide the ground via port pin P3.6 of IC1. The voltage does indeed rise a little above ground, but remains within reasonable limits.

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